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Symbol, Formula and Valency Chapter-2 Answer to the Short Questions (a) What is symbol? What is the significance of symbol? Ans.: A symbol of an element is an abbreviation of the full name of an element. Thus the symbol of Hydrogen is (H), Oxygen (O) and Nitrogen (N) etc. Significance of symbol: 1. The symbol of an element represents the name of the element in short e.g. hydrogen by H, oxygen by O, sodium by Na. 2. The symbol of an element indicates one atom of that element such as H stands for one atom of hydrogen. Likewise O stands for one atom oxygen, C for one atom carbon, Na for one atom of sodium etc. 3. The symbol represents one atom of an element and it also stands for the atomic weight of that element. For example, H means one part by weight of hydrogen and O means 16 parts by weight of oxygen. (b)Write down the symbol of: Carbon, Copper, Potassium, Iron and Calcium. Ans.: The symbols are as follows: Full Name Carbon Copper Potassium Iron
Symbol C Cu K Fe
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Calcium Ca (c) What is called a formula? Explain the significance of formula. Ans.: The abbreviated form of the molecule of elements or compounds is called formula. For example: the formula of water is H2O, calcium carbonate CaCO3 etc. Significance of formula: Formula has qualitative and quantitative significance. Qualitative significance: 1. It represents a particular substance. For example the formula H2O means the substance water. 2. It also denotes the different elements present in that substance. Quantitative significance: 1. Formula represents one molecule of a particular substance. For example, one molecule of sodium Chloride is represented by NaCl. 2. Formula represents the number of atoms of each of the constituent elements present in the molecule of the compound. 3. A formula represents molecular weight of a particular Substance and the molecular formula shows the relative weight of the elements presets in the molecular weight of the compound; for example, CO2 means the molecular weight of CO2 is 44 and it also represents relative weight of carbon and oxygen in the mol. Wt of the compound; i.e. in 44 parts by wt. of carbon dioxide there are 12 parts by weight of carbon and 32 parts by weight of oxygen. (d) What is meant by combining capacity of an element? Which element is taken as unit to determine the valency of an element? Ans.: The combing capacity of an element with others is called valency. The valency is measured as the number of hydrogen or equivalent atoms which is combined with or replaced by one atom of an element. Generally hydrogen or chlorine atom is combined with most of the element. For that reason hydrogen and chlorine atom is taken as the unit to determine the valency of an element. For example, one hydrogen atom is combined with one chlorine atom in hydrochloric acid (HCl) compound. So the valency of chlorine is one in HCl.
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Again in calcium chloride (CaCl2) compound two chlorine atoms combine with one calcium atom. So the valency of calcium is two in CaCl2 compound. Name of compound radical Sulphate Carbonate Nitrate By carbonate Phosphate
Formula SO4 CO3 NO3 HCO3 PO4
Valency 2 2 1 1 3
(e) What are compound radicals? Give examples of compound radicals. Ans.: Compound radicals are the group of atoms which act like atoms and participate in the chemical reaction as a group as like as atoms are called compound radicals. Example : Sulphate (SO4) is a radical which is composed with two different elements sulphur and oxygen. Here, four atoms of oxygen and one atom of sulphur is combined together and acts as like as an atom as a group. It is understood clearly from the following reaction: ZnSO4
+
2NaOH = Na2SO4 + Zn(OH) 2 unchanged Like this way one atom of carbon and three atoms of oxygen ed together to form (CO3) radical, one atom of phosphorus and four atoms of oxygen ed together to form phoshphate radical. (f) Write down the formula of the following compounds: Ferrous nitrate, Potassium sulphate, Ammonium carbonate, Copper sulphate, Magnesium hydrogen carbonate, Lead nitrate, Calcium oxide, Nitric acid, Sodium carbonate, Iron(II) sulphate, Magnesium hydroxide. Ans.: Name of the compound Formulae Ferrous nitrate Fe(NO3)2 Potassium sulphate K2SO4 Ammonium carbonate (NH4)2CO3 Copper sulphate CuSO4 Magnesium hydrogen carbonate Mg(HCO3)2
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Lead nitrate Calcium oxide Nitric acid Sodium carbonate Iron (II) sulphate Magnesium hydroxide
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Pb(NO3)2 CaO HNO3 Na2CO3 FeSO4 Mg(OH)2
(g) Write down the valency of the following compounds or radicals: Hydrogen carbonate, Nitrate, Carbonate, Sulphate, Carbon, Copper, Zinc, Iron, Aluminum, Oxide. Ans.: The valency of the following elements and radicals are as follows: Name of radicals/elements Hydrogen carbonates Nitrate Carbonate Sulphate Carbon Copper Zinc Aluminum Oxide
valency 1 1 2 2 4 2/1 2 3 2
Answer to the Essay Type Questions (a) Give the rules of writing symbols with examples. Ans.: The rules of the writing symbol are as follows: 1. Generally the symbol is represented by the first letter of its English name e.g. English name Symbol Hydrogen H Oxygen O Nitrogen N Carbon C Sulphur S
Symbol, Formula and Valency
Boron Phosphorus Uranium
19
B P U
2. When however, the names of two elements begin with the same letter, first letter in capital is used for one and for the others first letter along with another, prominently heard in pronouncing the name of the element , are used to represent the element. For example English neme Symbol Carbon C Calcium Ca Cobalt Co Cium Cd Chlorine Cl Boron B Barium Ba Bismuth Bi Bromine Br Beryllium Be 3. Some elements are represents by first letter or first two letters of its Latin names but there are also some exceptions. For example English name Sodium Potassium Copper Silver Tin
Latin name Natrium Kalium Cuprum Argentums Stanum
Symbol Na K Cu Ag Sn
(b) What is formula? Write down the differences between symbol and formula. Ans.: Symbol : The abbreviated form of the molecule of elements or compounds is called formula. That is, the shortened form of the molecule of any substance is called formula. To write the formula of a compound the symbols of the constituent elements are written side by side and the number of atoms of each
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element present in the molecule is mentioned as subscript on the right hand side of the symbols. For example: the formula of water = H2O, the formula of ammonia = NH3. The differences between symbol and formula are given below : Symbol Formula 1. Short name of an element is symbol. 1. Short name of an element or compound is formula. 2. The symbol of an element indicates 2. The formula of a substance indicates one atom. one molecule. 3. The symbol of element represents 3. Formula represents molecular weight the atomic weight of that element. of a particular substance. 4. Symbols contain only one type of 4. Formula represents the number of atoms. atoms of each of the constituent elements present in a molecule of the substance. (c) What is formula? Write down the in correct formulae given below and correct the incorrect one and write them. (i) H2S (ii) H2Cl (iii) Na(OH) 2 (iv) NaCO3 (v) H(NO3) 2 (vi) CaHCO3 (vii) (NH4) 2SO4 (viii) Fe2 (SO4) 3 (ix) BaCl (x) NO2Cl Ans.:
Formula (i)H2S (ii)H2Cl (iii) Na(OH)2 (iv) NaCO3 (v)H(NO3)3 (vi)CaHCO3 (vii)(NH4)2SO4 (viii)Fe2(SO4)3 (ix)BaCl (x)NO2Cl
Correct/Incorrect correct incorrect incorrect incorrect incorrect incorrect correct correct incorrect correct
Correct formula (if the given formula is not correct) HCl NaOH Na(CO3)2 HNO3 Ca(HCO3)2 BaCl3
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(d) (i) What type of information we get from the formula of H2SO4? (ii) What are the number of atoms in the molecule Fe2(SO4)3? Ans.: (i) The information we get from the above formula are as follows: Qualitative significance : 1. H2SO4 indicates a particular substance sulphuric acid. 2. The compound sulphuric acid (H2SO4) is composed of hydrogen, sulphur and oxygen. Quantitative significance : 1. The formula indicates H2SO4 is a molecule of sulphuric acid. 2. The formula also indicates the molecule contains two atoms of hydrogen, one atom of sulphur and four atoms of oxygen. 3. The molecular mass of the molecule is 98. 4. The formula means that there are 2 parts of hydrogen, 32 parts of sulphur and 64 parts of oxygen of the total mass of 98.
(ii) The number of atoms in the molecule are as follows: Atoms Fe S O
Number of atoms 2 3 12 Total atoms
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(e) The formulas of five oxides are MnO, Mn2O3, MnO2, MnO3, Mn2O7, from these formulae answer the following questions. a. What is the symbol of the manganese? b. In which oxide the valency of manganese is maximum? c. What are the different valencies of manganese? Ans.: (a) The symbol of manganese is Mn.
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(b) The oxide in which the valency of manganese is maximum is Mn 2O7. In this compound the valency of manganese is 7. (c) The different valencies of manganese are as follows : In MnO, the valency of Mn = 2 In Mn2O3, the valency of Mn = 3 In MnO2, the valency of Mn = 4 In MnO3, the valency of Mn = 6 In Mn2O7, the valency of Mn = 7.
Answer to the Extra Questions 1. What is the formula of element and compound? Ans.: Formula of elements : The formula of an element is represented by its symbol with the number of atoms of the element present in the molecule. When the molecule of an element consists of more than one atom, the fact is expressed in the formula by placing a number just against the right lower side of the symbol of the element. For example each hydrogen, nitrogen, oxygen, chlorine molecule has two atoms. So their formulae are H2, O2, N2, Cl2 etc. respectively. The formulae of ozone, sulphur and phosphorus are O3, S6, P4 respectively. Again the molecule of sodium, potassium etc. consists of one atom (Mono atomic). So their formulae are Na, K respectively. Formula of compounds : The molecule of a compound consists of atoms of more than one element, thus, a molecule of water consists of 2 atoms of hydrogen and one atom of oxygen. To write the formula of a compound, the symbol of the constituent elements are written side by side and the no. of atoms of each element present in a molecule is indicated by numerals placed slightly below and on the right hand side of the respective symbol. For example zinc oxide ZnO, sodium chloride NaCl etc. But when both the elements of a compound are nonmetal, then the more electronegative element is written later. For example H2O. 2. Why the formula of water is not HO and that of hydrogen chloride is not H2Cl?
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Ans.: The formula of water is not HO and that of hydrogen chloride is not H 2Cl because one atom of hydrogen combines with one atom chlorine to produce one molecule of hydrogen chloride. Two atoms of hydrogen will never combine with one atom of chlorine. Again one atom of hydrogen will never combine with one atom of oxygen for producing one molecule of water (H2O). That is, elements have the definite capability to combine with each other. 3. What is valency? Ans.: Valency : The combing capacity of an element with others is called valency. The valency is measured as the number of hydrogen or equivalent atoms which is combined with or replaced by one atom of an element. Generally hydrogen or chlorine atom is combined with most of the element. For that reason hydrogen and chlorine atom is taken as the unit to determine the valency of an element. For example one hydrogen atom is combined with one chlorine atom in hydrochloric acid (HCl) compound. So the valency of chlorine is one in HCl.
4. Write down the symbols of the following elements. Chlorine, Bismuth, Lead, Barium, Beryllium, Tin, Mercury, Silver, Iodine, Boron and Copper. Ans.: The symbols are given below : Name Symbol Chlorine Cl Lead Pb Barium Ba Beryllium Be Tin Sn Bismuth Bi
Name Mercury Silver Iodine Boron Copper Gold
Symbol Hg Ag I B Cu Au
5. Write down the following things for the formulae : (i) H2O (ii) 3H2SO4 (iii) 2Mg(HCO3)2 (iv) 5NH4OH. (a) Name (b) Number of molecules (c) Number of hydrogen atoms (d) Number of oxygen atoms.
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Ans.: Formulae
Name
H2O Water 3H2SO4 Sulphuric acid 2Mg(HCO3)2 Magnesium carbonate 5NH4OH Ammonium hydroxide
No. of molecules
bi
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1 3 2
No. of hydrogen atoms 2 6 4
No. of oxygen atoms 1 12 12
5
25
5
6. What are the differences between valency and formula? Ans.: The differences between valency and formula are as follows : Valency Formula (1) The combing capacity of an (1) Formula is the abbreviated form of element with others is called valency. elements or compounds. (2) The ratio of atoms or radicals in a (2) Formula indicates the atoms of compound is reciprocal proportion of different elements or one element and their valency. their numbers which remain in the compound. (3) Valency indicates the affinity of (3) Molecular weight and percentage chemical combination of an element. composition of elements can be understood by the formulae. (4) Valency indicates the electric (4) Electric charge of element or charge of element or radical. radical can not be known from formulae. 7. How formula can be written by using the valency? Ans.: When one element is combined with another element for making a compound, then valencies are exchanged in there between the two atoms. Valecies are written as subscript in the right side of the symbols of the elements. For exampleFormula of water is H1O2 = H2O Formula of sodium sulphate = Na2SO4.
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8. Which thing is comparable with the valency of element? Ans.: The element of an element can be compared with hand. For example the element with one hand means its valency is one. Therefore, one atom of one handed element will combine with another atom of one handed element. Both hydrogen and chlorine are one handed elements. So both have valency one. Again the valency of oxygen is two which means that one atom of oxygen has two hands. By these two hands oxygen can catch two hydrogen atoms which is shown below : H- O – H 9. What is molecular formula and empirical formula? Ans.: Molecular formula : Molecular formula is the abbreviated form of the molecules of elements or compounds which is represented by symbols. For example molecular formula of water is H2O. Empirical formula : Empirical formula is the simplest ratio of the number of elements remaining in a compound. For example empirical formula of benzene is CH. 10. What do you understand by double decomposition reaction? Give three examples of double decomposition reaction. Ans.: Double decomposition reaction : When two compounds exchange their acidic and basic portions, then the reaction is called double decomposition reaction. Example : 1. Hydrogen chloride + potassium iodide = hydrogen iodide + potassium chloride. 2. Silver nitrate + sodium chloride = silver chloride + sodium nitrate. 3. Calcium nitrate + potassium chloride = calcium chloride + potassium nitrate.
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11. Valency of oxygen is 2, valency of carbon is 4 and valency of sodium is 1 – explain. Ans.: Generally hydrogen and chlorine atom is taken as the unit of valency. We know, valency of any element is the number of hydrogen atoms which are combined with the one atom of that element. Valency of oxygen is 4, because methane (CH4) is produced by the reaction of hydrogen with carbon. It is observed from the structural formula of methane that four hydrogen atoms are attached with one atom of carbon. For that reason, the valency of carbon is 4. The structural formula of methane is shown below : H H–C–H H Valency of sodium is one because sodium chloride (NaCl) is formed due to reaction of sodium with chlorine. From the combination of sodium chloride, it is observed that one sodium atom is attached with one chlorine atom. For that reason valency of sodium is 1. The structural formula of sodium chloride is : Na – Cl 12. Why hydrogen is used as the unit for measuring the valency of any element? Ans.: It is observed from experiment that maximum elements have the greatest tendency or attraction to combine with hydrogen atoms. Moreover the valency of hydrogen is the lowest. For that reason hydrogen is used as the unit for measuring valency of any elements. 13. What is the significance of H2O? Ans.: The significance of H2O is as follows : Qualitative significance : (a) It indicates definite substance water. (b) It indicates molecule of water. (c) It also means that the molecule of water is composed of hydrogen and oxygen.
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Quantitative significance : (a) It indicates a molecule of water. (b) It is also indicated that one molecule of water is composed of two atoms of hydrogen and one atom of oxygen. (c) It indicates the molecular weight of water = 1.008 + 16 x 1= 18.016. (d) We can understand that 18.016 gm water is composed of 2.016 gm hydrogen and 16 gm oxygen. Or, in waterH = 2.016 x 100/18.016 = 11.19% O = 16 x 100/18.016 = 88.81%. 14. How the valency of any element is measured? Ans.: Atoms of different elements possess different capacities for combining with hydrogen atom. This combining capacity of hydrogen with other element is the lowest. It is observed by analyzing a lot of compounds that one atom of hydrogen can not combine with more than one atom of others elements. But one atom of others element can combine with more than one atom of hydrogen. For that reason the valency of an element is measured by the number of hydrogen atoms which combine with one atom of that element. In the case where any element does not combine with hydrogen, number of atoms of equivalent elements (example chlorine) of hydrogen is taken as unit. 15. Write down the structural formulae of the following compounds. (i) CO2 (ii) CO (iii) NH3 (iv) CH4 (v) H2O (Vi) NaCl Ans.: (i) O = C = O H
(iii) H –C – H H
(ii) H –N – H H (iv) H – O – H
16. What is the significance of symbol? Ans.: Significance of symbol:
(v) Na – Cl
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1. The symbol of an element represents the name of the element in short e.g. hydrogen by H, oxygen by O, sodium by Na. 2. The symbol of an element indicates one atom of that element such as H stands for one atom of hydrogen. Likewise O stands for one atom oxygen, C for one atom of carbon, Na for one atom of sodium etc. 3. The symbol represents one atom of an element and it also stands for the atomic weight of that element. For example, H means one part by weight of hydrogen and O means 16 parts by weight of oxygen. 17. What is the significance of formula? Ans.: Formula has two types of significances. These are (a) qualitative significance and (b) quantitative significance. Qualitative and quantitative significances are discussed below : (a)Qualitative significance : (i) a formula represents a particular substance. For example – H2O indicates water. (ii) Formula of a substance represents the atoms of different constituent elements of the substance. Quantitative significance : (i)formula represents one molecule of a particular substance. For example one molecule of sodium chloride is represented by NaCl. (ii) Formula represents the number of atoms of each of the constituent elements present in a molecule of the compound. (iii) a formula represents molecular weight of a particular substance and the molecular formula shows the relative weights of the elements present in the molecular weight of the compound; for example, CO2 means the molecular weight of CO2 is 44 and it also represents relative weight of carbon and oxygen in the mol. Wt of the compound; i.e. in 44 parts by wt. of carbon dioxide there are 12 parts of carbon and 32 parts by wt. of oxygen.
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Answer to the Objective Questions Multiple Choice Question : 1. Tick (√) correct answer : (1) What is called the ‘shortened’ form of the molecule of an element or compound? (a) Symbol √ (b) Formula (c) Valency (d) Combining capacity (2) Which is indicated by the symbol of an element? √ (a) One atom of an element (b) One letter of the name of an element (c) First letter of the name of an element (d) Latin name of an element (3) Which symbol is taken from Latin name? (a) Sulphur S (b) Phosphorus P √ (c) Copper Cu (d) Chlorine Cl (4) Which formula is incorrect? (a) CuSO4 (b) H2O √ (c) H2Cl (d) ZnCO3 (5) A metal M has valency 4? What is the correct formula of M (IV) sulphate of that metal? (a) M4SO4 (b) M(SO4) 4 (c) M2SO4 √ (d) M(SO4) 2 (6) What is the correct formula of iron (III) carbonate? (a) Fe4 (SO4) 2 (b) FeCO3 (c) Fe2(CO3) √ (d) Fe2(CO3) 3 (7) Which one is compound radical? (a) Fe (b) Cu (c) Br √ (d) HCO3 (8) The number of atoms present in a compound is expressed – (a) By the symbol of an element √ (b) By the formula of the compound (c) By the valency of an element (d) From radical.
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2. Fill in the blanks : (a) The symbol of chlorine is …..and formula is ….. (b)……..of any element or compound expressed in short is …… (c) In sulphuric acid (H2SO4) the valency of sulphate (SO4) is… (d) A group of atoms of different elements which acts like a single atom is called……. (e) To determine the valency of an element the valency of …..is taken as unit (f) The valency of nitrogen in ammonia is………. Ans.: (a) Cl, Cl2; (b) Molecule, formula; (c) 2; (d) radical; (e) hydrogen; (f) 3. 3. Match the left side sentences with those of right side : Left (a) Symbol of sodium (b) Formula of copper sulphate (c) The valency of oxygen (d) Carbonate; CO3 (e) Sodium, Potassium, Calcium (f) The valency of sodium is
Right (a) Fe2(SO4) 3 (b) Metal (c) Radical (d) Na (e) CuSO4 (f) 2 (g) 1
Ans.: (a) Symbol of sodium Na (b) Formula of copper sulphate CuSO4. (c) The valency of oxygen 2. (d) Carbonate; CO3 radical. (e) Sodium, Potassium, Calcium metal. (f) The valency of sodium is 1.
Symbol C Cu K Fe
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Calcium Ca (c) What is called a formula? Explain the significance of formula. Ans.: The abbreviated form of the molecule of elements or compounds is called formula. For example: the formula of water is H2O, calcium carbonate CaCO3 etc. Significance of formula: Formula has qualitative and quantitative significance. Qualitative significance: 1. It represents a particular substance. For example the formula H2O means the substance water. 2. It also denotes the different elements present in that substance. Quantitative significance: 1. Formula represents one molecule of a particular substance. For example, one molecule of sodium Chloride is represented by NaCl. 2. Formula represents the number of atoms of each of the constituent elements present in the molecule of the compound. 3. A formula represents molecular weight of a particular Substance and the molecular formula shows the relative weight of the elements presets in the molecular weight of the compound; for example, CO2 means the molecular weight of CO2 is 44 and it also represents relative weight of carbon and oxygen in the mol. Wt of the compound; i.e. in 44 parts by wt. of carbon dioxide there are 12 parts by weight of carbon and 32 parts by weight of oxygen. (d) What is meant by combining capacity of an element? Which element is taken as unit to determine the valency of an element? Ans.: The combing capacity of an element with others is called valency. The valency is measured as the number of hydrogen or equivalent atoms which is combined with or replaced by one atom of an element. Generally hydrogen or chlorine atom is combined with most of the element. For that reason hydrogen and chlorine atom is taken as the unit to determine the valency of an element. For example, one hydrogen atom is combined with one chlorine atom in hydrochloric acid (HCl) compound. So the valency of chlorine is one in HCl.
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Again in calcium chloride (CaCl2) compound two chlorine atoms combine with one calcium atom. So the valency of calcium is two in CaCl2 compound. Name of compound radical Sulphate Carbonate Nitrate By carbonate Phosphate
Formula SO4 CO3 NO3 HCO3 PO4
Valency 2 2 1 1 3
(e) What are compound radicals? Give examples of compound radicals. Ans.: Compound radicals are the group of atoms which act like atoms and participate in the chemical reaction as a group as like as atoms are called compound radicals. Example : Sulphate (SO4) is a radical which is composed with two different elements sulphur and oxygen. Here, four atoms of oxygen and one atom of sulphur is combined together and acts as like as an atom as a group. It is understood clearly from the following reaction: ZnSO4
+
2NaOH = Na2SO4 + Zn(OH) 2 unchanged Like this way one atom of carbon and three atoms of oxygen ed together to form (CO3) radical, one atom of phosphorus and four atoms of oxygen ed together to form phoshphate radical. (f) Write down the formula of the following compounds: Ferrous nitrate, Potassium sulphate, Ammonium carbonate, Copper sulphate, Magnesium hydrogen carbonate, Lead nitrate, Calcium oxide, Nitric acid, Sodium carbonate, Iron(II) sulphate, Magnesium hydroxide. Ans.: Name of the compound Formulae Ferrous nitrate Fe(NO3)2 Potassium sulphate K2SO4 Ammonium carbonate (NH4)2CO3 Copper sulphate CuSO4 Magnesium hydrogen carbonate Mg(HCO3)2
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Lead nitrate Calcium oxide Nitric acid Sodium carbonate Iron (II) sulphate Magnesium hydroxide
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Pb(NO3)2 CaO HNO3 Na2CO3 FeSO4 Mg(OH)2
(g) Write down the valency of the following compounds or radicals: Hydrogen carbonate, Nitrate, Carbonate, Sulphate, Carbon, Copper, Zinc, Iron, Aluminum, Oxide. Ans.: The valency of the following elements and radicals are as follows: Name of radicals/elements Hydrogen carbonates Nitrate Carbonate Sulphate Carbon Copper Zinc Aluminum Oxide
valency 1 1 2 2 4 2/1 2 3 2
Answer to the Essay Type Questions (a) Give the rules of writing symbols with examples. Ans.: The rules of the writing symbol are as follows: 1. Generally the symbol is represented by the first letter of its English name e.g. English name Symbol Hydrogen H Oxygen O Nitrogen N Carbon C Sulphur S
Symbol, Formula and Valency
Boron Phosphorus Uranium
19
B P U
2. When however, the names of two elements begin with the same letter, first letter in capital is used for one and for the others first letter along with another, prominently heard in pronouncing the name of the element , are used to represent the element. For example English neme Symbol Carbon C Calcium Ca Cobalt Co Cium Cd Chlorine Cl Boron B Barium Ba Bismuth Bi Bromine Br Beryllium Be 3. Some elements are represents by first letter or first two letters of its Latin names but there are also some exceptions. For example English name Sodium Potassium Copper Silver Tin
Latin name Natrium Kalium Cuprum Argentums Stanum
Symbol Na K Cu Ag Sn
(b) What is formula? Write down the differences between symbol and formula. Ans.: Symbol : The abbreviated form of the molecule of elements or compounds is called formula. That is, the shortened form of the molecule of any substance is called formula. To write the formula of a compound the symbols of the constituent elements are written side by side and the number of atoms of each
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element present in the molecule is mentioned as subscript on the right hand side of the symbols. For example: the formula of water = H2O, the formula of ammonia = NH3. The differences between symbol and formula are given below : Symbol Formula 1. Short name of an element is symbol. 1. Short name of an element or compound is formula. 2. The symbol of an element indicates 2. The formula of a substance indicates one atom. one molecule. 3. The symbol of element represents 3. Formula represents molecular weight the atomic weight of that element. of a particular substance. 4. Symbols contain only one type of 4. Formula represents the number of atoms. atoms of each of the constituent elements present in a molecule of the substance. (c) What is formula? Write down the in correct formulae given below and correct the incorrect one and write them. (i) H2S (ii) H2Cl (iii) Na(OH) 2 (iv) NaCO3 (v) H(NO3) 2 (vi) CaHCO3 (vii) (NH4) 2SO4 (viii) Fe2 (SO4) 3 (ix) BaCl (x) NO2Cl Ans.:
Formula (i)H2S (ii)H2Cl (iii) Na(OH)2 (iv) NaCO3 (v)H(NO3)3 (vi)CaHCO3 (vii)(NH4)2SO4 (viii)Fe2(SO4)3 (ix)BaCl (x)NO2Cl
Correct/Incorrect correct incorrect incorrect incorrect incorrect incorrect correct correct incorrect correct
Correct formula (if the given formula is not correct) HCl NaOH Na(CO3)2 HNO3 Ca(HCO3)2 BaCl3
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(d) (i) What type of information we get from the formula of H2SO4? (ii) What are the number of atoms in the molecule Fe2(SO4)3? Ans.: (i) The information we get from the above formula are as follows: Qualitative significance : 1. H2SO4 indicates a particular substance sulphuric acid. 2. The compound sulphuric acid (H2SO4) is composed of hydrogen, sulphur and oxygen. Quantitative significance : 1. The formula indicates H2SO4 is a molecule of sulphuric acid. 2. The formula also indicates the molecule contains two atoms of hydrogen, one atom of sulphur and four atoms of oxygen. 3. The molecular mass of the molecule is 98. 4. The formula means that there are 2 parts of hydrogen, 32 parts of sulphur and 64 parts of oxygen of the total mass of 98.
(ii) The number of atoms in the molecule are as follows: Atoms Fe S O
Number of atoms 2 3 12 Total atoms
17
(e) The formulas of five oxides are MnO, Mn2O3, MnO2, MnO3, Mn2O7, from these formulae answer the following questions. a. What is the symbol of the manganese? b. In which oxide the valency of manganese is maximum? c. What are the different valencies of manganese? Ans.: (a) The symbol of manganese is Mn.
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(b) The oxide in which the valency of manganese is maximum is Mn 2O7. In this compound the valency of manganese is 7. (c) The different valencies of manganese are as follows : In MnO, the valency of Mn = 2 In Mn2O3, the valency of Mn = 3 In MnO2, the valency of Mn = 4 In MnO3, the valency of Mn = 6 In Mn2O7, the valency of Mn = 7.
Answer to the Extra Questions 1. What is the formula of element and compound? Ans.: Formula of elements : The formula of an element is represented by its symbol with the number of atoms of the element present in the molecule. When the molecule of an element consists of more than one atom, the fact is expressed in the formula by placing a number just against the right lower side of the symbol of the element. For example each hydrogen, nitrogen, oxygen, chlorine molecule has two atoms. So their formulae are H2, O2, N2, Cl2 etc. respectively. The formulae of ozone, sulphur and phosphorus are O3, S6, P4 respectively. Again the molecule of sodium, potassium etc. consists of one atom (Mono atomic). So their formulae are Na, K respectively. Formula of compounds : The molecule of a compound consists of atoms of more than one element, thus, a molecule of water consists of 2 atoms of hydrogen and one atom of oxygen. To write the formula of a compound, the symbol of the constituent elements are written side by side and the no. of atoms of each element present in a molecule is indicated by numerals placed slightly below and on the right hand side of the respective symbol. For example zinc oxide ZnO, sodium chloride NaCl etc. But when both the elements of a compound are nonmetal, then the more electronegative element is written later. For example H2O. 2. Why the formula of water is not HO and that of hydrogen chloride is not H2Cl?
Symbol, Formula and Valency
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Ans.: The formula of water is not HO and that of hydrogen chloride is not H 2Cl because one atom of hydrogen combines with one atom chlorine to produce one molecule of hydrogen chloride. Two atoms of hydrogen will never combine with one atom of chlorine. Again one atom of hydrogen will never combine with one atom of oxygen for producing one molecule of water (H2O). That is, elements have the definite capability to combine with each other. 3. What is valency? Ans.: Valency : The combing capacity of an element with others is called valency. The valency is measured as the number of hydrogen or equivalent atoms which is combined with or replaced by one atom of an element. Generally hydrogen or chlorine atom is combined with most of the element. For that reason hydrogen and chlorine atom is taken as the unit to determine the valency of an element. For example one hydrogen atom is combined with one chlorine atom in hydrochloric acid (HCl) compound. So the valency of chlorine is one in HCl.
4. Write down the symbols of the following elements. Chlorine, Bismuth, Lead, Barium, Beryllium, Tin, Mercury, Silver, Iodine, Boron and Copper. Ans.: The symbols are given below : Name Symbol Chlorine Cl Lead Pb Barium Ba Beryllium Be Tin Sn Bismuth Bi
Name Mercury Silver Iodine Boron Copper Gold
Symbol Hg Ag I B Cu Au
5. Write down the following things for the formulae : (i) H2O (ii) 3H2SO4 (iii) 2Mg(HCO3)2 (iv) 5NH4OH. (a) Name (b) Number of molecules (c) Number of hydrogen atoms (d) Number of oxygen atoms.
General Science: Physics and Chemistry
Ans.: Formulae
Name
H2O Water 3H2SO4 Sulphuric acid 2Mg(HCO3)2 Magnesium carbonate 5NH4OH Ammonium hydroxide
No. of molecules
bi
24
1 3 2
No. of hydrogen atoms 2 6 4
No. of oxygen atoms 1 12 12
5
25
5
6. What are the differences between valency and formula? Ans.: The differences between valency and formula are as follows : Valency Formula (1) The combing capacity of an (1) Formula is the abbreviated form of element with others is called valency. elements or compounds. (2) The ratio of atoms or radicals in a (2) Formula indicates the atoms of compound is reciprocal proportion of different elements or one element and their valency. their numbers which remain in the compound. (3) Valency indicates the affinity of (3) Molecular weight and percentage chemical combination of an element. composition of elements can be understood by the formulae. (4) Valency indicates the electric (4) Electric charge of element or charge of element or radical. radical can not be known from formulae. 7. How formula can be written by using the valency? Ans.: When one element is combined with another element for making a compound, then valencies are exchanged in there between the two atoms. Valecies are written as subscript in the right side of the symbols of the elements. For exampleFormula of water is H1O2 = H2O Formula of sodium sulphate = Na2SO4.
Symbol, Formula and Valency
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8. Which thing is comparable with the valency of element? Ans.: The element of an element can be compared with hand. For example the element with one hand means its valency is one. Therefore, one atom of one handed element will combine with another atom of one handed element. Both hydrogen and chlorine are one handed elements. So both have valency one. Again the valency of oxygen is two which means that one atom of oxygen has two hands. By these two hands oxygen can catch two hydrogen atoms which is shown below : H- O – H 9. What is molecular formula and empirical formula? Ans.: Molecular formula : Molecular formula is the abbreviated form of the molecules of elements or compounds which is represented by symbols. For example molecular formula of water is H2O. Empirical formula : Empirical formula is the simplest ratio of the number of elements remaining in a compound. For example empirical formula of benzene is CH. 10. What do you understand by double decomposition reaction? Give three examples of double decomposition reaction. Ans.: Double decomposition reaction : When two compounds exchange their acidic and basic portions, then the reaction is called double decomposition reaction. Example : 1. Hydrogen chloride + potassium iodide = hydrogen iodide + potassium chloride. 2. Silver nitrate + sodium chloride = silver chloride + sodium nitrate. 3. Calcium nitrate + potassium chloride = calcium chloride + potassium nitrate.
General Science: Physics and Chemistry
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11. Valency of oxygen is 2, valency of carbon is 4 and valency of sodium is 1 – explain. Ans.: Generally hydrogen and chlorine atom is taken as the unit of valency. We know, valency of any element is the number of hydrogen atoms which are combined with the one atom of that element. Valency of oxygen is 4, because methane (CH4) is produced by the reaction of hydrogen with carbon. It is observed from the structural formula of methane that four hydrogen atoms are attached with one atom of carbon. For that reason, the valency of carbon is 4. The structural formula of methane is shown below : H H–C–H H Valency of sodium is one because sodium chloride (NaCl) is formed due to reaction of sodium with chlorine. From the combination of sodium chloride, it is observed that one sodium atom is attached with one chlorine atom. For that reason valency of sodium is 1. The structural formula of sodium chloride is : Na – Cl 12. Why hydrogen is used as the unit for measuring the valency of any element? Ans.: It is observed from experiment that maximum elements have the greatest tendency or attraction to combine with hydrogen atoms. Moreover the valency of hydrogen is the lowest. For that reason hydrogen is used as the unit for measuring valency of any elements. 13. What is the significance of H2O? Ans.: The significance of H2O is as follows : Qualitative significance : (a) It indicates definite substance water. (b) It indicates molecule of water. (c) It also means that the molecule of water is composed of hydrogen and oxygen.
Symbol, Formula and Valency
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Quantitative significance : (a) It indicates a molecule of water. (b) It is also indicated that one molecule of water is composed of two atoms of hydrogen and one atom of oxygen. (c) It indicates the molecular weight of water = 1.008 + 16 x 1= 18.016. (d) We can understand that 18.016 gm water is composed of 2.016 gm hydrogen and 16 gm oxygen. Or, in waterH = 2.016 x 100/18.016 = 11.19% O = 16 x 100/18.016 = 88.81%. 14. How the valency of any element is measured? Ans.: Atoms of different elements possess different capacities for combining with hydrogen atom. This combining capacity of hydrogen with other element is the lowest. It is observed by analyzing a lot of compounds that one atom of hydrogen can not combine with more than one atom of others elements. But one atom of others element can combine with more than one atom of hydrogen. For that reason the valency of an element is measured by the number of hydrogen atoms which combine with one atom of that element. In the case where any element does not combine with hydrogen, number of atoms of equivalent elements (example chlorine) of hydrogen is taken as unit. 15. Write down the structural formulae of the following compounds. (i) CO2 (ii) CO (iii) NH3 (iv) CH4 (v) H2O (Vi) NaCl Ans.: (i) O = C = O H
(iii) H –C – H H
(ii) H –N – H H (iv) H – O – H
16. What is the significance of symbol? Ans.: Significance of symbol:
(v) Na – Cl
General Science: Physics and Chemistry
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1. The symbol of an element represents the name of the element in short e.g. hydrogen by H, oxygen by O, sodium by Na. 2. The symbol of an element indicates one atom of that element such as H stands for one atom of hydrogen. Likewise O stands for one atom oxygen, C for one atom of carbon, Na for one atom of sodium etc. 3. The symbol represents one atom of an element and it also stands for the atomic weight of that element. For example, H means one part by weight of hydrogen and O means 16 parts by weight of oxygen. 17. What is the significance of formula? Ans.: Formula has two types of significances. These are (a) qualitative significance and (b) quantitative significance. Qualitative and quantitative significances are discussed below : (a)Qualitative significance : (i) a formula represents a particular substance. For example – H2O indicates water. (ii) Formula of a substance represents the atoms of different constituent elements of the substance. Quantitative significance : (i)formula represents one molecule of a particular substance. For example one molecule of sodium chloride is represented by NaCl. (ii) Formula represents the number of atoms of each of the constituent elements present in a molecule of the compound. (iii) a formula represents molecular weight of a particular substance and the molecular formula shows the relative weights of the elements present in the molecular weight of the compound; for example, CO2 means the molecular weight of CO2 is 44 and it also represents relative weight of carbon and oxygen in the mol. Wt of the compound; i.e. in 44 parts by wt. of carbon dioxide there are 12 parts of carbon and 32 parts by wt. of oxygen.
Symbol, Formula and Valency
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Answer to the Objective Questions Multiple Choice Question : 1. Tick (√) correct answer : (1) What is called the ‘shortened’ form of the molecule of an element or compound? (a) Symbol √ (b) Formula (c) Valency (d) Combining capacity (2) Which is indicated by the symbol of an element? √ (a) One atom of an element (b) One letter of the name of an element (c) First letter of the name of an element (d) Latin name of an element (3) Which symbol is taken from Latin name? (a) Sulphur S (b) Phosphorus P √ (c) Copper Cu (d) Chlorine Cl (4) Which formula is incorrect? (a) CuSO4 (b) H2O √ (c) H2Cl (d) ZnCO3 (5) A metal M has valency 4? What is the correct formula of M (IV) sulphate of that metal? (a) M4SO4 (b) M(SO4) 4 (c) M2SO4 √ (d) M(SO4) 2 (6) What is the correct formula of iron (III) carbonate? (a) Fe4 (SO4) 2 (b) FeCO3 (c) Fe2(CO3) √ (d) Fe2(CO3) 3 (7) Which one is compound radical? (a) Fe (b) Cu (c) Br √ (d) HCO3 (8) The number of atoms present in a compound is expressed – (a) By the symbol of an element √ (b) By the formula of the compound (c) By the valency of an element (d) From radical.
General Science: Physics and Chemistry
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2. Fill in the blanks : (a) The symbol of chlorine is …..and formula is ….. (b)……..of any element or compound expressed in short is …… (c) In sulphuric acid (H2SO4) the valency of sulphate (SO4) is… (d) A group of atoms of different elements which acts like a single atom is called……. (e) To determine the valency of an element the valency of …..is taken as unit (f) The valency of nitrogen in ammonia is………. Ans.: (a) Cl, Cl2; (b) Molecule, formula; (c) 2; (d) radical; (e) hydrogen; (f) 3. 3. Match the left side sentences with those of right side : Left (a) Symbol of sodium (b) Formula of copper sulphate (c) The valency of oxygen (d) Carbonate; CO3 (e) Sodium, Potassium, Calcium (f) The valency of sodium is
Right (a) Fe2(SO4) 3 (b) Metal (c) Radical (d) Na (e) CuSO4 (f) 2 (g) 1
Ans.: (a) Symbol of sodium Na (b) Formula of copper sulphate CuSO4. (c) The valency of oxygen 2. (d) Carbonate; CO3 radical. (e) Sodium, Potassium, Calcium metal. (f) The valency of sodium is 1.
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