Valency 1d4h5s

Valency Combining power of atoms

Valency of different elements 

Valency (historical meaning) - no. of bonds to be formed by an atom.



Historically, the values are deduced from the formulas of some chemicals. e.g. H2O, CH4, HCl, NH3, H2, HF, H2S, PH3, SiH4, HBr, Cl2

Valency deduction exercise 

Deduce the valency of each kind of atom from the following formulas. H2O, CH4, HCl, NH3, H2, HF, H2S, PH3, SiH4, HBr, Cl2

Atom Valency

H

O

C

Cl

N

F

S

P

Si Br

Valency deduction exercise 

Deduce the valency of each kind of atom from the following formulas. H2O, CH4, HCl, NH3, H2, HF, H2S, PH3, SiH4, HBr, Cl2

Atom

H

Valency

1

O

C

Cl

N

F

S

P

Si Br

Valency deduction exercise 

Deduce the valency of each kind of atom from the following formulas. H2O, CH4, HCl, NH3, H2, HF, H2S, PH3, SiH4, HBr, Cl2

Atom

H

O

C

Cl

N

F

S

P

Si Br

Valency

1

2

4

1

3

1

2

3

4

1

Valency of some atoms Valency

1

2

3

4

Atom

H F Cl Br

O S

N P

C Si

Positions of the first 20 Elements

Positions of the first 20 Elements

Is there any connection between the position of an element in the periodic table and the valency of the atom ? 

Yes Except H, the valency of an atom is the same as (8 – group no.) of the atom. i.e. Extra no. of electrons required to fill the outermost shell.

Valency of different elements 

Valency - no. of bond to be formed

Real meaning of valency 

Valency of chlorine : 1 Formula of chlorine molecule : Cl2

Covalent bond formation by electron sharing

co- means together; tly; -valent means outermost shell A non-metal atom can form bond by sharing outermost shell electrons with another atom.

Electrostatic nature of covalent bond electrostatic electrostatic attraction attraction

positive nucleus

negative bonding electrons

positive nucleus

No. of electrons to be shared 

No. of electrons to be shared = No. of electrons required to fill the outermost shell = 8 - no. of electrons present = 8 - group no.



Exception : H has only 1 electron to share.

No. of electrons to be shared 

No. of electrons to be shared by group VII atom e.g. F, Cl, Br, I = 8 - 7 = 1



No. of electrons to be shared by group VI atom e.g. O, S =8-6=2



No. of electrons to be shared by group V atom e.g. N, P =8-5=3



No. of electrons to be shared by group IV atom e.g. C, Si =8-4=4

No. of electrons to be shared  

Each electron to be shared can be considered as a hand to with other atoms. For a stable molecule, all hands have to be held together i.e. all atoms have completely filled outermost shell.

Real meaning of valency 

Valency of an atom = no. of bond to be formed = no. of electrons to be shared = 8 – group no.

Not all compounds are made of molecules, some have electrically charged groups of atoms called ions.

One of these is aluminium sulphate, known as alum, whose formula is Al2(SO4)3 and which contains positive aluminium ions and negative sulphate ions, the SO4 part of the formula.

There are a small number of these polyatomic ions whose formula we will need to know. Their names may be familiar:sulphate

SO42-

nitrate

NO3-

carbonate CO32-

hydroxide OH

-

where the negative signs indicate the electrical charge

The question now is:“How do we know how many of each atom are needed?”.

The question now is:“How do we know how many of each atom are needed?”. It turns out that atoms of each element (or poly-atomic ion) have a number associated with them which determines how they all link up together. This is sometimes called the valency or valence.

H hydrogen C carbon N nitrogen O oxygen Na sodium Mg magnesium Al aluminium Si silicon P phosphorus S sulphur

+1 ±4 -3 -2 +1 +2 +3 ±4 -3 -2

Cl chlorine -1 K potassium +1 Ca calcium +2 Cr chromium +3 Fe iron +2 or +3 Cu copper +2 Zn zinc +2 Br bromine -1 Ag silver +1 I iodine -1

What a difficult set of numbers to learn! Or are they?

Let’s re-arrange the list H Na K Mg Ca Al

hydrogen +1 sodium +1 potassium +1 magnesium +2 calcium +2 aluminium +3

Cr Fe Cu Zn Ag

chromium +3 iron +2 or +3 copper +2 zinc +2 silver +1

Cl Br I O S P N C Si

chlorine -1 bromine -1 iodine -1 oxygen -2 sulphur -2 phosphorus -3 nitrogen -3 carbon ±4 silicon ±4

NO3 OH CO3 SO4

nitrate hydroxide carbonate sulphate

-1 -1 -2 -2

So how do we use this table? Here are a few simple steps to follow in going from a compound’s name to its formula:I’ll use calcium chloride as a example 1 write the symbols implied by the name Ca and Cl (compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate) 2 Find the numbers on the list Ca +2 and Cl -1 3 The rule now is that “The numbers in the compound must add to zero”.  one Ca (+2) and two Cl (-1) will add to 0. 4 And so the formula is CaCl2

2

Now let’s try aluminium iodide 1 Al and I

2

Now let’s try aluminium iodide 1 Al and I 2 Al +3 and I -1

2

Now 1 2 3

let’s try aluminium iodide Al and I Al +3 and I -1 one Al will need 3 I

2

Now 1 2 3 4

let’s try aluminium iodide Al and I Al +3 and I -1 one Al will need 3 I formula is AlI3

2

Now 1 2 3 4

let’s try aluminium iodide Al and I Al +3 and I -1 one Al will need 3 I formula is AlI3

3

sodium oxide

2

Now 1 2 3 4

let’s try aluminium iodide Al and I Al +3 and I -1 one Al will need 3 I formula is AlI3

3

sodium oxide 1 Na and O

2

Now 1 2 3 4

let’s try aluminium iodide Al and I Al +3 and I -1 one Al will need 3 I formula is AlI3

3

sodium oxide 1 Na and O 2 Na +1 and O -2

2

Now 1 2 3 4

3

sodium oxide 1 Na and O 2 Na +1 and O -2 3 this time we need two Na for one

O

let’s try aluminium iodide Al and I Al +3 and I -1 one Al will need 3 I formula is AlI3

2

Now 1 2 3 4

3

sodium oxide 1 Na and O 2 Na +1 and O -2 3 this time we need two Na for one

O

4

let’s try aluminium iodide Al and I Al +3 and I -1 one Al will need 3 I formula is AlI3

formula is Na2O

2

3

O

Now let’s try aluminium iodide 1 Al and I 2 Al +3 and I -1 3 one Al will need 3 I 4 formula is AlI3 sodium oxide 1 Na and O 2 Na +1 and O -2 3 this time we need two Na for one 4

formula is Na2O

notice that in the name and the formula, the metal or hydrogen (things which have the positive numbers) come first.

A few more examples 3 calcium sulphate 1 Ca and SO4

A few more examples 3 calcium sulphate 1 Ca and SO4 2 Ca +2 and SO4 -2

A few more examples 3 calcium sulphate 1 Ca and SO4 2 Ca +2 and SO4 -2 3 one Ca for one SO4

A few more examples 3 calcium sulphate 1 Ca and SO4 2 Ca +2 and SO4 -2 3 one Ca for one SO4 4 formula is CaSO4

A few more examples 3 calcium sulphate 1 2 3 4

4

Ca and SO4 Ca +2 and SO4 -2 one Ca for one SO4 formula is CaSO4

magnesium hydroxide 1 Mg and OH 2 Mg is +2 and OH -1 3 one Mg for two OH 4 formula is Mg(OH)2

5 Aluminium carbonate 1 Al and CO3 2 Al +3 and CO3 -2 3 two Al for three CO3 4 formula is Al2(CO3)3

5 Aluminium carbonate 1 2 3 4

Al and CO3 Al +3 and CO3 -2 two Al for three CO3 formula is Al2(CO3)3

All these examples have had a metal as one part of the compound. If two non metals combine life is a bit more complicated.

5

Aluminium carbonate 1 Al and CO3 2 Al +3 and CO3 -2 3 two Al for three CO3 4 formula is Al2(CO3)3

All these examples have had a metal as one part of the compound. If two non metals combine life is a bit more complicated. We often need to just make sure that the numbers are equal for each element. It is easier than that all sounds!

1 Silicon dioxide

Let’s work out some examples

1 Silicon dioxide 1,2 Si 4, O 2

1 Silicon dioxide 1,2 Si 4, O 2 3 to make the values equal we would need two O for each Si

Silicon dioxide 1,2 Si 4, O 2 3 to make the values equal we would need two O for each Si 4 SiO2, quartz

Try Hydrogen nitride (usually called ammonia) 1,2 H 1, N 3 3 for equal values we need three H for each N 4  NH3, ammonia

3 Phosphorus oxide 1,2 P 3 and O 2 3 To make these the same for each element we need 2 P and 3 O 4  P2O3

You should now be able to write the formula for several hundred compounds. You will find that the number associated with each element is easy to with a bit of practice.

Writing Formulae.

Formulae of compounds can be written using either 1.

The name of the compound. “mono” “di” “tri” “tetra” “penta”

means means means means means

one two three four five

If any of these prefixes appears in the name of the compound we use this to write the formula. e.g. carbon monoxide is CO carbon dioxide is CO2

2.

Using Valency Rules

If the name does not give any clues about the formula, we use valency rules. e.g.

sodium oxide

write symbols

Na

O

write valency

1

2

cross over valency

2

1

cancel if necessary write formula

Na2O

Note:- For transition metals the valency is usually written as a Roman numeral in brackets after the name of the transition metal e.g. In iron(III) oxide, the iron has a valency of 3.

 3. Formulae involving Complex (group) Ions If the name of the compound ends in “-ate” or “ –ite” , it contains a complex ion (watch also for hydroxide). Complex ions are found in your data book (p4). e.g.

Copper(II) nitrate

write symbols or formula NO3write valency 1 (for complex ions, this is the charge)

Cu 2

 4. Formulae Showing Charges Sometimes you are asked to show the formula with charges (ionic formula)

e.g.

sodium carbonate

write symbols or formula CO32write valency

Na

1

2

(for complex ions, this is the charge) cross over valency

2

1

Calculations for you to try. 1.

Write the formula for:-

(a) Sulphur trioxide

SO3

(b) Dinitrogen monoxide

N2O

(c) Phosphorus pentachloride PCl5

(d)

Carbon terachloride.

2.

CCl4

Write the formula for:-

(a) Sodium oxide.

Na2O

(b) Aluminium iodide

AlI3

(c) Iron(III) oxide

Fe2O3

(d)

Ag2O

Silver(I) oxide.

3.

Write the formula for:-

(a) Sodium carbonate

Na2CO3

(b) Aluminium nitrate

Al(NO3)3

(c) Iron(III) hydroxide

Fe(OH)3

(d)

NiSO4

Nickel(II) sulphate

4.

Write the ionic formula for:-

(a) Lithium hydrogencarbonate

Li+HCO3-

(b) Aluminium hydroxide

Al3+(OH-)3

(c) Iron(II) sulphate

Fe2+SO42-

(d)

Cu2+(NO3-)2

Copper(II) nitrate.