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IUPAC Definition of Valency

The maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted is known as valency.
 
Valency of chlorine in HCl is 1.
 
Valency of oxygen in H2O is 2.
 
Valency of nitrogen in NH3 is 3.
 
Valency of carbon in CH4 is 4.
 
Similarly, the valency of sodium, magnesium, aluminium and carbon in the compounds NaCl, MgCl2, AlCl3 and CCl4 are 1, 2, 3 and 4, respectively.
 
For example, in aluminium oxide, Al2O3, 2 atoms of aluminium combine with 3 atoms of oxygen and, therefore, 1 atom of aluminium combines with 1.5 atoms of oxygen. Hence,
 
Valency of aluminium = Twice the number of oxygen atoms = 1.5 × 2 = 3
Valencies exhibited by some metals and non-metals are listed in Tables 5.1 and 5.2, respectively.
 

Table 5.1 Valencies Exhibited by Some Metals

Elements

Symbol

Valency

Sodium

Na

1

Potassium

K

1

Magnesium

Mg

2

Calcium

Ca

2

Barium

Ba

2

Aluminium

Al

3

Ferrum

Fe

2, 3

Chromium

Cr

3

Lead

Pb

2, 4

Manganese

Mn

2, 4

 

Table 5.2 Valencies Exhibited by Some Non-metals

Elements

Symbol

Valency

Fluorine

F

1

Chlorine

Cl

1

Bromine

Br

1

Iodine

I

1

Oxygen

O

2

Nitrogen

N

3, 5

Phosphorus

P

3, 5

Boron

B

3

Silicon

Si

4

Sulphur

S

4, 6

 

For elements that do not combine with hydrogen, the valency is the combining power of the element with another element whose valency is known.





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