Valency And Ions

Concept of Variable Valency
        
Most elements such as Na, K, Mg, Al, Ca, Ba etc..exhibit a fixed valency in all their compounds, Sometimes the same element may form two or more different compounds with another element by involving different number of electrons. Due to this the same element exhibits different valency which is called as Variable Valency and it is usually exhibited by Metals

Reason for showing Variable Valency
        The reason for variable valency in certain metals is due to involvement of electrons in outermost orbit (Valence shell ) as well as the electrons in shell of preceeding orbit (called as penultimate orbit).
For Example : Under normal conditions Iron looses 2 electrons from outermost orbit to form Fe²⁺ ion and shows Valency of 2, But under definite experimental conditions Iron looses 3 electrons ( 2 from outermost orbit and one more electron from penultimate orbit and forms Fe³⁺ and shows valency of 3

Naming of compounds showing variable valency
The compound in which the element shows the lower  valencies  is indicated by the  suffix -ous
The compound in which the element shows the higher valency  is indicated by the suffix - ic
Tin is divalent in stannous chloride (SnCl₂) and tetravalent in stannic chloride (SnCl₄).
 The valency of sulphur is 4 in SO₂ and 6 in SO₃
Exmples :- 
Iron exhibit two valencies 2 and 3
Iron in its lower valency 2 is named as ferrous  
(Ex: Ferrous Chloride or iron (II) chloride FeCl₂)
Iron in its higher valency 3, is named  as ferric  
(Ex: Ferric Chloride or iron (III) chloride FeCl₃)
Copper shows valency 1 (monovalent) in (Cuprous Chloride, CuCl ) and shows Valency 2 (Bivalent) in (Cupric Chloride, CuCl₂ )
Tin is divalent in stannous chloride (SnCl₂) and tetravalent in stannic chloride (SnCl₄).
The valency of sulphur is 4 in SO₂ and 6 in SO₃.