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Answer:
Explanation:
Valency is the combining capacity of an element. The number of electrons donated or accepted by an atom of an element so as to have electronic configuration of nearest noble gas is called its valency.
Sometimes, because of different conditions under which a chemical reaction takes place, the valency of the same element changes. Thus metals like lead, tin, copper, mercury, iron etc exhibit variable valency.
Normally, the metals donate electrons from their valence shell, so as to form positively charged ions. However, some metals lose electrons from the shell next to the valence shell. In such a situation the element exhibits more than one electropositive valency. Such elements are said to have variable valency.
The electrovalency of an element is equal to the number of electrons lost or gained by its atom during the formation of ionic bonds. The s-block elements do not show variable valency. However, p-block elements with higher atomic number transition and inner transition elements show variable valency.
There are two main reasons for showing variable valency
Inert pair effect in p-block elements.
Small energy difference between ns and (n-1)d sub shells in transition elements and ns and (n-2) sub shells in inner transition elements.
1. Inert Pair Effect
The electronic configurations of III A and IV A group elements are as
Variable Electrovalency
The elements of III A group show +3 oxidation state mainly. But as we go down the group the oxidation state +1 becomes more and more pronounced. Similarly the elements of IV A group show +4 oxidation state in general but as we go down the group the +2 state becomes more and more pronounced.
The two s electrons (ns2) in the valence shell tend to remain inert and do not participate in formation of bonds. This is called inert pair effect. Thus the inert pair effect is the main cause of variable valency in p block elements.
2. Energy Difference between Orbitals
In transition elements the variation in oxidation state is due to involvement of incomplete d-orbitals in addition to electrons of ns orbital. So the electrons from both the energy levels can be used for bonding.
Similarly, inner transition elements show variable valency due to incomplete filling of f-orbitals. So in addition to ns electrons, the electrons from f-orbitals may be used for chemical bond formation.
Variable Valency Examples
Certain elements have more than one valency. They exhibit variable valency. For example, copper, tin, iron and mercury exhibit variable valency. Copper combines with oxygen to form cuprous oxide (Cu2O) and cupric oxide (CuO).
Valency of oxygen being two, combining capacity of copper in the first case is one and that in the second case is two. The suffix “ous” is used to indicate the lower valency and the suffix “ic” to indicate the higher valency. The following table gives the names of some compounds with variable valencies of elements.
Compound Valency Ion
Cuprous (Cu2O) 1 Cu+
Cupric oxide (CuO) 2 Cu2+
Ferrous oxide (FeO) 2 Fe2+
Ferric oxide (Fe2O3) 3 Fe3+
Element Valency Ion
Mercury (Hg) 1 Mercurous (Hg+)
2 Mercuric (Hg2+)
Tin (Sn) 2 Stannous (Sn2+)
4 Stannic (Sn4+)
Another example is iron has two electrons in its valence shell. On donating these electrons, its valency is +2. However, under specific experimental conditions, it can lose another electron from the next valence shell. In such a situation the ion formed has +3 electropositive valency.
Determination of Valencies of Elements from the Formula of Compounds
We know that the valency of chlorine (Cl) is 1.
In sodium chloride (NaCl), the valency of sodium (Na) is 1.
In calcium chloride (CaCl2), the valency of calcium (Ca) is 2.
In ferric chloride (FeCl3), the valency of iron (Fe) is 3.
In phosphorus trichloride (PCl3), the valency of phosphorus (P) is 3.
The above example suggests a simple rule of determining the valency of elements in the compounds having two elements.
The number of units of valency of all the atoms of one element is equal to the number of units of valency of all the atoms of the other elements.
Solved Example
Find the valency of magnesium (Mn) in the compound Mn2O7
Solution:
No of valency units of 2 atoms of manganese (Mn) = No of valency units of 7 atoms of oxygen.
= 7 x 2
= 14 valency units
Therefore, No of valency units of one atom of Mn = 14/2 = 7
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