Before discussing what is variable valency, the concept of valency has to be understood. So, what is valency? Elements generally do not exist independently in nature, and combine with the other elements. Valency is the combining capacity of one element with another element. Elements combine with one another to attain a stable state. It determines the number of electrons an element can donate or accept to form a stable electronic configuration.
Types of Valency
1. Valency in ionic compounds
2. Valency in covalent compounds
3. Variable valency
Valency in Ionic Compounds or Electrovalent Compounds
Ionic compounds or electrovalent compounds are compounds that form by the combination of metal ions and non-metal ions. For example, sodium chloride (NaCl). In this compound, sodium (Na) is the metal, and chloride (Cl) is the non-metal. It is formed by the transfer of electrons. What is electro valency? This type of chemical bonding between metal and non-metal ions is called electro valency, and these compounds are called electrovalent compounds. Electro valency can be defined as the number of electrons lost or gained by the atoms in an ionic compound.
For Example: In NaCl
Na → Na+ + 1 e-
Cl + 1 e- → Cl-
Na+ + Cl- → NaCl
In the above example of sodium chloride (NaCl), the electro valency of sodium (Na) and chloride (Cl) is 1, as 1 electron is lost by sodium and gained by chloride ion.
Valency in Covalent Compounds
Covalent compounds are compounds formed by the chemical bonding of non-metals. These compounds are formed by the sharing of electrons between atoms.
Non-metal + Non-metal → covalent compounds.
The valency of these compounds can be defined as the number of bonds by which the atom is directly attached to the other atom. These compounds exhibit co-valency.
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The carbon is attached with the four hydrogens with the help of four single covalent bonds. Therefore, the co-valency of carbon in methane is four. One hydrogen is attached with the one-carbon by the single covalent bond and hence, the co-valency of hydrogen is one.
What is Variable Valency?
Some elements show more than one type of valency these types of valency are called variable valency. These types of compounds show a valency in one compound and another valency in other compounds. Variable valency is shown by elements like Iron, mercury, and copper. Transition elements show variable valency. For example: in some cases, iron shows a valency of 2 like ferrous sulphate (FeSO4), and in some, it shows valency of 3 like ferric chloride (FeCl3). Copper shows two types of valencies 1 and 2. Mercury shows two types of valency 1 and 2.
Why Do Some Elements Show Variable Valency?
Let’s see an example of iron:
The atomic number of iron = 26
Electronic configuration of iron = 1S2 2S2 2P6 3S2 3P6 4S2 3d6
4S2 3d6 shown electronic configuration makes iron an unstable molecule. The two electrons will be first removed from the 4S orbital. Then the electronic configuration of iron becomes 4S0 3d6. 3d6 is not a stable electronic configuration. After losing one electron from the d subshell it will become a half-filled subshell. This 3d5 is a stable electronic configuration. Hence the element of iron shows 2 and 3 valencies. These valencies are called iron variable valencies. The elements show variable valency to acquire stability. The half-filled subshell shows more exchange energy which further lowers the energy of the compound. Therefore, the compound becomes more stable.
Elements with Variable Valency
Copper (Cu) = Cuprous (Cu+2) and cupric (Cu+3)
Iron (Fe) = Ferrous (Fe+2) and Ferric (Fe+3)
Mercury = Mercurous (Hg +1) and mercuric (Hg +2)
Silver (Ag) = Argentous (Ag+1) and argentic (Ag+2.)
Stannum (Sn) = Stannous (Sn+2) and Stannic (Sn+3)
The element that exhibits lower valency will be suffixed with “ous”. While the element that exhibits higher valency will be suffixed with “ic”.
Did You Know?
The value of valency can never be zero.
The primary valency is the fixed property of elements.
Valency shows the bonding potential of the elements.