What Is Valency, Variable Valency, Covalent Bond And Coordinate Bond, Important Factors

What Is Valency

What Is Valency : Valency, a fundamental concept in chemistry, refers to the combining capacity of an element . It is a measure of how many electrons an atom can gain, lose, or share to achieve a stable noble gas electronic configuration. The valency of an element determines its ability to form chemical compounds and engage in reactions.

How To Determine The Valency Of Any Element?

Determining the Valency of any element is very simple, for IA to IVA group elements, valency is the equal number of valence shell electrons. And from VA to Zero group, it is:

[8-(Number of valence electrons)].

Valency = (No. of valence e ^– ) Valency = (8 – no. of valence e ^– )

Valence 1 2                  3                4                      5               6                 7                8

shell e ^–

Valency 1 2                   3                4                      3               2                 1                0

(8 – 5) = 3 (8 – 8) = 0

• All the elements of a group have the same valencies because they have the same number of valence shell electrons.

• Valency of any element ≤ 4

Covalency : Capacity to form covalent bond is known as covalency.

Variable valency

• Elements exhibiting variable valencies must possess vacant orbitals in their outermost shell.

• Lone pair electrons get excited in the subshell of the same shell to form the maximum number of unpaired electrons.

• The energy needed for electron excitation is referred to as promotion energy.

• This process follows the promotion rule, involving the excitation of electrons within the same orbit.

Examples Of Variable Valency

(a) Phosphorus → Ground state Covalency = 3

Ex: (PCl ₃ )

Phosphorus → Excited state Covalency = 5

Ex: (PCl ₅ )

(b) Sulphur → Ground state Covalency = 2

Ex: (SF ₂ )

Sulphur → Excited state

1 ^st excited state Covalency = 4

Ex: (SF ₄ )

2 ^nd excited state Covalency = 6

Ex: (SF ₆ )

So variable covalencies of S is 2, 4 & 6.

Covalent Bond

Covalent Bond : A covalent bond involves mutual sharing of valence electrons between two atoms. The sharing of two, four and six electron leads to the formation of a single, double and triple bond, respectively.

Electronegative element + Electronegative element Covalent bond

Examples Of Covalent Bond

Formation of Single Bond(s)

Cl ₂

Formation of Double Bond(s)

O ₂

Formation of Triple Bond(s)

N ₂

Note :

• Covalent Bonds are Directional in nature.

Coordinate Bond (Dative Bond)

Coordinate Bond (Dative Bond) : The bond formed between two atom in which contribution of an electron pair is made by one of them while the sharing is done by both.

In this type of Bonding,

• One species - must have lone pair - acts as donor known as Lewis’s base - acquire +ve charge.

• Another species - must have vacant orbital act as acceptor known as Lewis’s acid - acquire –ve charge.

• It is Shown by ‘ ’ & direction is from donor to acceptor.

Eg.

Similarly,

H ₂ O + H ⁺ → H ₃ O ⁺

BF ₃ + F ^– → BF ₄ ^–

NH ₃ + BF ₃ → F ₃ B. NH ₃

• During the formation of coordinate bond, structure & shape of the molecules gets changed.

Note :

Lewis’s base: Chemical species which can donate electron pairs.

Ex. F ^– , NH ₃ , H ₂ O, CN ^– , NO ₂ ^– , N(CN ₃ ) ₃ , SO ₄ ^2–

Lewis’s acid: Chemical species which can accept an electron pair.

Ex. BF ₃ , BCl ₃ , PCl ₃ , Fe ⁺² , H ⁺ , Me ₃ B, AlCl ₃ , SiF ₄ etc.