Basic assumption of electronic theory of valency

The outermost shell of an atom is known as the valence shell. The electrons present in the valence shell are called valence electrons.

K L M

Na - 11 - 2 8 1

Valence shell - M

Valence electrons - 1

Core electrons - 2 + 8 = 10

Question

The electronic configurations of some of the elements are detailed below. How many valence and core electrons do the following have?

• Al: 2, 8, 3

• Mg: 2, 8, 2

• Cl: 2, 8, 7

• Ca: 2, 8, 8, 2.

Solution

• Valence electrons = 3; core electrons = 10

• Valence electrons = 2; core electrons = 10

• Valence electrons = 7; core electrons = 10

• Valence electrons = 2; core electrons = 18

Significance of valence electrons

• Valence electrons of an atom are responsible for chemical reactions as they take part in them.

• Elements having same number of valence electrons in their atoms possess similar chemical properties. All alkali metals have one valence electron in their atom. Thus, their chemical properties are similar.

• Elements having 1, 2 or 3 electrons in the valence shell are metals. Exception is H and He. Elements having 4 to 7 electrons in their valence shell are non-metals.

Lets try out:

Classify the following elements as metals or non-metals:

A - 2,8,2 - It is a metal as it has 2 valence electrons

B - 2,8,6 - It is a non-metal as it has 6 valence electrons

C - 2,6 - It is a non-metal as it has 6 valence electrons

• The number of the valence shell in an atom determines the period number to which the elements belong in the Periodic Table.

Example

Na - 11 : 2, 8, 1

Number of the valence shell - 3

Therefore, sodium is placed in period 3 of the periodic table.

Example

Ca - 20 : 2, 8, 8, 2

Number of the valence shell - 4

Therefore, sodium belongs to period 4 of the periodic table.