Define valency by taking examples of silicon and oxygen.

Valency:

• The combining capacity of the atom of an element is called its valency.
• It is always a whole number.
• Valency varies from 1 to 8.
• Valency is always positive.
• An element with a valency is one called monovalent, with two called divalent.
• The valency of noble gases is zero.

Valency of Silicon:

• The atomic number of silicon is 14.
• The electronic configuration of the silicon is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^2$.
• Valency = 8 - valence shell electron.
• In a silicon atom, the outermost orbital contains 4 electrons, therefore its valency is 4.

Valency of Oxygen:

• The atomic number of oxygen is 8.
• The electronic configuration of oxygen is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^4$.
• In an oxygen atom, the outermost orbital contains 6 electrons.
• If an element contains more than 4 electrons in its outermost shell then the valency of an element can be calculated as:
• Valency = 8 - valence shell electron.
• Therefore, the valency of the oxygen atom is $8-6=2$.