What is valency and valence electron? Give valency of $\mathrm{Sodium},\mathrm{Magnesium},\mathrm{Aluminium},\mathrm{Chlorine},\mathrm{Oxygen}\mathrm{and}\mathrm{Nitrogen}$.

Valency:

• The capacity with which an atom of an element combines chemically with the atom(s) of another element is called the combining capacity or valency of the element.

Valence electron:

• Electrons in the outer shells that are not filled are called valence electrons.
• Valence is the number of electrons an atom must lose or gain to attain the nearest noble gas or inert gas electronic configuration.

The valency of an atom:

• It is described by the total number of electrons lost, gained, or shared to complete its octet.
• Valency of $\mathrm{Sodium}\left(\mathrm{Na}\right)$ is$+1$, As it loses $1$ valence electron for the bond formation.
• Valency of $\mathrm{Magnesium}\left(\mathrm{Mg}\right)$ is$+2$, As it loses $2$ valence electron for the bond formation.
• Valency of $\mathrm{Aluminium}\left(\mathrm{Al}\right)$is$+3$, As it loses $3$ valence electron for the bond formation.
• Valency of $\mathrm{Chlorine}\left(\mathrm{Cl}\right)$ is$-1$, As it gains $1$ electron in its valence shell for the bond formation.
• Valency of $\mathrm{Oxygen}\left(\mathrm{O}\right)$ is$-2$, As it gains $2$ electron in its valence shell for the bond formation.
• Valency of $\mathrm{Nitrogen}\left(\mathrm{N}\right)$ is$-3$, As it gains $3$ electron in its valence shell for the bond formation.