Question

Consider the elements Na, Mg and Al and their respective atomic numbers 11, 12 and 13. Write the electronic configuration of each element and answer the following question:

What is the number of valence electrons in each element?

Answer 1

Electronic configuration

Atomic numbers of Sodium ($\mathrm{Na}$), Magnesium ($\mathrm{Mg}$) and Aluminium ($\mathrm{Al}$) are 11, 12 and 13 respectively.

Electronic configuration of Sodium = (2, 8, 1)

Electronic configuration of Magnesium = (2, 8, 2)

Electronic configuration of Aluminium = (2, 8, 3)

All these elements belong to period 3.

Valence electrons

The electrons found in the valence shell of the element are valence electrons.

Sodium has 2 electrons in K-shell, 8 electrons in L-shell and 1 electron in M-shell, Since M-shell is valence shell, it has 1 valence electron.

Magnesium has 2 electrons in K-shell, 8 electrons in L-shell and 2 electrons in M-shell, Since M-shell is valence shell, it has 2 valence electrons.

Aluminium has 2 electrons in K-shell, 8 electrons in L-shell and 3 electrons in M-shell, Since M-shell is valence shell, it has 3 valence electrons.

Therefore, the valence electrons in Sodium, Magnesium and Aluminium are 1, 2 and 3 respectively.