# The first ionization energy

Question:

The first ionization energy (in $\mathrm{kJ} / \mathrm{mol}$ ) of $\mathrm{Na}, \mathrm{Mg}, \mathrm{Al}$ and $\mathrm{Si}$ respectively, are:

1. $496,737,577,786$

2. $496,577,737,786$

3. $786,737,577,496$

4. $496,577,786,737$

Correct Option: 1

Solution:

All given elements belongs to period III and generally their ionisation energy will increase along the period but Mg will show higher ionisation potential compared to $\mathrm{Al}$ due to its stable configuration. Thus correct order of ionisation energy will be : $\mathrm{Na}<\mathrm{Al}<$ $\mathrm{Mg}<$ Si.

Ionisation energy $(\mathrm{kJ} / \mathrm{mol})$ of the given metals are $\mathrm{Na}: 496 ; \mathrm{Al}: 577 ; \mathrm{Mg}: 737 ; \mathrm{Si}: 786$