# Why is a magnesium atom smaller than atoms of both sodium and calcium?

Jan 7, 2016

See explanation.

#### Explanation:

In order to answer this question, let us write the electron configuration of each element:

""_11Na:1s^(2)2s^(2)2p^(6)3s^(1)

""_12Mg:1s^(2)2s^(2)2p^(6)3s^(2)

""_20Ca:1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(2)

Looking at the electron configuration we conclude that:

• Magnesium atom is smaller than sodium atom because the nuclear charge of Magnesium ($12 +$) is higher than that of Sodium ($11 +$). Magnesium nucleus will attract the electrons more toward it and therefore, shrinking the size of the atom.
• Magnesium atom is smaller than Calcium atom because Calcium has more electron ($20 {e}^{-}$) which will occupy more energy levels ($n = 4$ for Calcium versus $n = 3$ for Magnesium).