Why is a magnesium atom smaller than atoms of both sodium and calcium?
In order to answer this question, let us write the electron configuration of each element:
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 (
#20e^-#) which will occupy more energy levels ( #n=4#for Calcium versus #n=3#for Magnesium).