Question

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

Answer 1

To answer this question we’ll have to remember the trends of atomic and ionic radius across the period and down the group. The change in the atomic radius across the period and going down the group will answer this question.

Complete answer:
We will first write the electronic configurations of Sodium, Magnesium and Calcium
${}_{11}Na = 1{s^2}2{s^2}2{p^6}3{s^1}$
${}_{12}Mg = 1{s^2}2{s^2}2{p^6}3{s^2}$
${}_{20}Ca = 1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}$
Sodium and Calcium both belong to the Group I of the periodic table and magnesium is from group II.
The trend of atomic radius across the period is: Across the period from left to right, the atomic radius decreases. This occurs because as we move across the period the atomic number increases, and an extra electron is added to the same shell. Along with electrons, protons are also added in the nucleus. Because of this extra addition of protons and electrons the effective nuclear charge of the atom increases. Because of this increase, the electrons are more strongly attracted towards the nucleus because of which the atomic size reduces. Hence sodium has more atomic size than magnesium
The trend of atomic radius down the group: The atomic radius increases as we move down the group. This is because the atomic number increases as we move down. The electrons are now added in a new shell, then that shell itself. With the addition of new shells to the atom, the distance of the outermost electron from the nucleus keeps on increasing. Also, the shielding of electrons plays an important role in increasing the size. Because of this the size of Calcium is more than both sodium and magnesium.
The order of atomic radius can be given as: $Ca > Na > Mg$

Note:
The Noble gases of the group XVIII of the periodic table, doesn’t obey the trend of across the period. The Noble gases have the largest size in any period. This is because the electronic configuration of these gases are stable. All orbitals are full because of which the shielding effect of electrons increases hereby causing the size to expand.