Group 2: Question 6

Worked solution

Why atomic radius and ionic radius both increase down the group

Going down Group 2 from magnesium to barium, each successive element has one more filled electron shell than the one above it. Extra shells of inner (shielding) electrons are added between the nucleus and the outermost $ns^2$ electrons, so both the atomic radius and the ionic radius of the resulting $\text{M}^{2+}$ ion get larger at each step down the group, even though the $\text{M}^{2+}$ ion always carries the same $2+$ charge.

Why first ionisation energy decreases down the group

First ionisation energy is the energy needed to remove one electron from an isolated gaseous atom. Down the group:

• The outer $ns^2$ electrons are further from the nucleus (larger atomic radius), so the nuclear attraction on them is weaker.
• There is more shielding from the extra inner shells of electrons.

Both effects make the outer electron easier to remove, even though the nuclear charge is also increasing. The shielding and distance effects dominate, so first ionisation energy decreases down the group, from magnesium to barium.

Why the other options are wrong

• B reverses all three trends.
• C gets atomic radius right but wrongly decreases ionic radius (it also increases) and wrongly increases first ionisation energy (it decreases).
• D gets atomic and ionic radius right but wrongly increases first ionisation energy instead of decreasing it.

Final answer

A. Atomic radius increases, ionic radius increases, and first ionisation energy decreases, going down Group 2 from magnesium to barium.