XSIQ
*

Chemistry - Trends in atomic radius

Trends in atomic radius

Atomic radius:

* decreases going across a period

* increases going down a group

The radii of cations and anions relative to their parent atoms are readily
explained. A cation usually has a smaller radius than its parent atom. An
anion usually has a larger radius than its parent atom.

Consider 19K - 1s2s2p3s3p4s and its associated cation 19K - 1s2s2p3s3p.
The cation K+ has a smaller radius than its parent atom K because there is
one less occupied electron shell in the cation.

The cation K+ has a smaller radius than its parent atom K because there is
one less occupied electron shell in the cation.

Consider 9F - 1s2s2p (core charge 9-2 = +7) and its associated anion 9F- -
1s2s2p (core charge 9-2 = +7).

The anion F- has a larger radius than its parent atom F because it has
more electrons in its outer shell. Contributing factors include more
electrons being acted upon by the same core charge, the outer-shell
electrons taking up more space, and the repulsion of four pairs of
electrons in the outer shell.

Previous | Next