Problem: Rank these atoms and ions according to radius.K-        K+       KRank these ions according to ionic radius.Sr2+        As3-      Se2-      Br-      Rb+ Rank these elements according to first ionization energy. Sb      N       As      P

We’re being asked to Rank these atoms and ions according to radius.

Recall that ionic radius is the size of an ion. The trend for ionic radius is as follows: it increases from right to left and down a period in the periodic table. Ionic radius also increases with increasing number of electrons.

K^-, K⁺  and K

• Since the element is the same, the charge could easily tell us the size. 
• more negative the charge→ more electrons → larger radius

Ranking from largest to smallest radius: 

K^- > K > K⁺

Sr²⁺ ,As^3-,Se^2-,Br^- and Rb⁺ 

• All the ions are isoelectronic with Kr (same number of electrons).

Because of this, the ionic radius will just depend on the charge of the ion: the more positive the charge of an ion is, the smaller the ion is.

–3 > –2 > –1 > 0 > +1 > +2 > +3

Ranking from largest to smallest radius: 

As^3- > Se^2- > Br^- > Rb⁺ > Sr²⁺