How Do Atom and Ion Sizes Compare in Various Elements?

[spiderflower]

Homework Statement

Order each of the following sets of five atoms and ions according to decreasing size.
(i.e. "1" largest and "5" smallest)

N, Br-, Cs, P, Mg2+

Homework Equations

When an atom loses electrons (becomes more +), its size decreases. Conversely, when an atom gains electrons (becomes more -), its size increases.

The Attempt at a Solution

I know that Br- will be the largest, and Mg2+ will be the smallest. Then Cs will probably be the second largest considering its position on the periodic table. But for N and P I'm not entirely sure. So my best bet is: Br- > Cs > P > N > Mg2+
Or should I switch the P and N around?

One little problem... simply by adding one electron to Br will increase its size so much that it'll be larger than Cs? I find it a little unreasonable... but my teacher says that if there is a negatively charged ion, whatever the other ones, the anion will be the largest. How come?

Thank you very much in advance. :)

 

[zhermes]

One little problem... simply by adding one electron to Br will increase its size so much that it'll be larger than Cs? I find it a little unreasonable...

As you move along a row in the periodic table, the radius of a neutral atom and equivalent ions decreases. As you move down the periodic table, the radius increases. Like you said, as you lose electrons---your radius also decreases (and therefore, gaining electrons increases the radius). So you are at least on the right track...

but my teacher says that if there is a negatively charged ion, whatever the other ones, the anion will be the largest. How come?

That's not true. I.e. Br- is most certainly larger than H, or He, or most other low atomic number elements. Anions are always larger than the neutral version of the same element. Thus I'm not familiar with any immediate way of knowing which is bigger, Cs or Br-