Relative Sizes of Atoms and Ions

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


Cs
N
Mg2+
Br-
P
My answer:
4
2
5
1
3
But it was wrong :( What did I do incorrectly?

Cs = period 6, very large atom
N = period 2, small atom
Mg2+ = period 2, -2 electrons, larger than Mg
Br- = period 4, +1 electron, smaller than Brº
P = period 3, medium sized

Cs = largest, then Br-, P, Mg2+, N
1, 5, 4, 2, 3