An Isoelectronic Series is a group of atoms/ions that have the same number of electrons.
N3-, O2-, F-, Ne, Na+, Mg2+, Al3+
This series each have 10 electrons.
P3-, S2-, Cl-, Ar, K+, Ca2+, Sc3+
This series each have 18 electrons.
A typical question about isoelectronic series usually involve size comparisons. Since the number of electrons are the same, size is determined by the number of protons. Al has 13 protons, therefore the nuclear charge is greatest and pulls the electrons closer, thus is the smallest. The N3- has 7 protons and the nuclear charge it the smallest of the series. Thus it has the least "pull" on the electrons and this species is the largest.