1.
\(a.\ Fe^{2+}: 1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4s 3d^{5} \ or \ [Ar]4s 3d^{5}\)
\(b.\ Co^{2+}: 1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4s^{2} 3d^{5} \ or \ [Ar]4s^{2} 3d^{5}\)
\(c.\ Ni^{2+}: 1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4s^{2} 3d^{6} \ or \ [Ar]4s^{2} 3d^{6}\)
2.
\(a.\ Cr^{3+}: 1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4s^{2} 3d \ or \ [Ar]4s^{2} 3d\)
\(b.\ Mg^{3+}: 1 s^{2} 2 s^{2} 2 p^{5}\)
\(c.\ Fe^{3+}: 1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4s^{2} 3d^{3} \ or \ [Ar]4s^{2} 3d^{3}\)
3.
\(a. \ Br^{-}\)
\(b. \ H^{-}\)
\(c.\ As^{3-}\)
\(d.\ Se^{2-}\)
4. Argon is a noble gas so it is stable. Ar, Cl-, S2- and P3- are all anions that give excess electrons to become stable like the nearest noble gas, which is in this case, Argon. So, all of these have the same electronic configuration:
\(1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6}\)
