Energy Levels

How are the electrons arranged in the energy levels?

Three Principles/Rules guide us with understanding this:

There are two exceptions: Cr and Cu:
Cr: 4s¹ 3d⁵
Cu: 4s¹ 3d¹⁰

One electron from the 4s sub-level "drops" into the 3d sub-level to make the electron arrangement more stable. A half-filled sub-level is stable due to "maximum spin". A complete 3d sub-level is also stable as it "fills" that energy level.

The last skill we need regarding electron configurations is to apply this to ion formation. What are the electron configurations of ions, and why are some coloured?