Energy Levels
We can read energy levels on a periodic table simply by going from left to right down each period.
1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶...
See the image below for a complete summary (taken from Wikibooks).
Each successive energy level is higher than the previous.
Thus, in the given list, 3d has the highest energy level.
