Flexi Says: Magnesium (Mg) has an electron configuration of 1s2 2s2 2p6 3s2. Each time a new principal energy level begins, as with the third element lithium, a new period is started on the periodic table. Note that 1s2 2s2 2p6 (Neon) completes the second period. As one moves across the third period, electrons are successively added. With magnesium, the 3s sublevel is complete.