Really basic question - what exactly are core electrons? ENC (effective nuclear charge) has a formula:
# of protons - # of core electrons = ENC
Mg has 12 protons and 10 core electrons, giving it a ENC of 2+.
How do I know that Mg has 10 core electrons?
1 answer
12Mg is 1s2 2s2 2p6 3s2. Those 3s2 electrons are the valence electrons. The other 10 are the core electrons.