What is the full electron configuration for ##"Mg"^(2+)##?

##"Mg"^(2+): 1s^2 2s^2 2p^6##

Your starting point for finding the of the magnesiu mcation, ##"Mg"^(2+)##, will be the electron configuration of the neutral magnesium atom, ##"Mg"##.

You know that magnesium is located in period 3, group 2 of , and that it has an equal to ##12##.

This means that a neutral magnesium atom will have a total of ##12## protons in its nucleus and ##12## electrons surrounding its nucleus. Its electron configuration would look like this

##"Mg: " 1s^2 2s^2 2p^6 3s^2##

Now, when the magnesium cation is formed, two electrons, hence the ##2+## charge of the cation, are lost by the magnesium atom.

These electrons, which are called , come from magnesium's outermost energy level. As you can see, magnesium has its outermost electrons located on the third energy level, ##n=3##.

This means that when the cation is formed, its electron configuration will look like this

##"Mg: " 1s^2 2s^2 2p^6 color(red)(cancel(color(black)(3s^2))) implies "Mg"^(2+): 1s^2 2s^2 2p^6##

Using , you will get

##"Mg: " ["Ne"]color(red)(cancel(color(black)(3s^2))) implies "Mg"^(2+): ["Ne"]##