electron configuration of an atom of 25Mg?

Answer 1

The isotope of an atom just changes the number of neutrons to change the mass, so the electron configuration for Mg is the same for all isotopes of Mg, since the number of protons and electrons don't change

Mg is atomic number 12, meaning it has 12 protons. As an element, Mg then also has 12 electrons.

Count up to 12. Each orbital holds 2 electrons. Level 1 holds an s level, which holds only one orbital(2 electrons). Level 2 holds an s and a p level, an s holds 1 orbital(2 electrons total), and a p holds 3 orbitals (6 electrons total). S always fills before p, and all levels have an s shell, so level 3 shell s will appear too (2 electrons total)

1s2 2s2 2p6 3s2 is Mg Add all the right side numbers, it makes 12.