Which electron configuration represents an

atom of magnesium in an excited state?

(1) 2-7-3 (3) 2-8-2

(2) 2-7-6 (4) 2-8-5

Atomic number of magnesium is 12. That implies that in ground state magnesium has 2 electrons in the first level, 8 electrons in the second level, and 2 electrons in the third level. That is represented by 2 8 2. That the atom is in an excited state means that one electron (at least) is a upper level than where it is in grounded state. That situation is represented by the option (1) 2 7 3, where one electron from the second level has been promoted to the third level.