# Question #b296a

$\text{_14"""Si}$: ${\text{1s"^2"2s"^2"2p"^6"3s"^2"3p}}^{2}$ (The valence electrons are in the 3rd energy level in sublevels 3s and 3p. The core electrons are all of the rest of the electrons in energy levels 1 and 2.
$\text{_20"""Ca}$: ${\text{1s"^2"2s"^2"2p"^6"3s"^2"3p"^6"4s}}^{2}$ (The valence electrons are in the 4th energy level in the 4s sublevel. The core electrons are all of the rest of the electrons in energy levels 1, 2, and 3.
$\text{_13"""Al}$: ${\text{1s"^2"2s"^2"2p"^6"3s"^2"3p}}^{1}$ (The valence electrons are in the 3rd energy level in the sublevels 3s and 3p. All of the rest of the electrons in energy levels 1 and 2 are core electrons.