# Question #b296a

##### 1 Answer
Oct 5, 2014

The electron configuration of an element typically indicates the number of electrons in the energy levels and sublevels in the atom's ground state (state of lowest energy). In a neutral atom, the numbers of protons and electrons are equal. The atomic number is the number of protons, so in a neutral atom it is also the number of electrons.

Valence electrons are the the electrons in the highest energy level in the s and p sublevels and are involved in chemical reactions (may vary with transition elements). The core electrons are all of the rest of the electrons in all of the previous energy levels.

$\text{_14"""Si}$: ${\text{1s"^2"2s"^2"2p"^6"3s"^2"3p}}^{2}$ (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.