An atom with an atomic number of 16 corresponds to sulfur (S). Sulfur has 16 electrons in total. When considering a -2 anion, it means that two additional electrons have been gained by the atom.
To distribute the electrons among the orbitals of the -2 anion, we start by filling the lower energy levels before moving to higher energy levels according to the Aufbau principle and Hund's rule.
Sulfur's electron configuration is 1s2 2s2 2p6 3s2 3p4. In the -2 anion, two additional electrons are added to the configuration, giving a total of 18 electrons.
The valence electrons are the electrons in the outermost energy level, which for sulfur are the electrons in the 3s and 3p orbitals. Therefore, sulfur in the -2 anion has 6 valence electrons.