# How do you find the number of valence electrons in a ion?

Dec 27, 2016

Let's take examples from Group 15 (nitrogen), Group 16 (oxygen), and Group 17 (fluorine). All of these are non-metals, and they tend to be (STRONGLY) oxidizing; that is they accept electron density to give ions that are isoelectronic with the next Noble Gas. Nitrogen thus forms a ${N}^{3 -}$ ion, oxygen an ${O}^{2 -}$ ion, and, fluorine, by reason of its electron count the most strongly oxidizing element on the table, a ${F}^{-}$ ion.
All of these ions have a full valence shell, and are isoelectronic with $N e$, which possesses a full valence configuration of $1 {s}^{2} 2 {s}^{2} 2 {p}^{6}$; this is the origin of the electron shell idea of $2 : 8 : 8$ etc. that we learnt in lower high school.