Introduction

While understanding periodic trends is very important, what is even more so is understanding what this means for how elements react. This topic goes over core and valence electrons, and how knowledge of the number of valence electrons, and the periodic trends, can be used to predict how elements will react based on their location in the periodic table.

Valence and Core Electrons

Electrons can typically be split into two different categories. Core electrons are any electrons within filled shells. The core electrons in a chlorine ($\mathrm{Cl}$) atom are all of the electrons in the $1\mathrm{s}$, $2\mathrm{s}$, and $2\mathrm{p}$ subshells. Valence electrons are the electrons found in the outermost shell, so for chlorine ($\mathrm{Cl}$), this would be the $3\mathrm{s}$ and $3\mathrm{p}$ subshells. 

Looking at the groups (or columns) on the periodic table, we can see patterns in the number of valence electrons. Group 1 all have 1 valence electron, group 2 have 2 valence electrons, group 13 has 3 valence electrons, and another valence electron is added all the way to row 18, with 8 valence electrons. Groups 3-12 consist of the transition metals, and the electrons that are added will usually be added to a $\mathrm{d}$ or $\mathrm{f}$ subshell in a lower shell. (The electron configurations of the transition metals have many exceptions and are not tested in AP Chemistry.)

Trends for Valence Electrons

The number of valence electrons can tell you a lot about how an element will react and form bonds. In general, atoms want to have 8 valence electrons, as it is the most stable state for an atom to be in, and will gain or lose electrons in order to do so. For example, elements in group 1 will want to get rid of their 1 valence electron, effectively making the electrons in the lower shell the valence electrons, and giving the atom 8 valence electrons. On the other side of the periodic table, group 17 elements want to gain an electron to go from 7 valence electrons to 8 valence electrons. Elements typically try to reach 8 valence electrons with the least change in electrons possible, so groups 1, 2, and 13 will want to lose 1, 2, and 3 electrons respectively, and groups 15, 16, and 17 will want to gain 3, 2, and 1 electrons. Group 18 elements already have 8 valence electrons, and so don’t need to gain or lose electrons. Group 14 elements are the same number of electrons away from 8 valence electrons by gaining or losing, and so while they can do either, it takes so much energy that the base state is much more stable.

Forming Ionic Compounds

Ionic compounds form when one or more electrons are transferred between two atoms. The two atoms then become charged ions, and their opposite charges draw them together, which forms a crystalline structure on a large scale. Using our knowledge on the trends of valence electrons, we can predict which elements will form ionic compounds together. For example, the element sodium ($\mathrm{Na}$) wants to lose 1 electron, and the element chlorine ($\mathrm{Cl}$) wants to gain 1 electron. The sodium ($\mathrm{Na}$) then gives its electron to chlorine ($\mathrm{Cl}$), making them the ions $\mathrm{Na}{}^{+}$ and $\mathrm{Cl}{}^{-}$, which form the compound sodium chloride, or $\mathrm{NaCl}$.

Ionic compounds can also form between more than 2 atoms. Calcium ($\mathrm{Ca}$) can lose both of its valence electrons and give them to 2 different bromine ($\mathrm{Br}$) atoms, getting them each to 8 valence electrons and forming the compound calcium bromide, or $\mathrm{CaBr}{}_2$.