Determine valence electrons using the periodic table

An atom's valence electrons are the electrons in its outermost shell. In the chlorine model below, the valence electrons are shown in $\mathbf{\text{red}}$‍.

The number of valence electrons determines most of an atom's chemical behaviors. So, it's important to be able to identify how many valence electrons atoms of different elements have.

Fortunately, we can use the periodic table to quickly determine the number of valence electrons for main group elements.

Bohr models for the first three periods of the periodic table are shown below. The valence electrons are shown in $\mathbf{\text{red}}$‍.

The number of valence electrons increases going left to right across a period. But, with the exception of $\mathrm{He}$‍, the number of valence electrons remains the same going down a group.

For example, $\mathrm{H}$‍, $\mathrm{Li}$‍, and $\mathrm{Na}$‍ are in group 1 and all have one valence electron. Both $\mathrm{N}$‍ and $\mathrm{P}$‍ are in group 15 and have five valence electrons.

This is why elements in the same group have similar chemical properties—they have the same number of valence electrons!

This pattern allows us to quickly identify the number of valence electrons for main group elements on the periodic table, even if we don't have Bohr models to reference. The diagram below shows the number of valence electrons (VE) for the main group elements.

To summarize, we can use an element's group on the periodic table to determine how many valence electrons its atoms have:

*except helium, which has 2