Valence electron. Image credit: Flickr
Valence electron. Image credit: Flickr

A valence electron is an electron located in an atom’s outermost shell, called valence shell. It is the farthest from the nucleus and is therefore least attracted to it. Valence electrons participate actively in a chemical reaction such as a chemical bond where an atom gains/loses or shares a valence electron. The word “valence” is inspired from the word “valiant”, which means strong and courageous.

The presence or lack of valence electrons determines an atom’s chemical characteristics, and its willingness to participate in a chemical reaction. In other words, if an atom has all electron spots in its valence shell filled, it’s stable and would not be interested in bonding with other atoms. But if there are vacant spaces, the atom is supposedly reactive (chemically) and could loan in a few or share electrons with another atom. An atom that has excessive valence electrons would share the electrons with another atom or donate them to another atom.

Periodic Table Grouping

Valence electrons have the highest energy of all electrons within an atom and, as a result, are the most reactive. This also explains why elements that have an identical set of valence electrons have similar chemical traits. Elements with the same electron configuration are placed in a particular group in the periodic table. For instance, group 1 elements have a single valence electron. Group 2 elements comprise 2 valence electrons.

Generally, atoms have a nucleus and shells surrounding the nucleus. The shells have subshells, which have orbitals. Orbitals house electrons – a single orbital cannot house more than 2 electrons. Shell 1 or the shell nearest to the nucleus has one subshell, called subshell s. This subshell has one orbital. Therefore, shell 1 can hold only 2 electrons. Shell 2 has 2 subshells: s and p. Subshell p has 3 orbitals. This means the second subshell can house 8 electrons (2 electrons courtesy subshell s and 6 electrons courtesy subshell p). Shell 3 has 3 subshells: s, p and d. Subshell d has 5 orbitals. This takes subshell d’s electron accommodating capacity to 18 electrons (if you do the math).

Let’s understand this with an example. Aluminium has 13 electrons in total. As per the orbital configuration rule, the shell closest to the nucleus should not have more than 2 electrons. The second closest shell can house up to 8 electrons. The third shell would need 10 electrons. In the case of aluminium, the third shell would have 3 electrons.