Electronic structure of atoms

Electrons in an atom are arranged in energy levels, often called shells, around the nucleus. Electrons fill the lowest available energy level first, so shells nearest the nucleus are occupied before electrons enter shells farther out.

An atom’s electronic structure shows how many electrons are in each shell. It can be written as numbers separated by commas, starting with the shell nearest the nucleus. For example, sodium has the electronic structure $$2,8,1$$: it has two electrons in its first shell, eight in its second and one in its third.

The same structure can be shown in a shell diagram. The shells are drawn as concentric rings around the nucleus, with electrons placed on the rings. A sodium diagram would show two electrons on the innermost ring, eight on the next and one on the outermost ring.

The terms shell and energy level can be used interchangeably when describing electronic structure.

Constructing electronic structures for the first twenty elements

A neutral atom has the same number of electrons as protons. Its atomic number gives the number of protons, so it also gives the number of electrons to place in shells.

To construct the electronic structure of one of the first twenty elements, place electrons in shells in this order:

1. Place up to two electrons in the first shell.
2. Place up to eight in the second shell.
3. Place up to eight in the third shell.
4. Place any remaining electrons in the fourth shell. For the first twenty elements, this shell contains at most two electrons.

Stop when all the atom’s electrons have been placed. A shell does not have to be full if there are no electrons left.

Electronic structures in shell order

Write the number of electrons in each occupied shell, starting with the shell closest to the nucleus.

Chlorine has atomic number 17, so a neutral chlorine atom has 17 electrons. Two go in the first shell and eight in the second, leaving seven for the third. Its electronic structure is $$2,8,7$$.

Calcium has atomic number 20, so a neutral calcium atom has 20 electrons. After placing two in the first shell, eight in the second and eight in the third, two remain for the fourth. Its electronic structure is $$2,8,8,2$$.

The third shell can hold more than eight electrons. However, for the first twenty elements, the fourth shell starts to fill once the third contains eight.

Electronic structures in diagrams

The same structure can be shown using concentric shells around the nucleus. Each dot or cross on a shell represents one electron. For chlorine, place two marks on the first shell, eight on the second and seven on the third. The numbers on the diagram must match the numbers in the shell-order notation.