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.

Electronic configurations of atoms and ions

Electrons occupy shells around an atom’s nucleus. An electronic configuration shows how many electrons are in each shell, starting with the shell closest to the nucleus. For example, $$2,8,3$$ means that there are 2 electrons in the first shell, 8 in the second and 3 in the third.

The shell counts must add up to the total number of electrons in the atom or ion.

Filling electron shells

For elements with proton numbers from 1 to 20, electrons fill the shells in this order:

1. The first shell holds up to 2 electrons.
2. The second shell holds up to 8 electrons.
3. The third shell fills up to 8 electrons before the fourth shell begins to fill.

For example, a neutral calcium atom has 20 electrons. Its configuration is $$2,8,8,2$$: after the first three shells contain 2, 8 and 8 electrons, the remaining 2 enter the fourth shell. The rule of filling the third shell to 8 before starting the fourth applies to the elements in this range; it is not a general statement that the third shell can never hold more than 8 electrons.

Configurations of atoms and ions

A neutral atom has equal numbers of protons and electrons. Its proton number therefore tells us how many electrons to distribute among its shells. For example, a sodium atom, $$\ce{Na}$$, has proton number 11 and 11 electrons, so its configuration is $$2,8,1$$.

An ion forms when an atom loses or gains electrons. Its proton number does not change:

• A positive ion has lost electrons, so it has fewer electrons than protons.
• A negative ion has gained electrons, so it has more electrons than protons.

A sodium atom can lose its outer electron to form $$\ce{Na+}$$. The ion still has 11 protons, but it has 10 electrons and the configuration $$2,8$$.

A chlorine atom, $$\ce{Cl}$$, has 17 protons and 17 electrons, arranged as $$2,8,7$$. It can gain one electron to form $$\ce{Cl-}$$. The ion still has 17 protons, but its 18 electrons are arranged as $$2,8,8$$.