What are electron shells?

Shells: layers of electrons

The electrons around a nucleus are found in shells, also called energy levels, which work a bit like the floors of a building. The first shell is closest to the nucleus and lowest in energy, and each shell after it is farther out and higher in energy.

Shells are numbered 1, 2, 3 and so on, outward from the nucleus, and they also have letter names: K, L, M and N for the first four. An element’s period number tells you how many shells its atoms use, so sodium, in period 3, has electrons in three shells.

How many electrons fit in each shell

Each shell can hold a maximum number of electrons, given by the formula 2n², where n is the shell number. The first shell holds 2 × 1² = 2, the second holds 2 × 2² = 8, the third holds 2 × 3² = 18 and the fourth holds 2 × 4² = 32.

Electrons fill the lowest-energy shell first, and when a shell is full, the next electron starts a new shell farther out. Helium’s 2 electrons fill the first shell, so lithium’s third electron must go into the second shell, giving the arrangement 2, 1.

The 2-8-8 pattern for the first 20 elements

For the first 20 elements, one simple rule gives the electron arrangement: put 2 electrons in the first shell, up to 8 in the second, up to 8 in the third, and any left over in the fourth. Carbon (6 electrons) is 2, 4. Sodium (11) is 2, 8, 1. Chlorine (17) is 2, 8, 7. Argon (18) is 2, 8, 8.

The third shell could hold 18 electrons, yet potassium (19) is 2, 8, 8, 1 and calcium (20) is 2, 8, 8, 2, because the first part of the fourth shell is slightly lower in energy than the rest of the third shell. From scandium (21) onward, electrons go back to filling the third shell, so the 2-8-8 rule only works up to calcium.

Drawing a Bohr diagram

A Bohr diagram is a quick sketch of an atom’s shells. Draw a small circle for the nucleus with the element’s symbol inside, then one ring for each shell, and place the electrons on the rings as dots.

Worked example: sodium. Its atomic number is 11, so a neutral sodium atom has 11 electrons. Put 2 on the first ring, which leaves 9. Put 8 on the second ring, which leaves 1, and that last electron sits alone on the third ring. The diagram shows 2, 8, 1.

Why the outer shell matters

The electrons in the outermost shell, the valence electrons, decide how an atom reacts. Atoms with a full outer shell, such as neon (2, 8) and argon (2, 8, 8), hardly react at all. Lithium (2, 1), sodium (2, 8, 1) and potassium (2, 8, 8, 1) each have a single outer electron that they lose easily, so all three are reactive metals.

The Bohr model and its limits

The Danish physicist Niels Bohr proposed this model in 1913. He suggested that electrons travel in fixed orbits with fixed energies and give off light when they drop from a higher orbit to a lower one. This explained the exact colors of light from glowing hydrogen gas, a big success at the time.

The model has limits, though. It only predicts the light from hydrogen and other one-electron atoms correctly, and electrons do not really circle the nucleus like planets around the Sun. In the modern quantum model, each electron occupies an orbital, a region where it is likely to be found, and each shell is split into subshells called s, p, d and f. It is still a handy tool for counting electrons, as long as you remember it is simplified.