Electron shells

Neon, argon and helium hardly react with anything. The reason is how their electrons are arranged.

In the simple shell model, electrons fill levels around the nucleus. For the first 20 elements the levels hold 2, then 8, then 8, then the rest. Real electrons do not travel on neat circles, but the model predicts a lot of chemistry. Niels Bohr introduced the idea of fixed energy levels in 1913, and chemists still use this picture to explain why elements react the way they do.

Filling the shells

Electrons go into the innermost shell first, because that is where they are held most tightly. Hydrogen has 1 electron and helium 2, which fills the first shell. Lithium, with 3, has to start a second shell: 2, 1. The second shell is full at neon, with 2, 8. Sodium starts the third shell, and argon fills it to 8 at 2, 8, 8. Potassium and calcium then put their last electrons into a fourth shell.

Outer shells can hold more than 8 in heavier atoms. The general limit for shell number n is 2 × n × n, so 2, 8, 18 and 32. For the first 20 elements, though, the third shell stops at 8 before the fourth one starts, and that is all you need for this course.

The outer shell

Electrons in the outermost shell are the ones that take part in reactions. An atom with a full outer shell, like neon (2, 8), is stable. Sodium (2, 8, 1) easily gives away its single outer electron, and chlorine (2, 8, 7) easily takes one. That is why the two combine into table salt, sodium chloride: the sodium becomes a +1 ion, the chlorine a −1 ion, and opposite charges hold them together.

The same idea predicts the charge of many ions. An atom with one, two or three outer electrons tends to lose them and is left with the full shell underneath. Magnesium (2, 8, 2) loses two and becomes Mg²⁺. An atom that is one or two electrons short of a full shell tends to gain them. Oxygen (2, 6) gains two and becomes O²⁻, and chlorine gains one to become Cl⁻. Count the outer electrons and you can usually guess the charge before you look it up.

Shells and the periodic table

The shell model explains the layout of the periodic table. Each row, called a period, starts a new shell: hydrogen and helium in row 1 use one shell, lithium to neon in row 2 use two, sodium to argon in row 3 use three. Elements in the same column, called a group, have the same number of outer electrons, so they behave alike. Lithium, sodium and potassium each have one outer electron, and all three react strongly with water. Helium, neon and argon sit in the last column with full outer shells, and they are the noble gases that hardly react at all.