ELECTRON SHELLS

Imagine a ladder. You can stand on rung 1, rung 2, or rung 3 — but never on rung 1.5.Electrons work the same way. They exist at specific, fixed energy levels around the nucleus. They cannot be between levels — they must be on a rung. This property, that energy comes only in discrete, allowed values rather than a continuous spectrum, is called quantization.

These energy levels are called electron shells. They are labelled K, L, M, N, and so on, moving outward from the nucleus. The K shell is the closest — and the lowest energy. The L shell is next, then M, then N.

Lower shells are lower energy, and — just like a ball rolling to the bottom of a hill — electrons prefer to be in the lowest energy state available to them.

Each shell can only hold a certain number of electrons. Think of it like seats in a row at a theatre — each row has a fixed number of seats, and once a row is full, the next person has to move to the row behind. Electrons fill from the innermost shell outward, only starting a new shell when the previous one is full.

Use the diagram below to see exactly how this plays out for the first 20 elements. Step through them one by one and watch the shells fill.

A few things to notice as you step through the elements:

• Hydrogen (Z=1) has just one electron, alone in the K shell.
• Helium (Z=2) fills the K shell completely with 2 electrons. Its first and only shell is full.
• Lithium (Z=3) can't fit its third electron in the K shell — so it starts a brand new L shell.
• Neon (Z=10) has a completely full K shell (2) and a completely full L shell (8) — 2 + 8 = 10.
• Sodium (Z=11) must start a third shell, the M shell, for its 11th electron.

Not all electrons in an atom are equal. We call electrons in the outermost shell of an atom (closest to the outside world) valence electrons. As shown above, which shell is the outermost changes depending on the element. In hydrogen the valence electron sits in the K shell, whereas in carbon the valence electrons sit in the L shell.

Sodium (Z=11), for example, has the configuration 2, 8, 1 — two electrons in the K shell, eight in the L shell, and one lone electron in the M shell. That single outermost electron is sodium's valence electron, and it is almost entirely responsible for how sodium behaves chemically.

One of the most useful patterns in the periodic table is that elements in the same column share the same number of valence electrons. That is why they behave so similarly — lithium, sodium, and potassium are all in Group 1, all have one valence electron, and all react in strikingly similar ways.

Pay special attention to helium (Z=2), neon (Z=10), and argon (Z=18). All three have something in common: their outermost shell is completely full.

A full outer shell is an exceptionally stable configuration. Elements in this state have almost no tendency to react with other atoms — they have no electron vacancies to fill and nothing to gain from bonding. These elements are called the noble gases, since they are above the squabbles of ordinary reactive elements. They sit in the rightmost column of the periodic table.

If you have ever seen a glowing neon sign or a helium balloon that doesn't catch fire, you are witnessing the stability of a full outer shell. This pattern — the way electrons fill shells and create stable full-shell configurations — is one of the deepest organising principles in all of chemistry. Almost every reaction you will ever study can be traced back to atoms trying, in one way or another, to achieve a full outer shell.