Bohr's shells

To explain why atoms don't collapse, Niels Bohr proposed a new model in 1913.

Bohr said electrons can move only in fixed circular paths called shells (also orbits, or stationary states). In each shell an electron has a fixed energy, so shells are also called energy levels.

Shells are named K, L, M, N…, or numbered n = 1, 2, 3, 4…. K (n = 1) is closest to the nucleus and has the least energy. The farther out the shell, the more energy it has.

Think of seats at a cricket stadium, sold by price. You can sit only in a row your ticket allows. You can't sit between rows. To move to a better row, you must pay the exact difference; moving back gives you change. Electrons are the same:

• an electron inside a shell does not lose energy;
• it can jump to another shell only by taking in or giving out exactly the energy difference between the two shells;
• each shell holds only a limited number of electrons.

Bohr postulated (proposed, as a starting rule) that in a shell the electron's energy stays constant, even though it is moving. So it does not spiral in, and the atom is stable. His model explained many observations, and he won the Nobel Prize in 1922.

"Why start at K, not A?" Charles Barkla named the first X-ray line he found "K", leaving room for earlier letters in case more were found. None ever was.

Later still, the quantum mechanical model replaced Bohr's. It pictures electrons as clouds rather than as tiny planets on tracks. You'll meet it in higher classes.