Quantum numbers
Four numbers describe each electron in an atom. n is the shell (size and energy), l is the subshell shape, mₗ is the orbital direction and mₛ is the spin, +½ or −½.
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Analogy
Like an address: building (n), floor type (l), room (mₗ) and which of two beds (mₛ).
Example
n = 1, 2, 3 ...l = 0 to n − 1mₗ = −l to +lmₛ = +½ or −½
In practice
Quantum numbers tell you why shell n = 2 holds 8 electrons and shell n = 3 holds 18.
Practice
Flashcards
- What does the quantum number n describe?The shell: its size and main energy level.
Questions people ask
What values can l take for n = 3?
For n = 3, l can be 0, 1 or 2, which is always 0 up to n − 1. These match the s, p and d subshells. So the third shell contains one s, three p and five d orbitals, which gives 9 orbitals and room for 18 electrons.
Why are there only two spin values?
Experiments show an electron behaves as though it has a tiny built-in magnet that can point in only two directions, called spin up and spin down. We label them +½ and −½. No third direction is ever seen, so mₛ has just two values.