A-Level Chemistry OCR Notes

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Electronic Configuration

Electrons orbit the central nucleus in shells. Each shell can hold 2n2 electrons, where n is the principal quantum number.
Electron shells are made up of atomic orbitals, which are regions in space where electrons may be found.
Each shell is composed of one or more orbitals and each orbital can hold one pair of electrons.

There are four main types of orbitals: s-, p-, d-, and f-
There are 1 s-orbital, 3 p-orbitals, 5 d-orbitals and 7-f orbitals possible in each subshell.
Shell/quantum number n = 1 can have the occupancy 1s2

n = 2 can have the occupancy 2s^22p^6
n = 3 can have the occupancy 3s^23p^63d^10
n = 4 can have the occupancy 4s^24p^64d^104f^14

Within each shell, orbitals that are of the same energy level are grouped together in sub-shells.

Sub-shells have different energy levels. Note that 4s is lower in energy than 3d, so 4s will fill first.
Shells and sub-shells are filled with electrons according to a set of rules:
- Atomic orbitals with the same energy fill individually first before pairing
- Aufbau principle – the lowest available energy level is filled first

Electron configuration is written with n representing principal quantum number. X is the type of orbital and y is the number of electrons in the orbitals of the subshell

e.g. potassium has 19 electrons and its electron configuration is written as

1s^22s^22p^63s^23p^64s^1

Electronic configurations of ions can be determined by using the same building up principle.

e.g. K+ is the same as potassium but one electron has been removed, therefore it is:

1s^22s^22p^63s^23p^64s^1

Take care with d-block ions, as 4s fills before 3d, but it also empties before 3d when forming ions

Electrons have an intrinsic property (spin). For two electrons in the same orbital, the spin must be opposite to minimise the repulsion.

No more than two electrons can fill an atomic orbital

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