Writing Electron Configurations (DP IB Chemistry): Revision Note
Writing Electron Configurations
The electron configuration gives information about the number of electrons in each shell, subshell and orbital of an atom
The subshells are filled in order of increasing energy
Electron Configuration Key

Electrons can be imagined as small spinning charges which rotate around their own axis in either a clockwise or anticlockwise direction
Spin pair repulsion diagram

Electrons with the same spin repel each other (spin–pair repulsion):
Electrons occupy separate orbitals within a subshell first to minimise repulsion, with spins aligned
They pair only after all orbitals are singly occupied, with opposite spins
This is known as Hund’s Rule:
In a p subshell, electrons fill px, py, and pz orbitals singly before pairing.
Hund's Rule

Hund’s Rule:
Electrons fill degenerate orbitals (same energy) singly first, with parallel spins, to minimise repulsion
The principal quantum number (n) indicates the energy level of an electron:
For example, 2p electrons are in the second shell, n = 2
Pauli Exclusion Principle:
An orbital holds two electrons with opposite spins only
Electrons pair only when no empty orbital of the same energy is available
Pairing costs less energy than jumping to a higher orbital
Each box represents an atomic orbital
The boxes are arranged in order of increasing energy from lower to higher (i.e. starting from closest to the nucleus)
The electrons are represented by opposite arrows to show the spin of the electrons
E.g. the box notation for titanium is shown below
Electron box notation for titanium diagram

How to write electronic configurations
Electron configuration shows how electrons are arranged in shells, subshells, and orbitals.
There are two formats:
Full configuration; lists all electrons from 1s onward
Shorthand configuration; uses the symbol of the nearest noble gas in brackets to represent inner electrons (e.g. [Ar])
Ions form when atoms gain or lose electrons:
Anions (negative) form by adding electrons to the outer shell
Cations (positive) form by removing electrons from the outer shell
Transition metals:
Fill the 4s before 3d when neutral
Lose electrons from 4s first, not 3d, when forming ions
In the Periodic Table the elements are grouped into blocks based on their valence subshell:
s-block: valence electrons in an s orbital
p-block: valence electrons in a p orbital
d-block: valence electrons in a d orbital
f-block: valence electrons in an f orbital
s, p, d and f blocks in the Periodic Table

Examples
Electronic configuration of Fe
Atomic number = 26 so there are 26 electrons
Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Shorthand: [Ar] 4s2 3d6
Electronic configuration of Fe2+
Atomic number = 26 so there are 26 electrons, but the Fe2+ ion only has 24 electrons
Electrons are removed from the 4s orbital before the 3d
Full configuration: 1s2 2s2 2p6 3s2 3p6 3d6
Shorthand: [Ar] 3d6
Exceptions to the Aufbau Principle
Chromium and copper have the following electron configurations:
Cr is [Ar] 3d5 4s1 not [Ar] 3d4 4s2
Cu is [Ar] 3d10 4s1 not [Ar] 3d9 4s2
This is because the [Ar] 3d5 4s1 and [Ar] 3d10 4s1 configurations are energetically favourable
By promoting an electron from 4s to 3d, these atoms achieve a half full or full d-subshell, respectively
Worked Example
Write down the full and shorthand electron configuration of the following elements:
Potassium
Calcium
Gallium
Ca2+
Answer:
Potassium
Electrons: 19
Full configuration: 1s2 2s2 2p6 3s2 3p6 4s¹
Shorthand: [Ar] 4s¹
Note: Argon (Ar) has 18 electrons and is the previous noble gas
Calcium
Electrons: 20
Full configuration: 1s2 2s2 2p63s2 3p6 4s2
Shorthand: [Ar] 4s²
Note: 4s is filled before 3d because it is lower in energy
Gallium
Electrons: 31
Full configuration: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p1
Shorthand: [Ar] 3d10 4s2 4p1
Note: Includes filled 3d subshell after argon
Calcium 2+ ion
Electrons: 18 (after losing two 4s electrons)
Full configuration: 1s2 2s2 2p6 3s2 3p6
Shorthand: [Ar]
Note: Ca²⁺ has the same configuration as argon
Examiner Tips and Tricks
Orbital spin diagrams can be drawn horizontally or vertically, going up or down the page - there is no hard and fast rule about this
The important thing is that you label the boxes and have the right number of electrons shown
The arrows you use for electrons can be full or half-headed arrows, but they must be in opposite directions in the same box.
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