Writing Electron Configurations (DP IB Chemistry): Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Richard Boole

Updated on

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

Diagram explaining electron configuration 1s¹: '1' indicates principal quantum number, 's' denotes subshell, and '¹' specifies number of electrons.
The electron configuration shows the number of electrons occupying a subshell in a specific shell
  • 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

Two blue spheres depict magnetic fields; the left has a north arrow up, the right has a south arrow up. Both show anti-clockwise rotation arrows.
Electrons can spin either in a clockwise or anticlockwise direction around their own axis. The spin creates a tiny magnetic field with N-S pole pointing up or down, although you are not required to know this for the exam
  • 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

Lowercase 'p' followed by three blocks, each containing a half headed arrow.
Electron configuration: three electrons in a p subshell
  • 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

Diagram of electron configuration shows orbitals: 1s, 2s, 2p, 3s, 3p, 4s, and 3d, each with arrows representing electron spins.
The electrons in titanium are arranged in their orbitals as shown. Electrons occupy the lowest energy levels first before filling those with higher energy

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

Diagram showing periodic table blocks: s-block (blue), d-block (orange), p-block (green), f-block (red) with respective subshell notations.
The elements can be divided into four blocks according to their outer shell electron configuration

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:

  1. Potassium

  2. Calcium

  3. Gallium

  4. 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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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Richard Boole

Reviewer: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

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