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3.1.1 Atomic Structure

Electron Configuration

AQA A-level Chemistry

Shells and sub-shells

Electrons occupy shells, divided into sub-shells, which are made of orbitals. An orbital holds 2 electrons.

Sub-shellOrbitalsElectrons heldFirst shell
s121
p362
d5103

The order the sub-shells fill

SUB-SHELL ENERGY energy MAX 4p 6 3d 10 4s 2 3p 6 3s 2 2p 6 2s 2 1s 2 4s is below 3d, so 4s fills first fill from the bottom up: 1s first, 4p last THE ION RULE 4s fills first but empties first 4s electrons leave from here first 3d 3d keeps its electrons Ca2+ 1s22s22p63s23p6 (4s0) Fe2+ 1s22s22p63s23p63d6 no 4s electrons in either ion
The 4s line sits below the 3d line, and filling runs upwards from 1s.
Method: full configuration of an atom
  1. Number of electrons = the atomic number, Z.
  2. Fill in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p, so 4s fills before 3d because it is lower in energy.
  3. Stop at Z electrons and check the superscripts add up to Z.

Writing the configuration of an ion

Method: full configuration of an ion
  1. Write the full configuration of the neutral atom.
  2. Positive ion: remove from 4s before 3d, so every transition metal ion has an empty 4s.
  3. Negative ion: add into the next space in the filling order.
  4. Check the superscripts add to Z minus the charge.
ATOM FIRST, THEN CHANGE IT Fe26 electrons 1s22s22p63s23p63d64s2 the 4s pair is the one that goes lose 2 e− Fe2+24 electrons 1s22s22p63s23p63d6 3d untouched, 4s empty Cl17 electrons 1s22s22p63s23p5 3p has one space left gain 1 e− Cl−18 electrons 1s22s22p63s23p6 3p now full
Build the atom first, then change it.
IonFull electron configurationWhere the electrons went
Ca2+1s22s22p63s23p6both 4s
Al3+1s22s22p6the 3p, then both 3s
Cr3+1s22s22p63s23p63d3the 4s, then two 3d
Fe2+1s22s22p63s23p63d6both 4s, 3d untouched
Co2+1s22s22p63s23p63d7both 4s
Cu2+1s22s22p63s23p63d9the 4s and one 3d, from 3d104s1
F−1s22s22p6one into 2p
Cl−1s22s22p63s23p6one into 3p

Full configurations and the noble gas core

Write every sub-shell out from 1s, with no noble gas core. [Ar]3d9 is not a full configuration.

Chromium and copper

AtomFull electron configuration
Cr1s22s22p63s23p63d54s1
Cu1s22s22p63s23p63d104s1

Each has a single 4s electron, and either order of 3d and 4s may be written.

The one wrong answer for chromium

3d44s2 is what the filling order alone predicts, and it is wrong. Chromium is 3d54s1.

Deducing the element from a configuration

Method: configuration to element
  1. Add the superscripts for the number of electrons.
  2. For an ion, add the charge back: the electrons shown are not the protons.
  3. That total is Z. Read the symbol off the periodic table.
  • 2+ ion with krypton's configuration: Sr2+.
  • 2+ ion with 1s22s22p63s23p63d5: Mn, never V.
  • Compound of a 2+ and a 3− ion both isoelectronic with argon: Ca3P2.

Isoelectronic species

Species that are isoelectronic have the same electron arrangement. Na+ and F− are both 1s22s22p6.

IonProtons and attractionSize
Mg2+more protons with the same electron arrangement, so a stronger attraction between the nucleus and the outer electronssmaller
F−fewer protons, so a weaker attractionlarger

Shielding, charge density or atomic radius in place of the proton comparison is not an answer. Compare the protons first, then say the electron arrangement is the same.

Two ways this answer goes wrong

The species is an ion, so write fluoride ion or F−, never fluorine. Never write that the ions have different numbers of electrons: isoelectronic means the electrons are the thing that is the same.

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