Ionisation energies and electron shells

Definition

The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions:

Na(g) -> Na+(g) + e-

The second ionisation energy is Na+(g) -> Na2+(g) + e-, and so on. All are endothermic because energy is needed to overcome the attraction between the nucleus and the electron.

What affects it

• Nuclear charge: more protons, stronger attraction, higher IE.
• Distance from the nucleus: an outer electron further away is held less strongly.
• Shielding: inner shells of electrons repel the outer electron, reducing the attraction.
• The sub-shell the electron is in, and whether it is paired.

Successive ionisation energies

Each successive IE is larger, because the electron is removed from an increasingly positive ion. A big jump shows the electron is now coming from a shell closer to the nucleus. So the number of electrons removed before the first big jump gives the group number. Plotting log10 of the IE against electron number makes the jumps easy to see; for potassium the jumps show electrons arranged in shells of 2, 8, 8 and 1.

Evidence for sub-shells

Across Period 2, first IE generally rises, but falls from Be to B (the B electron is in the 2p sub-shell, higher in energy and slightly shielded by 2s) and from N to O (in O, two electrons share a 2p orbital and repel). Period 3 shows the same dips at Al and S.

Worked example

The first five ionisation energies of an element are 578, 1817, 2745, 11 577 and 14 842 kJ/mol. Which group is it in?

1. The big jump comes between the 3rd and 4th IE.
2. So 3 electrons are in the outer shell.

Answer: Group 13 (aluminium).

Check yourself

1. Which equation represents the first ionisation energy of calcium?
   • Ca(s) -> Ca+(g) + e-
   • Ca(g) -> Ca2+(g) + 2e-
   • Ca(g) -> Ca+(g) + e-
   • Ca(g) + e- -> Ca-(g)
   Show answer

   Ca(g) -> Ca+(g) + e-. One mole of gaseous atoms each lose one electron.

2. Successive IEs (kJ/mol) are 738, 1451, 7733, 10 543. Which group is the element in?
   • Group 2
   • Group 1
   • Group 3
   • Group 4
   Show answer

   Group 2. The jump is between the 2nd and 3rd IE, so 2 outer electrons.

3. Why is the first IE of oxygen lower than that of nitrogen?
   • Oxygen has fewer protons than nitrogen
   • Oxygen's outer electron is in the 3s sub-shell
   • In oxygen two electrons are paired in one 2p orbital and repel each other
   • Oxygen has more shielding from inner shells
   Show answer

   In oxygen two electrons are paired in one 2p orbital and repel each other. N has three unpaired 2p electrons; O's fourth 2p electron is paired, and the repulsion makes it easier to remove.

4. Why is the first IE of boron lower than that of beryllium?
   • Boron has a smaller nuclear charge
   • Boron's outer electron is in a 2p orbital, which is higher in energy than 2s
   • Boron's outer electron is paired
   • Boron has an extra full shell
   Show answer

   Boron's outer electron is in a 2p orbital, which is higher in energy than 2s. The 2p electron is higher in energy and shielded a little by the 2s electrons, so less energy is needed.

5. Why does first IE decrease down Group 1?
   • The nuclear charge decreases
   • The atoms have fewer electrons
   • The outer electron is in a p orbital
   • The outer electron is further from the nucleus and more shielded
   Show answer

   The outer electron is further from the nucleus and more shielded. The extra protons are outweighed by greater distance and shielding.

6. Why are all ionisation energies endothermic?
   • Energy is released when bonds form
   • Energy is needed to overcome the attraction between the nucleus and the electron
   • Gaseous ions are lower in energy than atoms
   • The electron gains energy from the nucleus
   Show answer

   Energy is needed to overcome the attraction between the nucleus and the electron. Removing a negative electron from a positive nucleus takes energy.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.