In order to be able to explain the trends of the first ionisation energies across a period of the periodic table we first need to know about the first ionisation energy itself.

The first ionisation energy of an element is the energy needed to remove one electron from each atom in one mole of atoms of the element in the gaseous state to form one mole of gaseous 1+ ions.

The first ionisation energy is influenced by the following factors:

  • The size of the nuclear charge – ionisation energy increases as the proton number increases.
  • Distance of outer electrons from the nucleus – the further the outer electron shell is from the nucleus, the lower the ionisation energy.
  • Shielding effect of inner electrons – the ionisation energy decreases as the number of full electron shells between the outer electrons and the nucleus increases.

Explaining patterns in the first ionisation energies across Period 2

Period 2 is from lithium to neon and the nuclear charge increases as the number of protons in the nucleus increases across the period. Since all the Period 2 elements have 2 shell each, the electron to be removed comes from the same shell, which is the second shell.

The force of attraction between the positive nucleus and the
outer negative electrons increases across the period as

  • the nuclear charge increases
  • but the distance between the nucleus and the outer electron,
  • and the shielding by inner shells remains almost constant.

Across Period 2, the first ionisation energy increases as the nuclear charge increases.

However, from beryllium (1s2 2s2) to boron (1s2 2s2 2p1), there is a slight decrease. This is because, even though boron has a greater nuclear charge, the outer electron of boron is in the 2p subshell, which is slightly further away from the nucleus than the 2s subshell from which the beryllium electron is removed. There is less attraction between the 2p electron and the nucleus than between the 2s electron and the nucleus, hence the decrease.

  • There is another slight decrease between nitrogen (1s2 2s2 2p3) and oxygen (1s2 2s2 2p4). Oxygen has a higher nuclear charge than nitrogen and the electron to be removed is in the same 2p subshell. An increase in first ionisation energy is expected, but however,
  • the electron removed from the nitrogen is from an orbital that contains an unpaired electron.
  • the electron removed from the oxygen is from the orbital that contains a pair of electrons.

The electron pair repulsion in the electron to be removed in oxygen results in less energy being needed to remove the electron, which makes the first ionisation energy of oxygen is lower than expected.

Trends of first ionisation energies across a Period

Explaining patterns in the first ionisation energies across Period 3

Period 3 is from sodium to argon and the nuclear charge increases as the number of protons in the nucleus increases across the period. All the Period 3 elements have 3 shells each and the electron to be removed comes from the same shell, which is the third shell.

The force of attraction between the positive nucleus and the
outer negative electrons increases across the period as

  • the nuclear charge increases
  • but the distance between the nucleus and the outer electron,
  • and the shielding by inner shells remains almost constant.

The first ionisation energy increases as the nuclear charge increases.

However, from magnesium (1s2 2s2 2p6 3s2) to aluminium (1s2 2s2 2p6 3s2 3p1), there is a slight decrease. This is due to the fact that even though aluminium has a greater nuclear charge, the outer electron of aluminium is in the 3p subshell, which is slightly further away from the nucleus than the 3s subshell from which the magnesium electron is removed. There is less attraction between the 3p electron and the nucleus than between the 3s electron and the nucleus, hence the decrease.

There is another slight decrease in first ionisation energy between phosphorus (1s2 2s2 2p6 3s2 3p3) and sulphur (1s2 2s2 2p6 3s2 3p4). Sulphur has a higher nuclear charge than phosphorus and the electron to be removed is in the same 3p subshell. An increase in first ionisation energy is expected, but however,

  • the electron removed from the phosphorus is from an orbital that contains an unpaired electron.
  • the electron removed from the sulphur is from the orbital that contains a pair of electrons.

The electron pair repulsion in the electron to be removed in sulphur results in less energy being needed to remove the electron, which makes the first ionisation energy of sulphur lower than expected.


Sydney Chako

Mathematics, Chemistry and Physics teacher at Sytech Learning Academy. From Junior Secondary School to Tertiary Level Engineering Mathematics and Engineering Science.

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