Ionisation Energies

Page 1

Ionisation energies provide the best evidence for the existence of electron shells.

Ionisation energy is the energy required to remove an electron from an element to form a positive ion.

First Ionisation Energy (∆Hᵢₑ): first ionisation energy of an element is the energy needed to remove one electron from one mole of gaseous atoms to form one mole of gaseous unipositive ions.

All ionisations require energy as they are endothermic processes and have positive enthalpy change values.

Three factors influence ionisation energies:

Size of positive nuclear charge: force of attraction between negatively charged electrons and positively charged protons in nucleus. Increase in nuclear charge with atomic number leads to increase in ionisation energies.

Distance of electron from nucleus: as principal energy levels of electron shells increases, distance between nucleus and outermost electrons increases so forces of attraction between the nucleus and outermost electrons decreases. Decrease in ionisation energies.

Shielding effect by electrons in filled inner shells: filled inner shells will shield positive nuclear charge. Greater this shielding effect on electron, easier it is to remove so lower its ionisation energy. All elements in same period experience the same shielding effect.

Trends across a period - first ionisation energy increases across a period. This is because:

Size of positive nuclear charge increases across period. More protons in nucleus.

No additional shielding.

Stronger attraction of the ion to lost electron.

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First ionisation energy equations of lithium and magnesium:

Li (g) --> Li⁺ (g) + e⁻

Mg (g) --> Mg ⁺ (g) + e⁻

Magnesium has a higher ionisation energy than sodium because:

Electron being removed experiences a greater positive nuclear charge.

No additional shielding.

Greater attraction of ion to lost electron.

First ionisation energy of aluminium is less than magnesium despite increase in nuclear charge because:

Electron removed from aluminium is removed from 3p orbital.

3p orbital higher in energy than 3s.

Weaker attraction between positive ion and lost electron.

First ionisation energy of sulphur is less than phosphorus despite sulphur having a larger nuclear charge because:

Pair of electrons in 3p orbital.

Causes repulsion so less energy required to remove electron.

Trends down a group - ionisation energies decrease down a group.

Electron being removed from successively higher principal energy level.

Further from nucleus.

Increase in shielding as more filled inner shells.

Weaker attraction of the ion to the lost electron.

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Once an electron has been removed from an atom, remaining electrons pulled closer to nucleus and more tightly held.

As a result, successive ionisation energies show increase in value.

Second ionisation energy is the energy required to remove a second electron from each ion in a mole of gaseous ions.

e.g. Ca⁺ (g) --> Ca²⁺ (g) + e⁻

Second ionisation energy value always greater than first ionisation energy.