7.2 - Ionisation energies

Question 1

What is ionisation energy?

Answer 1

The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 2

Write down the first ionisation energy for sodium

Answer 2

Na(g) -> Na⁺(g) + e⁻

Question 3

What are the three factors affecting ionisation energy?

Answer 3

1. Atomic radius, bigger radius = lesser attraction
2. Nuclear charge, more protons = greater attraction
3. Electron shielding, inner-shell electrons repel outer-shell electrons which is called shielding and reduces the attraction

Question 4

Describe the trend in ionisation energies across a period

Answer 4

• Increases overall has some exceptions, B and O, and Al and S
• B and Al: outer electron in 2p sub-shell has a higher energy than 2-s subshell so requires less energy to remove the outer electron
• O and S: electron repulsion theory, one of the 2p orbitals contains a pair of electrons which repel each other so requires less energy to remove the outer electron

Question 5

Describe the trend in ionisation energies down a group

Answer 5

• Decreases due to atomic radius and electron shells
• The nuclear charge also increases down a group but its affect is offset by the other factors

Question 6

Write two successive ionisation energies for helium

Answer 6

He(g) -> He⁺(g) + e⁻
He⁺(g) -> He²⁺(g) + e⁻

Question 7

Why does fluorine have a large difference in ionisation energy from the 7th to the 8th successive number?

Answer 7

Removal of electron from inner shell (1) which experiences a greater attraction towards the nucleus