First Ionisation Energy

Question 1

What is ionisation energy?

Answer 1

• The first ionisation energy is the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms..
• You have to put in energy to ionise so it is endothermic.
• O(g) -> O(g)(+) + e-
• gas state symbol
• refer to 1 mole of atoms than single atom
• lower the ionisation energy, easier it is to form an ion

Question 2

Factors affecting ionisation energy

Answer 2

• The first electron lost will be in highest energy level and will experience least attraction from nucleus.

1) Nuclear charge: more protons in nucleus=greater the attraction between nucleus and outer electrons.
2) Atomic radius: greater the distance=less the nuclear attraction. Force of attraction falls of sharply with distance.
3) Shielding: electrons are negative so inner-shell electrons repel outer-shell electrons. This repulsion is called shielding effect and it reduces the attraction between the nucleus and outer electrons.

Question 3

Trend in first ionisation energy down a group

Answer 3

• 1st ionisation energy decreases down a group.
• Elements have more shells down the group so atomic radius is larger which reduces attraction.
• Extra inner shells also shield outer electrons from attraction of nucleus.
• These factors make it easier to remove outer electrons which gives lower ionisation energy.
• Although pos. charge of nucleus increases down group, this is overridden by effect of more shells.

Question 4

Trend in ionisation energy across a period

Answer 4

• General increases in first ionisation energy across the first three periods.
• This is due to increases in nuclear charge and similar shielding as they have the same no. of shells. This means nuclear attraction increases and atomic radius decreases.

Question 5

Decreases in ionisation energy across groups

Answer 5

• Between group 2 and group 3 there is a small decreases due to outer electrons in g3 being in p-orbital than s-orbital. E.g Boron and Beryllium. Boron’s 2p is easier to remove that 2s in beryllium.
• In group 5 and 6 there is drop due to electron repulsions. E.g Nitrogen and Oxygen. Both have same highest energy electrons (2p). But Oxygen has paired electrons in one of 2p orbitals which repel one another, making it easier to remove.