TOPIC 3: Ionisation Energy

Atomic Structure and Periodic Table

Define ‘ionisation’: 

Removing electrons from an atom makes a positive ion 

What kind of process is ionisation?

Endothermic

It requires energy to overcome the strong attractive force between the oppositely charged nucleus & electrons

Define ‘first ionisation energy’:

Th energy required to remove one mole of electrons from one mole gaseous atoms to form one mole of 1+ ions in the gaseous state

Define ‘second ionisation energy’:

The energy required to remove one mole of electrons from one mole of gaseous 1+ ions to form one mole of 2+ ions in the gaseous state

What are the two competing forces in an atom?

1) Electrostatic attraction between the positive nucleus & negatively charged electrons.

2) Repulsion between the negatively charged electrons.

What factors affect how much energy is needed to remove an electron?

N- nuclear charge → more protons= more attraction

A- atomic radius → closer outer electron = outer electron

S- shielding → fewer inner shells = less shielding = more attraction

A- attraction

What happens to the number of protons across a period?

Increase

What happens to the attraction felt by the outer shell electron? (across period)

Increase

What happens to the amount of energy required to remove the outer shell electron? (across period)

Increase

Describe the trend in atomic radius across a period?

Decreases

Explain the trend of atomic radius as you go across the period

• Number of protons across period increase

• Attraction felt b outer shell increases

→ This attraction pulls the outer shell electron inwards - decreasing atomic radius - increasing ionisation energy

Describe the trend in atomic radius as you go down the group

Increases

Explain the trend in atomic radius as you go down the group

• Number of shells increases down the group

• Attraction felt by the outer shell electron decreases

→ Ionisation energy decreases

What is shielding?

When the repulsion due to inner electrons shield the nucleus from the outer electrons

It increases with increasing number of occupied shells