Trends in First Ionisation Energy

Question 1

What happens to ionisation energies as you go down a group?

Answer 1

Ionisation energies decrease down a group

Question 2

What happens to ionisation energies as you go across a period?

Answer 2

Ionisation energies increase across a period

Question 3

Why do ionisation energies decrease as you go down Group 2?

Answer 3

Each element down Group 2 has an extra electron shell compared to the one above, the extra inner shells will shield the outer electrons from the attraction of the nucleus. The extra shells also mean that the outer electrons are further away from the nucleus, so the nucleus’ attraction will be reduced. Both of these factors make it easier to remove an outer electron, therefore a lower ionisation energy

Question 4

Why do ionisation energies increase across Period 3?

Answer 4

As you move across a period, the number of protons is increasing, which means a stronger nuclear attraction. All off the extra electrons are at roughly the same energy level even if the outer electrons are in different orbital types. This means there’s little shielding or extra distance to lessen the attraction from the nucleus. These factors mean it is harder to remove the outer electrons so ionisation energies increase

Question 5

What does the drop in ionisation energy between Group 2 and 3 mean?

Answer 5

Group 3’s outer electron is in a 3p orbital rather than a 3s. The 3p orbital has a slightly higher energy than the 3s orbital, so the electron is further from the nucleus. The 3p orbital has additional shielding from the 3s electrons. Both of these factors are strong enough to override the effect of increased nuclear charge, resulting in ionisation energy dropping slightly. This is evidence of sub-shell structure

Question 6

Explain the drop in ionisation energy between groups 5 and 6

Answer 6

The drop provides evidence for electron repulsion. The shielding is identical in both atoms and the electron being removed is from an identical orbital. In Group 5, the electron is being removed from a singly-occupied orbital. In Group 6, the electron is being removed from an orbital containing two electrons. The repulsion between 2 electrons in an orbital means it’s easier to remove electrons from shared orbitals