8.3 - Ionisation Energies

Question 1

What is ionisation?

Answer 1

• the process of removing one or more electrons from an atom or molecule
• endothermic process

Question 2

What is the first ionisation energy?

Answer 2

the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

Question 3

Factors affecting ionisation energy? (3)

Answer 3

1. Nuclear charge- Atoms with more protons in their nucleus have a stronger positive charge. This creates stronger electrostatic attraction between the nucleus and the outer electrons.
2. Atomic radius- Electrostatic attraction drop off steeply with increasing distance. Electrons in smaller atoms are held closer to the nucleus so the attraction is greater.
3. Electron shielding - Inner electron shells shield the outermost electrons from the full attractive force of the nucleus, reducing the effective nuclear charge experienced by the outer electrons. More electron shells provide more shielding.

Question 4

Going down groups what are the first ionisation energy trends:

Answer 4

• Nuclear charge- Increases down the group as more protons are added, increasing attraction for electrons.
• Atomic radius- Increases down the group as more electron shells are added, moving electrons away from nucleus.
• Electron shielding - Increases down group as more inner electron shells reduce nuclear attraction.

The atomic radius and shielding effects down groups are greater than the nuclear charge effect, leading to an overall decrease in ionisation energies as you move down a group

Question 5

Going across periods what are the first ionisation energy trends:

Answer 5

• Nuclear charge- Increases as more protons added across periods.
• Atomic radius- Decreases across periods as extra electrons added to same shell.
• Electron shielding - Stays similar across periods with no extra inner shells.

Question 6

second ionisation energy of magnesium equation

Answer 6

Mg+(g)→ Mg2+(g)+ e− ΔHIE2= +1,451 kJ mol−1

Question 7

How do the successive ionisation energies increase within the same shell?

Answer 7

• As successive electrons are removed from the same shell, the remaining electrons experience greater electrostatic attraction to the increasingly positive nucleus. This increased nuclear attraction requires more energy to remove the next electron from that shell.
• For magnesium, the second ionisation energy (1,450 kJ mol-1) is slightly higher than the first (740 kJ mol-1) as these electrons are both being removed from the 3s subshell.

Question 8

Large jumps in successive ionisation energies between the shell

Answer 8

When reaching a new inner electron shell, there is a big increase in the ionisation energy needed to remove the first electron in that new shell. This happens because the attraction to the nucleus is much greater for inner shell electrons closer to the nucleus.
For magnesium, there is a large jump from the second ionisation energy (1,450 kJ mol-1) to the third (7,730 kJ mol-1) as the 3s subshell is now full, requiring the next electron to be removed from the inner 2p subshell.