Atomic Structure: Electron Configuration - Ionisation Energies

Question 1

First Ionisation Energy

Answer 1

• The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of +1 ions
• X(g) → X+(g) + e‒

Question 2

Second Ionisation Energy

Answer 2

• The second ionisation energy involves the removal of a second electron
• X+(g) → X2+(g) + e‒

Question 3

Factors Affecting Ionisation Energies

Answer 3

• A high ionisation energy means there’s a high attraction between the electron and the nucleus, so more energy is needed to remove the electron

• Nuclear charge
- The more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons

• Distance from the nucleus

• Attraction falls off very rapidly with distance
• An electron close to the nucleus will be much more strongly attracted than one further away

• Shielding

• As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction to the nucleus
• This lessening of the pull thanks to the inner electrons is called shielding

Question 4

Explain drastic increases in ionisation energies

Answer 4

• Large jumps in the ionisation energy reveal where electrons are being removed from a lower energy principal energy level
• Also less shielding makes the electron harder to remove
• Successive ionisation energies give us evidence for principal energy levels

Question 5

Ionisation Energy Across a Period

Answer 5

• Across a period, there is a general increase in the ionisation energies

• The number of protons increase, so there is a stronger nuclear attraction
• The shielding effect is similar as the outer electrons are at roughly the same energy level

Question 6

Ionisation Energy Down a Group

Answer 6

• The first ionisation energy always decreases down a group

• The nuclear charge increases down the group
• But the electrons are being removed from successively higher principal energy levels, which are on average:
• Further from the nucleus
• More shielded by inner electron shells
• And are therefore less attracted to the nucleus