Ionisation Energies

Question 1

Define Ionisation:

Answer 1

Removing one or more electrons from an atom.

Question 2

Define First Ionisation Energy:

Answer 2

Energy required to remove 1 mole of electrons from 1 mole of gaseous atoms.

Question 3

Is Ionisation Endothermic or Exothermic?

Answer 3

It is endothermic (energy taken in).

Question 4

Show an equation for the first ionisation energy of oxygen:

Answer 4

O → O⁺ + e⁻

1st IE = +1314 KJmol⁻¹

Question 5

What are the three rules to remember for ionisation energies?

Answer 5

1) Gas state symbols must be used (IE is measured for gaseous atoms).
2) Always refer to one mole of atoms.
3) The lower the IE, the easier an ion forms.

Question 6

What three factors affect Ionisation Energy?

Answer 6

1) Nuclear charge.
2) Atomic Radius.
3) Shielding.

Question 7

Explain Nuclear Charge:

Answer 7

The more protons there are in the nucleus, the more positively charged it is and the stronger the attraction for the electrons.

Question 8

Explain Atomic Radius:

Answer 8

Attraction falls off very rapidly with distance. As such an electron close to the nucleus will be much more strongly attracted than one further away.

Question 9

Explain Shielding:

Answer 9

As the number of electrons increases between the nucleus and outer electron, the outer electron feels less attraction to the nucleus.

Question 10

What does a high ionisation energy mean?

Answer 10

A high ionisation energy means that there is a strong attraction between the electrons and nucleus.

As such more energy is needed to overcome the attraction and remove the outer electron.

Question 11

What happens to Ionisation energy going down a group and why?

Answer 11

It decreases down a group.

Elements further down a group have extra electron shells. This increases atomic radius so outer electrons are further from the nucleus, reducing N ↔︎ E attraction.

Extra inner shells also shield outer electrons from nucleus (extra shells override effect of more protons).

Question 12

A decrease in ionisation energy down a group proves ______

Answer 12

shells.

Question 13

What happens to the ionisation energy across a Period?

Answer 13

Ionisation energy increases.

Across a period, number of protons increases. This causes an increase in charge which pulls the electrons closer to the nucleus and causes atomic radius to decrease.

The extra electrons gained are added to the outer energy level so don’t provide any shielding effects.

Question 14

What are the two exceptions to the overall trend of ionisation energies moving across a period?

Answer 14

Between groups 2 & 3 and 5 & 6, ionisation energy decreases.

Question 15

Explain the drop between Groups 2 and 3.

Answer 15

Occurs due to sub shell structure:

• The outer electrons in Group 3 are in a p-orbital rather than an s-orbital.
• P-Orbitals have a slightly higher energy than s-orbitals so are further from nucleus.
• S electrons also provide additional shielding.

All these factors override the increase in nuclear charge, resulting in a drop in Ionisation Energy.

Question 16

Explain the drop between Groups 5 and 6.

Answer 16

Occurs due to p-orbital repulsion

Group 5 elements have their electron removed from a singly occupied orbital. Group 6 elements have their electron removed from an orbital with two electrons.

The repulsion between two electrons in an orbital means these electrons in shared orbitals (Group 6) are easier to remove.

Question 17

Explain Successive Ionisation Energies:

Answer 17

All the electrons from an atom can be removed, leaving only the nucleus. Each time an electron is removed, it counts as an SIE:

O⁺ → O²⁺ + e⁻

Question 18

Why do Successive Ionisation Energies gradually increase?

Answer 18

Within a shell, SIE increases since electrons are removed from an increasingly positive ion, and the repulsion between remaining electrons is weaker.

This results in a stronger N ↔︎ E attraction.

Question 19

Why do jumps in Ionisation Energy occur?

Answer 19

The jumps occur as a new shell is being broken into that is closer to the nucleus.

Question 20

How do you find a Group Number from an Ionisation Energy Graph?

Answer 20

Count how many electrons are removed before the first big jump.

Question 21

How do you find a Electronic Structure from an Ionisation Energy Graph?

Answer 21

From right to left, count how many points there are before each jump to find the amount of electrons in each shell.