Ionisation energies

Question 1

Define the second ionisation energy of Mg.

Answer 1

The energy required to remove 1 mole of electrons from 1 mole of gaseous Mg 1+ ions turning them into 1 mole of gaseous Mg 2+ ions.

Question 2

The first ionisation energy of sodium is 494kJ/mol, while the first ionisation energy of rubidium is 402 kJ/mol. Explain why rubidium has a lower first ionisation energy than sodium. Both sodium and rubidium are in group 1.

Answer 2

• outer e- is in a higher energy shell
• so outer e- is further away from the nucleus, experiences more shielding, and experiences less attraction
• so less energy required to remove

Question 3

Predict whether an atom of 88Sr will have an atomic radius that is larger than, smaller than or the same as the atomic radius of 87Rb. Explain your answer. (Same period, Sr is in group 2 but Rb in group 1.)

Answer 3

88Sr will have a smaller atomic radius, because:
- 88Sr has an additional proton so a greater nuclear charge
- but the outer e- has the same shielding
- so the outer e- is pulled in more tightly in 88Sr than in 87Rb

Question 4

A Neon atom and a Mg 2+ ion have the same number of electrons. Why is the first ionisation energy of Mg2+ greater than that of neon?

Answer 4

• Mg2+ has greater nuclear charge
• outer e- is in same energy level and experiences same shielding
• so greater attraction of e- to nucleus
• so outer e- harder to remove

Question 5

This question is about periodicity.
State and explain the general trend in the first
ionisation energies of the period 3 elements Na - Ar.
Explain this trend.

Answer 5

• as you go from left to right, FIE increases
• increasing number of protons, so increasing nuclear charge
• outer e- in same energy level, same shielding
• greater force of attraction for e-, to the nucleus
• harder to remove

Question 6

State and explain the trend in the melting points of the period 3 metals Na, Mg and Al.

Answer 6

• the melting points increases as you go from left to right, since
• increasing number of protons means increasing charge of positive metal ions/increased number of delocalised e-s
• so increased electrostatic force of attraction between metal ions and delocalised electrons/ more forces to overcome
• metallic bonding is stronger

Question 7

Explain why the atomic radius decreases across period 3, from sodium to chlorine.

Answer 7

• As you go from left to right:
• nuclear charge increases
• outer e- in same energy level, experiences same shielding
• so outer e- experiences a greater force of attraction to the nucleus and is pulled in closer

Question 8

Define periodicity.

Answer 8

a repeating pattern of physical or chemical properties

Question 9

Identify the element in period 4 with the largest atomic radius. Explain your answer.

Answer 9

K, potassium
- smallest nuclear charge in period 4
- outer e- experiences same shielding as other elements, in same energy level
- so least force of attraction between outer e- and nucleus

Question 10

The elements in Period 2 show periodic trends.

Identify the period 2 element, from carbon to flourine, that has the largest atomic radius.

Answer 10

• Carbon
• smallest nuclear charge
• same shielding as other elements, outer e- in same energy level
• so least force of attraction between outer e- and nucleus

Question 11

There is a general trend for an increase in ionisation energy across period 3.

Give one example of an element that deviates from this trend.

Explain why.

Answer 11

Example 1:
- Aluminium, since
- the outer e- in aluminium is in a higher energy sublevel (3p vs 3s)
- so, slightly more shielding, further away from nucleus
- so weaker force of attraction between outer e- and nucleus
- outer e- easier to remove

Example 2:
- Sulfur, since
- the e-s in the 3p sublevel begin to pair
- repulsion decreases energy required to remove

Question 12

Explain why the melting point of sulfur (S8) is greater than that for phosphorous (P4).

Answer 12

• S8 has a greater Mr, and is a larger molecule with a larger SA
• so van der waals forces are stronger between molecules

Question 13

This question is about melting points.

Identify the period 3 element that has the highest melting point. Explain why.

Answer 13

• Silicon.
• since there are many strong covalent bonds
• which require lots of energy to overcome.

Question 14

This question is about period 2.

From lithium to Neon, the melting points follow a pattern.
Nitrogen’s melting point is lower than Li, Be, B and C.
Explain, in terms of structure and bonding, why the melting point of nitrogen is so low.

Answer 14

• nitrogen has a simple molecular structure
• so there are only weak IMF between molecules
• that require little energy to overcome