Period 3 and Group 2 Ionisation Energy Trends and Electronic Structure

Q1. State and explain the general trend in first ionisation energy across Period 3.

Increases across the period.

Number of protons increases (greater nuclear charge).

Electrons added to the same shell → shielding remains similar.

Atomic radius decreases.

Stronger attraction between outer electron and nucleus, so more energy needed to remove.

Q2. Explain why the first ionisation energy decreases between Mg and Al.

Outer electron in Al is in a 3p orbital, whereas Mg's is in a 3s orbital.

3p orbital has higher energy and is slightly further from the nucleus.

Additional shielding from the 3s electrons.

These factors outweigh increased nuclear charge, so less energy required to remove Al's outer electron.

Q3. Explain why the first ionisation energy decreases between P and S.

In S, an electron is removed from a doubly-occupied 3p orbital, whereas in P all 3p orbitals contain only one electron.

Paired electrons experience electron-electron repulsion.

This repulsion makes it easier to remove an electron from sulfur.

Q4. State and explain the trend in first ionisation energy down Group 2.

First ionisation energy decreases.

Atomic radius increases (outer electrons further from nucleus).

More inner electron shells → increased shielding.

Nuclear attraction to outer electron decreases, so less energy required to remove.

Q5. Explain why successive ionisation energies increase for an element.

Electrons are being removed from an increasingly positive ion.

Fewer electrons → less repulsion between them.

Remaining electrons are held more strongly by the nucleus.

Q6. Why is there a large jump in ionisation energy at certain points?

A new electron shell is broken into.

Electrons in an inner shell are closer to the nucleus and experience less shielding.

Much more energy is required to remove them.

. Sodium has successive ionisation energies showing a large jump after the first electron is removed. What does this tell you about its electronic structure and its group in the periodic table?

Sodium has 1 electron in its outer shell (the big jump occurs after 1 electron is removed).

Therefore, it is in Group 1.

How can successive ionisation energies provide evidence for the existence of electron shells?

Large jumps in ionisation energy occur when electrons are removed from a shell closer to the nucleus.

The pattern of increases shows that electrons are arranged in discrete energy levels (shells).

Phosphorus has a higher first ionisation energy than sulfur. Explain why, using electronic configurations.

P: 1s² 2s² 2p⁶ 3s² 3p³

S: 1s² 2s² 2p⁶ 3s² 3p⁴

In P, outer electrons are singly occupied in 3p orbitals.

In S, one 3p orbital contains a pair → electron-electron repulsion.

Easier to remove paired electron in S → lower ionisation energy.

Q10. Aluminium has a lower first ionisation energy than magnesium. Explain why, in terms of sub-shells.

Mg: electron removed from 3s orbital.

Al: electron removed from 3p orbital.

3p orbital is higher in energy and slightly further from the nucleus.

Additional shielding from 3s electrons.

Outweighs increased nuclear charge → lower ionisation energy in Al.