Ionisation Energies

Ionisation energy is a quantitative measure of how strongly an atom holds onto its electrons. Studying how these energies change across elements provides solid evidence for the existence of principal shells and sub-shells.

First Ionisation Energy

All ionisation processes are endothermic (\( \Delta H \) is positive) because energy must be supplied to overcome the electrostatic attraction between the negatively charged electron and the positively charged nucleus.

Factors Affecting Ionisation Energy

The magnitude of the attraction between the nucleus and the outermost electron is determined by three main factors:

1. Nuclear Charge: The number of protons in the nucleus. More protons mean a larger positive charge, which exerts a stronger pull on the outer electrons, increasing ionisation energy.
2. Electron Shielding: Inner shells of electrons repel the outer shell electrons. This shielding reduces the net electrostatic attraction felt by outer electrons, decreasing ionisation energy.
3. Atomic Radius (Distance): The distance of the outer electron from the positive nucleus. Electrostatic attraction drops off rapidly with distance. A larger radius means weaker attraction, decreasing ionisation energy.

Trends in First Ionisation Energy

1. Trend Down a Group

First ionisation energy decreases down any group. Although the nuclear charge increases (more protons), this is completely outweighed by two factors:

• The outer electron is in a **new shell**, so it is **further away** from the nucleus.
• There is **more shielding** from inner electron shells.

As a result, the electrostatic attraction is weaker, and the electron is easier to remove.

2. Trend Across a Period

First ionisation energy generally increases across a period. This is because:

• The nuclear charge increases (more protons).
• The shielding remains relatively constant (electrons are added to the same principal shell).
• Therefore, the atomic radius decreases as the outer shell is pulled closer, increasing the attraction on the outer electron.

Exceptions to the Period Trend (Crucial for Exams)

While the overall trend across Period 3 is an increase, there are two distinct dips that provide key evidence for sub-shell structure:

Successive Ionisation Energies

Successive ionisation energies are the energies required to remove subsequent electrons from the same atom one after the other:

\[ \text{1st: } \text{X(g)} \rightarrow \text{X}^+\text{(g)} + \text{e}^- \]

\[ \text{2nd: } \text{X}^+\text{(g)} \rightarrow \text{X}^{2+}\text{(g)} + \text{e}^- \]

\[ \text{3rd: } \text{X}^{2+}\text{(g)} \rightarrow \text{X}^{3+}\text{(g)} + \text{e}^- \]

Successive ionisation energies always increase because as electrons are removed, the remaining electrons experience a stronger net pull from the positive nucleus (reduced shielding and electron repulsion).

Identifying Shell Structure from Jumps

When plotting successive ionisation energies on a logarithmic scale, we observe **sharp jumps** in energy. A large jump indicates that an electron is being removed from a shell that is **closer to the nucleus** (a new principal energy level) with significantly less shielding.