Trends of Period 3 Elements: First Ionisation Energy

There is a general increase in first ionisation energy as you move from Sodium ($Na$) to Argon ($Ar$).

This increase is driven by the increasing nuclear charge; as the number of protons increases, the nucleus exerts a stronger pull on the outer electrons.

Simultaneously, the atomic radius decreases because the stronger nuclear pull draws the electron shells closer to the center.

The shielding effect remains relatively constant because all elements in Period 3 have their outer electrons in the same third principal energy level ($n=3$).

A small decrease (dip) in $IE_1$ occurs between Magnesium ($[Ne] 3s^2$) and Aluminium ($[Ne] 3s^2 3p^1$).

The outer electron in Aluminium is in a $3p$ orbital, which is at a slightly higher energy level than the $3s$ orbital used by Magnesium.

This $3p$ electron is also slightly further from the nucleus and experiences additional shielding from the $3s$ subshell electrons.

These factors combined make the $3p$ electron in Aluminium easier to remove than the $3s$ electron in Magnesium, despite Aluminium's higher nuclear charge.

Another dip in $IE_1$ is observed between Phosphorus ($[Ne] 3s^2 3p^3$) and Sulphur ($[Ne] 3s^2 3p^4$).

In Phosphorus, each of the three $3p$ orbitals contains a single electron (Hund's Rule), minimizing repulsion.

In Sulphur, the fourth $3p$ electron must pair up in an orbital that already contains one electron, leading to electron-pair repulsion.

This repulsion between the two electrons in the same orbital makes it easier to remove one of them, resulting in a lower ionisation energy for Sulphur compared to Phosphorus.

State Symbols: Always include $(g)$ for both the atom and the ion in equations; examiners often penalize the omission of state symbols.

Three-Point Explanation: When explaining the general trend, always mention: 1. Nuclear charge increases, 2. Atomic radius decreases, 3. Shielding remains constant.

Anomaly Identification: If asked about Magnesium vs. Aluminium, focus on the orbital energy level ($3s$ vs $3p$). If asked about Phosphorus vs. Sulphur, focus on electron-pair repulsion in the $3p$ orbital.

Common Mistake: Do not say shielding increases significantly across a period; it only increases significantly when moving down a group.