Periodic Trends: Atomic Radius and Ionisation Energy

Learning Objectives (OCR Spec 3.1.1)

By the end of this lesson you should be able to:

• Describe and explain the trend in atomic radius across Periods 2 and 3 and down a group
• Define first ionisation energy and write a general equation for it
• Describe and explain the trend in first ionisation energy across Periods 2 and 3 and down Group 2 and Group 7
• Explain anomalies in the trend (e.g. B/Be, O/N, Al/Mg, S/P) in terms of sub-shell structure and electron pairing repulsion
• Apply the four factors: nuclear charge, atomic radius (distance), shielding, sub-shell structure

Atomic Radius

Atomic radius is a measure of the size of an atom - conventionally the distance from the nucleus to the outermost electron. Because electrons occupy diffuse clouds, it is usually defined as half the distance between two bonded nuclei of the same element.

Trend Across a Period

graph LR
  A[Move left to right across a period] --> B[Nuclear charge increases]
  A --> C[Electrons added to same shell]
  B --> D[Stronger attraction from nucleus on outer electrons]
  C --> E[Shielding roughly constant]
  D --> F[Atomic radius DECREASES]
  E --> F

Period 3 element	Na	Mg	Al	Si	P	S	Cl	Ar
Atomic radius / pm	186	160	143	117	110	104	99	(71, van der Waals)

Explanation: As you move across Period 3, the number of protons in the nucleus increases (from 11 for Na to 17 for Cl), so the nuclear charge rises. Each successive electron is added to the same outer (3rd) shell, so the shielding provided by inner shells stays roughly constant. The outer electrons therefore experience a stronger effective nuclear attraction and are drawn closer to the nucleus.

Trend Down a Group

Moving down a group:

• More shells are occupied → outer electrons are further from the nucleus (atomic radius increases)
• More inner electrons → more shielding of the outer electron from the nuclear charge
• Although nuclear charge increases too, the effects of distance and shielding dominate

Hence atomic radius increases down a group.

First Ionisation Energy

The first ionisation energy (IE1) is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions:

X(g) → X+(g) + e- ΔH = IE1

Key points to include in every OCR definition:

1. Energy required
2. Per mole
3. Gaseous atoms → gaseous 1+ ions

The units are kJ mol-1 and the value is always endothermic (positive) because energy must be supplied to overcome the electrostatic attraction between the electron and the nucleus.

Four Factors Affecting Ionisation Energy

OCR expects you to quote four factors when explaining IE trends:

Factor	Effect on IE
Nuclear charge	Higher nuclear charge → stronger attraction on outer e- → higher IE
Atomic radius (distance)	Greater distance → weaker attraction (1/r2 dependence) → lower IE
Shielding	More inner electrons shield the outer e- from the full nuclear charge → lower IE
Sub-shell structure (spin pairing)	Paired electrons in the same orbital repel each other → electron easier to remove → lower IE

Trend Across Period 3

graph TD
  A[IE1 across Period 3] --> B[General rise Na to Ar]
  A --> C[Dip at Al]
  A --> D[Dip at S]
  B --> B1[Nuclear charge increases, radius decreases]
  C --> C1[Al: outer e- in 3p, higher energy/further than 3s of Mg]
  D --> D1[S: first paired electron in 3p, repulsion aids removal]