Atomic Radius Explained: H2 Chemistry Guide (9476)

Quick answer: Atomic radius decreases across a period (rising nuclear charge, shielding barely changes) and increases down a group (extra occupied shells outweigh the higher nuclear charge). Sodium’s atomic radius is 0.157 nm versus chlorine’s 0.099 nm across Period 3; magnesium’s is 0.160 nm versus barium’s 0.217 nm down Group 2. Ionic radius follows the same logic but with a twist: cations are smaller than their parent atom, anions are larger — and comparing ions with the same electron count (an isoelectronic series) is the cleanest way to isolate nuclear charge as the only variable.

Atomic radius looks like one of the simpler periodic trends in H2 Chemistry, but exam questions rarely stop at “smaller” or “larger” — they expect a reasoned comparison using nuclear charge, electron shells, and shielding, sometimes with real Data Booklet values to interpret. This guide covers the trend with actual figures, a worked isoelectronic radius comparison, and the specific exceptions students tend to lose marks on. For the underlying nuclear-charge-and-shielding mechanism itself, see our guide to shielding — this page builds directly on it. This kind of data-led, exam-focused explanation is what H2 Chemistry tuition at Pamela’s Place is built around, in line with the H2 Chemistry syllabus (9476).

Table of Contents

What Is Atomic Radius?

An atom has no sharp outer edge, so atomic radius is a practical measurement rather than an exact boundary — commonly taken as half the distance between the nuclei of two identical, bonded atoms. For H2 Chemistry, the relative size and the reasoning behind it matter more than the precise definition: the further the outer electrons sit from the nucleus, the larger the atomic radius; the stronger the nuclear attraction, the more that electron cloud is pulled inward.

Struggling With Chemistry?

Everything you need to score higher in O Level & A Level Chemistry — from exam tips to study strategies.

Speak to Us · Reserve an Exclusive Slot

The Main Factors Affecting Atomic Radius

Three factors decide atomic radius, and they’re the same three that decide every periodic trend in H2 Chemistry:

• Nuclear charge — more protons pull the outer electrons in more strongly, provided shielding is comparable.
• Number of occupied shells — an extra shell puts the outer electron further out, increasing radius.
• Shielding — inner-shell electrons reduce the attraction felt by the outer electron, allowing the atom to be larger than nuclear charge alone would suggest.

How these three combine into nuclear charge, shielding, and effective nuclear charge is covered in full in the shielding guide — this page picks up from there and applies it specifically to size.

Atomic Radius Across Period 3: Real Data

Element	Na	Mg	Al	Si	P	S	Cl
Atomic radius (nm)	0.157	0.136	0.125	0.117	0.110	0.104	0.099

Across the period, each added electron enters the same third principal shell while proton number rises by one each time — so shielding stays roughly constant and the increasing nuclear charge pulls the electron cloud in progressively tighter.

Exam-style comparison. Which is smaller, magnesium or sulfur? Sulfur (0.104 nm) is smaller than magnesium (0.136 nm). Both are in Period 3, so their outer electrons occupy the same principal shell with broadly similar shielding — sulfur’s greater nuclear charge is the decisive factor, pulling its outer electrons in more strongly.

Atomic Radius Down Group 2: Real Data

Element	Mg	Ca	Sr	Ba
Atomic radius (nm)	0.160	0.197	0.215	0.217

Each step down the group adds a new occupied shell. Nuclear charge does increase too, but the extra shell and the additional shielding it brings outweigh that increase, so the outer electron ends up further out and less tightly held overall.

Exam-style comparison. Which is larger, magnesium or barium? Barium (0.217 nm) is larger than magnesium (0.160 nm). Barium has more occupied electron shells and greater shielding from its additional inner shells, and these effects outweigh its higher nuclear charge.

Ionic Radius: Cations vs Anions

Ionic radius follows the same nuclear-charge-and-shielding logic as atomic radius, but ionisation changes the electron count without changing the number of protons, which shifts the balance:

Ion	Na⁺	Mg²⁺	Al³⁺	Si⁴⁺	P³⁻	S²⁻	Cl⁻
Ionic radius (nm)	0.102	0.072	0.054	0.041	0.212	0.184	0.181

Cations are smaller than their parent atom — removing electrons (sometimes an entire outer shell) leaves the remaining electrons more tightly held by the same nuclear charge. Anions are larger than their parent atom — adding electrons to the existing outer shell increases electron-electron repulsion, so the electron cloud expands even though nuclear charge is unchanged.

Worked Example: The Isoelectronic 10-Electron Series

An isoelectronic series holds particles with the same electron count, which removes electron count as a variable entirely — making it the cleanest type of radius comparison to reason through precisely.

Ion	O²⁻	F⁻	Na⁺	Mg²⁺	Al³⁺
Protons	8	9	11	12	13
Ionic radius (nm)	0.140	0.133	0.102	0.072	0.054

Every particle in this series has ten electrons and the same electron configuration — [Ne]. Shielding and occupied shells are identical across all five, so proton number alone decides the outcome: as proton number rises from 8 to 13, the same ten electrons are pulled in progressively harder, and ionic radius falls steadily from O²⁻ down to Al³⁺.

A complete answer names the series as isoelectronic first, states that electron count and shielding are fixed, then attributes the entire size difference to the rising nuclear charge — the same structure this site’s Effective Nuclear Charge guide scores its sample answers against.

Where This Page Fits

The complete Periodic Table guide for H2 Chemistry covers atomic radius alongside melting point, conductivity, and electronegativity at an overview level across the full Period 3 and Group 2/17 property set. This page is the fuller breakdown of atomic and ionic radius specifically — real figures, the isoelectronic worked example, and the exceptions below — the same relationship Ionisation Energy Explained has to that same pillar for its own trend.

Noble Gases and Measurement Caveats

Noble gases are often left off atomic radius tables entirely, or given a radius measured a different way (van der Waals rather than covalent/metallic), because they don’t form the same bonds used to measure other atoms. Treating a noble gas’s tabulated radius as directly comparable to its neighbours without checking which definition was used is a common source of confusion — it isn’t that the trend “breaks” at the noble gas, it’s that the measurement itself changed.

Common Atomic Radius Misconceptions

• “More protons always means a smaller atom.” Only within a fixed number of shells. Down a group, added shells win out despite the higher nuclear charge.
• “A greater nuclear charge in a lower period means it must be smaller.” Comparing atoms across different periods without accounting for shell number can give the wrong answer — a lower-period atom can be larger despite a smaller nuclear charge.
• “Atomic radius is an exact physical boundary.” Electron clouds have no hard edge; different measurement conventions give different numbers, which is why trend-based reasoning matters more than memorising single values.

How to Answer Atomic Radius Questions

The structure is the same nuclear-charge-and-shielding discipline covered in the effective nuclear charge framework, applied specifically to size:

1. Identify whether the comparison is across a period, down a group, or an isoelectronic series — each has a different decisive factor.
2. State nuclear charge and shielding for both particles as a direct comparison, not two separate facts.
3. Say which effect wins when they point in different directions.
4. Conclude with the size comparison in one clause, using “attraction” language rather than restating the size difference as the explanation.

How Pamela’s Place Supports H2 Chemistry Revision

At Pamela’s Place, atomic and ionic radius are taught from real Data Booklet figures rather than trend arrows alone, so students can interpret unfamiliar numerical comparisons rather than only reciting the general rule. This sits inside the same small Omakase groups, capped at 7 students, used across our Chemistry programmes.

Atomic Radius FAQs