Atomic Radius Trend
Atoms shrink across a period and grow down a group.
About this topic
Atomic radius decreases across a period because the nuclear charge increases while electrons are added to the same shell, pulling them closer. It increases down a group because each new period adds a shell, putting outer electrons farther from the nucleus.
Two effects fight each other: nuclear charge pulls electrons in, while inner electrons shield the outer ones from that pull. Across a period, shielding stays about constant, so the effective nuclear charge grows and the atom shrinks. Down a group, extra inner shells increase shielding and distance, so the atom grows even though the nuclear charge is larger. Radius is usually measured as half the distance between two bonded nuclei, in picometres.
Example: in period 3, sodium has a radius of about 186 pm while chlorine has about 99 pm. Going down group 1, lithium is about 152 pm and caesium about 265 pm. Cations are smaller than their atoms and anions larger, so Na+ is much smaller than Na, and Cl- is bigger than Cl.
This trend underlies ionisation energy, electronegativity and reactivity. A common error is to think that more protons always means a bigger atom. Check the number of shells first, then the nuclear charge.
Key ideas
- More protons pull electrons tighter across a period
- More shells increase size down a group
- Cations are smaller than atoms
Quick check
Use cases: where Atomic Radius Trend helps
- Homework: review the key idea behind Atomic Radius Trend before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Atomic Radius Trend without notes.
- Visual learning: play with the interactive lab to see how Atomic Radius Trend changes when you adjust its inputs.
- Teaching a friend: explain Atomic Radius Trend in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Atomic Radius Trend?
Atoms shrink across a period and grow down a group.
What will I learn in Atomic Radius Trend?
More protons pull electrons tighter across a period. More shells increase size down a group. Cations are smaller than atoms.
Is Atomic Radius Trend part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Atomic Radius Trend lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Atomic Radius Trend?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.