Atomic and ionic radius trends

5 minAS Level1.1.7

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can state and explain how atomic radius and ionic radius change across a period and down a group.

Three things decide the size

The radius depends on how strongly the nucleus pulls on the outer electrons. Three factors:

Nuclear charge: more protons pull harder.
Number of shells: more shells put the outer electrons further out.
Shielding: inner electrons repel the outer ones, reducing the pull they feel.

Atoms across a period and down a group

Across a period, atomic radius decreases.

Nuclear charge goes up by one each step.
The new electrons go into the same outer shell, so shielding stays about the same.
So the outer electrons are pulled in closer.

Down a group, atomic radius increases.

Each element has one more shell, so the outer electrons are further from the nucleus.
There's more shielding from the extra inner shell.
These outweigh the increase in nuclear charge.

Quick check

Multiple choiceQuestion 1 · Recall

Why does atomic radius decrease from sodium to chlorine?

AEach element has one more shell than the one before it.BShielding increases across the period, so the electrons are pulled closer.CNuclear charge increases while electrons are added to the same shell with similar shielding, so the outer electrons are pulled closer.DAtoms with more electrons have more repulsion, which makes them shrink.

Fill in the blanksQuestion 2 · Recall

Down a group, atomic radius because each element has one more of electrons, and the extra distance and shielding outweigh the increase in nuclear charge.

Answer the 2 questions above to continue.

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