Result:

Atoms become smaller across a period.

Example (Period 3):
Na > Mg > Al > Si > P > S > Cl

This shrinking size explains:

• Increasing ionization energy
• Increasing electronegativity
• Stronger ability to attract electrons in bonding

Trend 2: Down a Group (top → bottom): Atomic Radius Increases

Why it happens:

• New electron shells are added
• Increased distance from the nucleus
• Increased shielding from inner electrons
• Reduced effective nuclear attraction for outer electrons

The number of protons also increases, but the effect of additional shells dominates.

Result:

Atoms become larger down a group.

Example (Group 1):
Li < Na < K < Rb < Cs

This explains why alkali metals become more reactive down the group—they lose electrons more easily because their valence electrons are farther from the nucleus.

Effective Nuclear Charge: The Key Idea

The phrase “effective nuclear charge” (Zₑff) means the actual pull the outer electrons feel from the nucleus after accounting for shielding.

Zₑff = protons − shielding

Across a period:

• Protons increase
• Shielding stays the same
• Zₑff increases
• Atoms shrink

Down a group:

• Shielding increases a lot
• Zₑff barely changes
• Atoms expand

This idea summarizes the entire trend.

Exceptions to the Trend

Atomic radius trends are mostly smooth, but small irregularities can occur due to:

• Subshell structure
• Electron repulsion

For example:

• Ga is slightly smaller than Al despite being below it
• Transition metals have similar radii across the series because 3d electrons shield poorly

IB rarely tests these exceptions, but they help explain transition metal chemistry.

Atomic Radius and Reactivity

Group 1 Metals:

Larger atoms → weaker hold on electrons → more reactive down the group.

Group 17 Halogens:

Smaller atoms → stronger attraction for electrons → more reactive at the top of the group.

Atomic size directly influences reactivity patterns across the periodic table.

Atomic Radius vs Ionic Radius

Atoms and ions follow related but distinct trends:

• Cations (positive ions) are smaller than their atoms (loss of electron shell).
• Anions (negative ions) are larger than their atoms (more electron–electron repulsion).

For isoelectronic ions, more protons = smaller radius.

Example:
O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺

Common IB Misunderstandings

“Atomic radius increases across a period.”

Incorrect—it decreases across a period.

“Shielding is the same down a group.”

No—each new energy level increases shielding.

“Bigger atoms hold electrons more strongly.”

The opposite is true—larger radius means weaker attraction.

“Nuclear charge decreases across a period.”

It always increases.

FAQs

Why doesn’t shielding increase across a period?

Because electrons are added to the same energy level, so inner shells remain unchanged.

Why are noble gases so large?

Their filled electron shells create increased electron–electron repulsion.

Why don’t transition metals show a strong radius trend?

Because d-electrons shield poorly, causing “d-block contraction.”

Conclusion

Atomic radius decreases across a period due to increasing nuclear charge and increases down a group due to increased electron shells and shielding. These trends govern reactivity, bonding, and many chemical behaviors in the periodic table. Understanding the causes behind atomic radius trends is essential for mastering periodicity in IB Chemistry.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.