Atomic Radius Trends — Chemistry Mnemonic
Target: mnemonic for atomic radius trends periodic table
Why is this hard to memorize?
Atomic radius is the most fundamental periodic trend — it drives all other trends (IE, EA, EN). The rules are: radius DECREASES across a period (more protons pull electrons closer) and INCREASES down a group (new shells added). But exam questions go deeper: comparing ionic radii (Na⁺ < Na, Cl⁻ > Cl), isoelectronic species (O²⁻ > F⁻ > Na⁺ > Mg²⁺), and the effect of lanthanide contraction on 5d elements. Understanding size trends unlocks everything else in periodicity.
Classic mnemonics you should know
"Across → Shrinks (more protons, same shell). Down → Grows (new shell added)."
Across a period: nuclear charge increases but electrons go into the same shell. More pull, same distance → atoms shrink. Down a group: new electron shell → radius jumps up. Opposite of IE trends!
"Cations < parent atom (lost electrons). Anions > parent atom (gained electrons). "Cat shrinks, An grows""
Na(186pm) → Na⁺(95pm): lost outer shell → much smaller. Cl(99pm) → Cl⁻(181pm): gained electron → more repulsion → expands. These size changes explain ionic compound properties.
"Same electrons, more protons → smaller. O²⁻ > F⁻ > Ne > Na⁺ > Mg²⁺ > Al³⁺"
All have 10 electrons, but O has 8 protons, F has 9, Na has 11, Mg has 12... More protons pulling same number of electrons → smaller radius. This is a NEET/JEE favourite question type.
The complete list
- Atomic radius decreases across period
- Atomic radius increases down group
- Cation < neutral atom
- Anion > neutral atom
- Isoelectronic: more protons = smaller
- Lanthanide contraction affects 5d elements
Frequently asked questions
Why does atomic radius decrease across a period?
Each step right adds +1 proton and +1 electron. The electron goes into the same shell (same shielding), but the extra proton increases nuclear pull. More pull on same-shell electrons → smaller radius. Na(186pm) → Cl(99pm) across Period 3.
How do I compare sizes of isoelectronic species?
Isoelectronic species have the same number of electrons. The one with MORE protons is SMALLER (more nuclear pull). For the 10-electron series: O²⁻(140pm) > F⁻(136pm) > Na⁺(95pm) > Mg²⁺(65pm) > Al³⁺(50pm). Always order by increasing atomic number = decreasing size.
What is lanthanide contraction?
As you fill the 4f orbital (Ce to Lu), each added 4f electron shields poorly (f-orbitals have poor penetration). So nuclear charge increases but shielding barely increases → atoms shrink across the lanthanides. Result: 5d elements (Hf, Ta, W...) are almost the same size as their 4d counterparts (Zr, Nb, Mo...), giving them similar chemistry.
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