Learn · Periodic Trends
📈 Periodic Trends
How atomic radius, ionisation enthalpy, electron gain enthalpy and electronegativity vary across periods and down groups — with the exact exceptions JEE/NEET loves to test.
Open full interactive periodic table⚪ Atomic Radius(pm)
Half the distance between the nuclei of two like atoms bonded together (covalent radius for most; van der Waals radius for noble gases).
Decreases left → right across a period. Nuclear charge rises while electrons enter the same shell, so the effective nuclear charge pulls the cloud inward.
Increases top → bottom down a group. Each step adds a new principal shell, and increased shielding outweighs the higher nuclear charge.
Why
Radius is the balance between attractive nuclear pull (Zₑff) and the number of shells / electron–electron shielding.
High-yield exceptions
Noble gases look 'large'
Their radii are van der Waals radii (no bonded neighbour), so they are not directly comparable to the covalent radii of other elements.
d-block contraction
Across a transition series the radius changes only slightly — added d-electrons shield poorly, nearly cancelling the rise in nuclear charge.
Lanthanide contraction
Poor shielding by 4f electrons makes 5d-series atoms (e.g. Hf, Ta) almost the same size as their 4d counterparts (Zr, Nb).
Quick facts
- • Cation < parent atom (lost a shell / higher Zₑff).
- • Anion > parent atom (added electron, more repulsion).
- • Cs is among the largest atoms; He the smallest.
Periodic table coloured by radius
tap a tile for its valueHighest
Cs
Caesium
298 pm
Lowest
He
Helium
31 pm