Periodic TrendsJEE Main

Ionization energy & radius across a period — interactive Chemistry simulation for IIT-JEE.

Concept

Across a period, the nuclear charge grows while electrons fill the same shell — pulling electrons in tighter. So atomic radius shrinks and ionization energy rises left to right, with a couple of famous dips that reveal sub-shell structure.

Key formula

\text{Z}_{\text{eff}} \uparrow \;\Rightarrow\; \text{radius} \downarrow,\; \text{IE} \uparrow \quad (\text{across a period})

Derivation

Effective nuclear charge $Z_{eff}$ increases across a period because added electrons shield poorly within the same shell. Stronger pull means smaller radius and harder-to-remove electrons (higher IE).

Two dips break the IE trend: B < Be (a 2p electron is easier to remove than a filled 2s²) and O < N (pairing in 2p⁴ adds repulsion). These exceptions are classic JEE fodder.

Scenarios to explore

Periodic Trends — Ionization energy & atomic radius across a period.

Real-world applications

Predicting reactivity (metals vs non-metals).
Explaining ionic sizes and lattice energies.
Rationalising oxidation states down groups.

JEE exam tips

Down a group: radius increases, IE decreases (new shells dominate over $Z_{eff}$ ).
Half-filled (p³) and fully-filled (p⁶) sub-shells are extra stable — driving the anomalies.

Common mistakes

Expecting a perfectly smooth IE trend — remember the Be→B and N→O dips.
Confusing the across-period trend with the down-group one (opposite for radius).
Forgetting that cations are smaller and anions larger than their parent atoms.

Exam traps to avoid

Ionization energy of oxygen is lower than nitrogen despite higher Z.
Noble gases have the highest IE in their period, not the halogens.