Periodic trends: atomic radius, ionisation energy, electron affinity and electronegativity
Across a period atoms shrink and pull electrons harder. Down a group they grow and pull less. Radius, ionisation energy, electron affinity and electronegativity follow this pattern.
One idea explains all four
Across a period, each element has one more proton, so the nuclear charge (the positive pull of the nucleus) rises. The new electron goes into the same shell, so the shell is pulled closer. Down a group, a new shell is added at each step, so the outer electrons sit farther away. They are also shielded (screened) by the inner shells. A small atom with a strong pull holds and attracts electrons well. A large atom does not.
The four trends
| Property | Meaning | Across a period (left to right) | Down a group |
|---|---|---|---|
| Atomic radius | Distance from the centre of the nucleus to the outermost shell | Decreases: more nuclear charge, same shell | Increases: a new shell is added each time |
| Ionisation energy | Minimum energy needed to remove an electron from the outermost shell of an isolated gaseous atom | Increases: smaller atom holds its electrons tighter | Decreases: outer electron is farther away and shielded |
| Electron affinity | Energy released when an electron is added to the outermost shell of an isolated gaseous neutral atom | Increases (halogens are highest) | Decreases |
| Electronegativity | Relative tendency of an atom to attract the shared pair of electrons in a covalent bond | Increases | Decreases |
Ionisation energy vs electron affinity
Both are measured on isolated atoms in the gas state. Noble gases have the highest ionisation energy in each period and almost no tendency to gain electrons.
Examples to quote
- Group 1 atomic radius grows in the order Li < Na < K < Rb < Cs.
- In period 2, lithium has the largest atom and fluorine the smallest (leaving out neon).
- First ionisation energies fall down group 1: Li 520, Na 496, K 419 kJ mol⁻¹.
- Fluorine is the most electronegative element; caesium is among the least.
Sodium gives up its outer electron easily (low ionisation energy), so it is never found free in nature. Chlorine, with a high electron affinity, takes that electron. Together they form the common salt collected in the salt pans of Thoothukudi.
2 marks: “Define ionisation energy.” The minimum energy needed to remove an electron from the outermost shell of an isolated gaseous atom.
2 marks: “Why does atomic radius increase down a group?” A new shell is added at each step down, so the outer electrons are farther from the nucleus.
4 marks: “How do atomic radius and electronegativity vary along a period and down a group? Give reasons.” Radius decreases across (more nuclear charge, same shell) and increases down (new shells). Electronegativity increases across and decreases down, for the same reasons.
Moving right and up, atoms get small and greedy: radius falls; ionisation energy, electron affinity and electronegativity rise.