Atomic size, ionisation enthalpy, electron gain enthalpy and electronegativity all follow from effective nuclear charge and the shell number of the outermost electrons.
How it's asked: Ordering ionisation enthalpies with the Be/B and N/O exceptions, radii of isoelectronic species, Cl vs F electron gain enthalpy, nature of oxides across a period, and IUPAC names for Z > 100.
Why this is true
Across a period electrons are added to the same shell while the nuclear charge rises, and electrons in the same shell shield each other poorly, so effective nuclear charge increases. The outer electrons are pulled closer: atomic radius falls and ionisation enthalpy, electron gain enthalpy (more negative) and electronegativity rise. Down a group a new shell is added each time, which outweighs the rising nuclear charge, so size increases and these properties fall. The exceptions come from subshell stability: B has lower ionisation enthalpy than Be because its 2p electron is less penetrating than a 2s electron, and O is lower than N because removing one of the paired 2p electrons relieves electron–electron repulsion, whereas N's half-filled 2p³ is extra stable. Fluorine's electron gain enthalpy is less negative than chlorine's because the incoming electron enters the very compact 2p shell and is repelled by the electrons already there.
Key points
Key formulae
Effective nuclear charge
Zeff = Z − σ
σ = shielding by inner electrons
Isoelectronic size
radius ∝ 1/Zeff
Pauling electronegativity (fluorine)
χ(F) = 4.0
Trends in the main group
| Property | Across a period (→) | Down a group (↓) |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionisation enthalpy | Increases (exceptions: group 13 < 2, 16 < 15) | Decreases |
| Electron gain enthalpy | More negative | Less negative (Cl > F exception) |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
Common traps
- Writing F as having the most negative electron gain enthalpy — it is Cl.
- Ranking isoelectronic ions by mass instead of by nuclear charge.
Worked example
Arrange Be, B, N and O in increasing order of first ionisation enthalpy and justify the two inversions.
High-yield
IE: B < Be, O < N; most negative ΔegH: Cl; highest electronegativity: F; isoelectronic ions shrink as Z increases.
Quick check
Q1.Which of the following has the largest radius?
Q2.The element with the most negative electron gain enthalpy is: