Periodic Trends From One Idea: Effective Nuclear Charge
Nearly every trend across a period or down a group follows from how strongly the outer electrons feel the nucleus.
The controlling quantity
An outer electron does not feel the full nuclear charge, because inner electrons screen it. What it feels is the effective nuclear charge — the actual charge reduced by shielding.
| Direction | Protons | Shielding | Effective charge |
|---|---|---|---|
| Across a period | Increases | Barely changes — electrons enter the same shell | Increases |
| Down a group | Increases | Increases correspondingly — a new shell each time | Roughly constant |
The trends that follow
| Property | Across a period | Down a group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionisation energy | Increases | Decreases |
| Electron affinity | Becomes more negative | Becomes less negative |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
All five reduce to the same cause: outer electrons held more tightly across a period, and further away with comparable attraction down a group.
The exceptions, and why they occur
Exceptions are asked more often than the trends themselves, because they test whether the reasoning is understood.
- Ionisation energy dips where an electron enters a new subshell that is higher in energy, or where a paired electron in an otherwise half-filled subshell is removed. Half-filled and fully filled subshells have extra stability from exchange energy, so removing an electron from the following element is easier than expected.
- Electron affinity of the first element in a group is often smaller than the second, because the small size means strong repulsion between the incoming electron and those already present.
- The lanthanide contraction makes third-row transition metals almost identical in size to second-row ones, breaking the expected increase down a group.
The first of these is the most examined, and the answer must name the stability of half-filled and filled subshells rather than simply asserting an anomaly.
Ionic radii
A cation is always smaller than its parent atom, because losing electrons reduces repulsion and often removes an entire shell. An anion is always larger, because added electrons increase repulsion while the nuclear charge is unchanged.
For an isoelectronic series — species with the same electron count — size decreases as nuclear charge increases, since the same electrons are pulled by more protons. Ordering an isoelectronic series by size is a standard question, answered by simply ordering by atomic number.
Diagonal relationships
An element resembles the one diagonally below and to the right more than its own group members, in the top corner of the periodic table. The reason is that moving right increases charge and moving down increases size, and the two effects roughly cancel to give similar charge density.
Charge density is what governs polarising power, and polarising power governs how covalent a compound is — which is why the diagonal pairs show similar chemistry.
Frequently asked questions
Why does effective nuclear charge stay roughly constant down a group?
Because each new shell adds both protons and shielding electrons, so the increases roughly offset. The dominant change is the greater distance of the outer shell.
Why is there a dip in ionisation energy at certain points?
Because a half-filled or fully filled subshell has extra stability, so removing an electron from the next element, which breaks that arrangement, requires less energy than the trend predicts.
Why is a cation smaller than its parent atom?
Because losing electrons reduces electron–electron repulsion and frequently removes the outermost shell entirely.
How do I order an isoelectronic series by size?
By nuclear charge. With the same number of electrons, more protons means stronger attraction and a smaller radius.
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