Lesson 1 of 6
Electronic structure and atomic trends
Compare radius, ionisation energy and electronegativity across and down.
A-Level 9476 (2026-2027)
I can recognise electronic-configuration patterns.Syllabus
Syllabus 9476, 5(a). Recognise electronic-configuration patterns.
I can explain atomic and ionic size, first IE and electronegativity trends.Syllabus
Syllabus 9476, 5(b). Explain atomic and ionic size, first IE and electronegativity trends.
A period fills a shell; a group preserves an outer pattern
Outer configurations explain recurring chemistry, but subshell and structure changes interrupt simple trends.
| Element | Outer configuration | Typical highest oxidation number in the specified compounds |
|---|---|---|
| Na | 3s1 | +1 |
| Mg | 3s2 | +2 |
| Al | 3s23p1 | +3 |
| Si | 3s23p2 | +4 |
| P | 3s23p3 | +5 |
| S | 3s23p4 | +6 |
| Cl | 3s23p5 | Seven outer electrons; chlorine oxides are outside the specified oxide list |
Across this period the inner [Ne] core stays the same while nuclear charge increases and electrons enter the third shell. Down Group 2 from Mg to Ba, the outer configuration remains ns2. Down Group 17 from Cl to I, it remains ns2np5. The value of n increases down each group, so the outer electrons occupy more distant shells.
Use these patterns to predict typical ions: Group 2 loses two electrons to give M2+; Group 17 gains one to give X-. This does not mean that every compound consists of ions. Period-3 non-metals also share electrons in covalent compounds.
Check your understanding
What stays the same, and what changes, in the outer configuration down Group 17?
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The ns2 np5 pattern stays the same; n rises by one each period.
Cl is 3s2 3p5 and Br is 4s2 4p5. The same pattern explains similar reactions, and a larger n puts the outer electrons further from the nucleus.
Compare attraction, shielding and shell number together
Across a period attraction generally strengthens; down a group extra shells usually dominate.
Make a guess. You are not marked.
How does atomic radius change from sodium to chlorine across Period 3?
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It decreases, as nuclear charge rises
Nuclear charge rises by one each step while the electrons go into the same shell with similar shielding, so the outer electrons are pulled closer.
| Property | Across Na to Cl | Down Group 2 or Group 17 |
|---|---|---|
| Atomic radius | Generally decreases: rising nuclear charge with similar inner-shell shielding attracts the outer shell more strongly. | Increases: additional shells put outer electrons farther from the nucleus. |
| First ionisation energy | Generally increases, with subshell and pairing exceptions. | Generally decreases: greater distance and shielding outweigh the increased nuclear charge. |
| Electronegativity | Generally increases: the atom attracts a shared pair more strongly. | Generally decreases: a bonding pair is farther from the more shielded nucleus. |
The ionisation-energy decrease from Mg to Al occurs because Al loses a higher-energy 3p electron, whereas Mg loses a 3s electron. The decrease from P to S occurs because S has a paired 3p electron; repulsion within that pair makes removal easier than from P's three singly occupied p orbitals. A general trend is not a monotonic rule.
Cations are usually smaller than their parent atoms: electron loss reduces repulsion and may remove the outer shell entirely. Anions are larger than their parent atoms because added electrons increase repulsion while nuclear charge is unchanged. Comparing isoelectronic ions isolates nuclear charge: Na+, Mg2+ and Al3+ each have ten electrons, and radius decreases in that order.
Worked exampleDo not join two different ionic series into one smooth trend
Compare Al3+ with P3-, then P3-, S2- and Cl-.
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- Al3+ has ten electrons in two occupied shells; P3- has eighteen electrons in three shells. The change in shell number produces a large increase.
- P3-, S2- and Cl- are isoelectronic. Their proton numbers rise from 15 to 17.
- Greater nuclear attraction contracts the same eighteen-electron arrangement.
P3- is much larger than Al3+; within the anion series, P3- > S2- > Cl- in radius.
Down Group 2, M2+ radii increase because the ions have progressively more occupied shells. The same applies to Cl-, Br- and I-. Always state whether a graph shows atomic radius or the radius of a particular ion.
Exam-style question
Explain why atomic radius decreases from sodium to chlorine across Period 3. [2]
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