Trends Across Period 3: Radius, Ionisation Energy and Melting Point
Across a period the nuclear charge climbs while the outer shell stays the same, so atoms shrink and get harder to ionise. Melting point follows structure instead, which is why silicon towers over both of its neighbours.
What a learner can do afterwards
- Explains the fall in atomic radius across period 3
- Explains the general rise in first ionisation energy and accounts for the two dips
- Explains the melting point pattern by naming the structure of each element in the row
- Places an unknown period 3 element from two of its measured properties
1 · Read
Walk from sodium to argon and every atom gets smaller. The outer electrons stay in shell 3 the whole way, but each step adds a proton, so the nuclear charge climbs from plus 11 toward plus 18. The same shell pulled by a bigger charge shrinks, and electrons within a shell barely shield each other.
First ionisation energy climbs across the row for the same reason radius falls: a rising effective charge grips the outer electron tighter. But the climb has two famous dips. From magnesium to aluminium it drops, since the aluminium outer electron sits in a higher 3p orbital. From phosphorus to sulfur it dips again, since sulfur starts pairing and the shared-orbital repulsion frees one electron more cheaply.
Melting point ignores the charge story and follows structure instead. Sodium, magnesium and aluminium are metallic lattices that strengthen across the three, so melting rises. Then silicon towers near 1400 C, since its giant covalent lattice locks every atom with strong bonds. Past silicon the floor falls out: phosphorus, sulfur and chlorine are small molecules, and phosphorus melts near 44 C. Argon is lone atoms with the weakest forces of all.
To place an unknown, ask which story its properties tell. Radius and ionisation point along the charge climb, while melting points at the structure. Two measured properties are usually enough to seat it.
Radius and ionisation follow the rising charge, while melting follows the structure.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.