---
title: "Trends Across Period 3: Radius, Ionisation Energy and Melting Point"
description: "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"
canonical: https://lightmysky.com/learn/science/trends-across-period-3-radius-ionisation-energy-and-melting-point-mt_stFFQ6GPIh
source: https://lightmysky.com/learn/science/trends-across-period-3-radius-ionisation-energy-and-melting-point-mt_stFFQ6GPIh.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Chemistry · Ages 16 to 17
Page: https://lightmysky.com/learn/science/trends-across-period-3-radius-ionisation-energy-and-melting-point-mt_stFFQ6GPIh

## Ready when they can

- 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

## Lesson: Reading the third row

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.

**Example.** 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.

**Tip.** 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.

**Recap.** Radius and ionisation follow the rising charge, while melting follows the structure.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Orbitals, Sub-Shells and Electron Configuration](https://lightmysky.com/learn/science/orbitals-sub-shells-and-electron-configuration-mt_sRMo1f5PfJ)
- [Reading Structure from Properties](https://lightmysky.com/learn/science/reading-structure-from-properties-mt_yCA66GjWRw)

## Opens up

- [Period 3 Oxides and the Acid-Base Divide](https://lightmysky.com/learn/science/period-3-oxides-and-the-acid-base-divide-mt_3kC5hTavIQ)
- [Many-Electron Atoms: Spin, Antisymmetry and Effective Charge](https://lightmysky.com/learn/science/many-electron-atoms-spin-antisymmetry-and-effective-charge-mt_WPesw2f8T3)
- [Main-Group Chemistry: What Changes Down a p-Block Group](https://lightmysky.com/learn/science/main-group-chemistry-what-changes-down-a-p-block-group-mt_wq15-pBrI5)