Walk from sodium to argon and the elements change from soft reactive metals, through a giant covalent semiconductor, to small molecules and a noble gas. Every physical trend follows from the nuclear charge and the structure.
Across Period 3 the nuclear charge rises while electrons enter the same shell, so shielding is almost constant: the atomic radius decreases, and first ionisation energy and electronegativity generally increase. Melting point depends on structure: it rises from Na to Al as metallic bonding strengthens, peaks at silicon (giant covalent), then drops sharply for the simple molecular P4, S8, Cl2 and monatomic Ar. Conductivity rises from Na to Al (more delocalised electrons); Si is a semiconductor and the rest are insulators.
Step across Period 3 in the simulator and watch the atom shrink. Then compare the ions: the metal ions lose their third shell, the non-metal ions gain electrons in it.
| Property | Trend Na → Ar | Reason |
|---|---|---|
| atomic radius | decreases | more protons, same shell, similar shielding |
| cation radius | Na+ > Mg2+ > Al3+ | same electron configuration (2,8), rising nuclear charge |
| anion radius | P3− > S2− > Cl− | three shells (2,8,8), so much larger than the cations |
| electronegativity | increases (Ar not assigned) | greater attraction for a bonding pair |
Switch to the structure view. Each element's melting point and conductivity follow from the kind of lattice it forms.
| Element | Structure | m.p. / °C | Conductivity |
|---|---|---|---|
| Na, Mg, Al | giant metallic | 98, 650, 660 | good, rising Na < Mg < Al |
| Si | giant covalent (macromolecular) | 1410 | semiconductor |
| P4, S8, Cl2 | simple molecular | 44, 115, −101 | none |
| Ar | monatomic | −189 | none |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
How does atomic radius change from sodium to chlorine?
Which Period 3 element has the highest melting point?
Why is the first ionisation energy of aluminium lower than that of magnesium?
Which is the correct order of melting points?
The general rise across the period has two dips. Explain each one by the electron being removed.
Silicon's high melting point is due to its giant covalent structure, not to being a metal. And the fall from sulfur to chlorine to argon in melting point is about intermolecular forces between small molecules or atoms; no covalent bonds are broken when P, S or Cl2 melt.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why is the first ionisation energy of sulfur lower than that of phosphorus?
Which ion has the smallest radius?
Why does electrical conductivity increase from sodium to aluminium?
Why does phosphorus melt at a much lower temperature than silicon?
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