Explain the decrease in atomic radius across Period 3.

Nuclear charge increases while electrons enter the same shell with similar shielding, so the outer electrons are pulled closer.

What is the general trend in first ionisation energy across Period 3?

A general increase from Na to Ar.

Why is the first ionisation energy of Al lower than that of Mg?

Al's outer electron is in a 3p sub-shell, which is higher in energy and shielded by the 3s electrons.

Why is the first ionisation energy of S lower than that of P?

In S two electrons share a 3p orbital and repel each other, so one is easier to remove.

Define first ionisation energy.

The energy to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions.

Which bonding is present in Na, Mg and Al?

Metallic bonding: positive ions attracted to delocalised electrons.

Why does melting point rise from Na to Al?

Greater ionic charge, more delocalised electrons and smaller ions, so stronger attraction in the lattice.

Which Period 3 element has the highest melting point and why?

Silicon, because it has a giant covalent structure with many strong covalent bonds to break.

What is overcome when P₄, S₈ or Cl₂ melts?

The weak London forces between molecules, not covalent bonds.

Why does S₈ have a higher melting point than P₄?

S₈ has more electrons, so stronger London forces.

What kind of structure does argon have?

Monatomic, so it has only weak London forces between atoms.

Physical properties of Period 3 elementsAQA A-Level Chemistry: Flashcards

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How does atomic radius change from Na to Cl?

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How does atomic radius change from Na to Cl?: It decreases.
Explain the decrease in atomic radius across Period 3.: Nuclear charge increases while electrons enter the same shell with similar shielding, so the outer electrons are pulled closer.
What is the general trend in first ionisation energy across Period 3?: A general increase from Na to Ar.
Why is the first ionisation energy of Al lower than that of Mg?: Al's outer electron is in a 3p sub-shell, which is higher in energy and shielded by the 3s electrons.
Why is the first ionisation energy of S lower than that of P?: In S two electrons share a 3p orbital and repel each other, so one is easier to remove.
Define first ionisation energy.: The energy to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions.
Which bonding is present in Na, Mg and Al?: Metallic bonding: positive ions attracted to delocalised electrons.
Why does melting point rise from Na to Al?: Greater ionic charge, more delocalised electrons and smaller ions, so stronger attraction in the lattice.
Which Period 3 element has the highest melting point and why?: Silicon, because it has a giant covalent structure with many strong covalent bonds to break.
What is overcome when P₄, S₈ or Cl₂ melts?: The weak London forces between molecules, not covalent bonds.
Why does S₈ have a higher melting point than P₄?: S₈ has more electrons, so stronger London forces.
What is the order of melting points of S₈, P₄, Cl₂ and Ar?: S₈ > P₄ > Cl₂ > Ar.
What kind of structure does argon have?: Monatomic, so it has only weak London forces between atoms.

Exam questions on Physical properties of Period 3 elements

The atomic radius of the elements decreases across Period 3, from 0.186 nm for sodium to 0.099 nm for chlorine.
Explain why the atomic radius decreases from sodium to chlorine.2 marks
The first ionisation energies of the Period 3 elements (in kJ mol⁻¹) are: Na 496, Mg 738, Al 578, Si 786, P 1012, S 1000, Cl 1251, Ar 1521.
Explain why the first ionisation energy of argon is much higher than that of sodium.2 marks
The melting points of four Period 3 elements are: sodium 371 K, magnesium 923 K, aluminium 933 K and silicon 1687 K.
Explain why magnesium has a higher melting point than sodium.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).