Periodicity of physical properties of the elements in Period 3

AS Level Chemistry · The Periodic Table: chemical periodicity. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Periodicity means a pattern of properties that repeats in each period. Across Period 3, from sodium to argon, each atom has one more proton and one more electron than the one before, and the new electron goes into the same shell.

The nuclear charge rises while the shielding stays almost the same, so the outer electrons are pulled in more strongly and the atoms get smaller.

Melting point and electrical conductivity depend on structure and bonding, which change across the period: giant metallic for Na, Mg and Al, giant molecular for Si, then simple molecular for P₄, S₈ and Cl₂, and single atoms for Ar.

Rules to remember

Atomic radius decreases from Na to Cl: more protons, same number of shells, similar shielding.
Positive ions Na⁺, Mg²⁺, Al³⁺ have 2 shells and get smaller as nuclear charge rises; Al³⁺ is the smallest.
Negative ions P³⁻, S²⁻, Cl⁻ have 3 shells, so they are much larger than the positive ions; they also get smaller from P³⁻ to Cl⁻.
Melting point rises Na < Mg < Al: higher ion charge and more delocalised electrons give stronger metallic bonding.
Si has the highest melting point: giant molecular, many strong covalent bonds to break.
P₄, S₈, Cl₂ and Ar have low melting points (only id-id forces to overcome); order S₈ > P₄ > Cl₂ > Ar, following the number of electrons.
Conductivity increases Na < Mg < Al; Si is a semiconductor; P, S, Cl and Ar do not conduct.

Common mistake

Students expect atoms to get bigger across the period because they have more electrons. The extra electrons go into the same shell while the nuclear charge increases, so the radius decreases.

Worked example

Which atom has the smaller radius, magnesium (proton number 12) or sulfur (proton number 16)? Explain.

Both atoms have three occupied shells, with the same inner shells, so the shielding is about the same.
Sulfur has 16 protons and magnesium has 12, so sulfur has the greater nuclear charge.
The outer electrons of sulfur are attracted more strongly and are pulled closer to the nucleus.

Answer: Sulfur has the smaller atomic radius.

Practice questions

Try each one, then open the answer.

1. Which Period 3 element is the best conductor of electricity?

A
sodium
B
silicon
C
aluminium
D
sulfur

Show answer

Answer: C. Conductivity rises from Na to Al because each atom releases more delocalised electrons per ion (one, two, three). Silicon is a semiconductor and sulfur, a molecular solid, does not conduct.

2. Which Period 3 element is a semiconductor?

A
aluminium
B
phosphorus
C
silicon
D
sulfur

Show answer

Answer: C. Silicon conducts far less well than the metals Na, Mg and Al but better than the non-metals; it is a semiconductor with a giant covalent structure.

3. Why do the melting points increase from sodium to magnesium to aluminium?

A
the metallic bonding gets stronger because the ions have a higher charge, are smaller and supply more delocalised electrons
B
the elements change from metallic to giant covalent
C
the intermolecular forces increase as the number of electrons increases
D
the atoms get larger, so there is more contact between them

Show answer

Answer: A. Na, Mg and Al release 1, 2 and 3 electrons per atom into the delocalised sea, and the ions get smaller, so the attraction between the ions and the delocalised electrons increases.

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