Periodicity: trends across Periods 2 and 3

Melting and boiling points across Period 3

• Na, Mg, Al: giant metallic. Melting point rises as the number of delocalised electrons per ion rises and ions get smaller and more charged.
• Si: giant covalent, the highest; many strong covalent bonds must break.
• P4, S8, Cl2: simple molecular, low; only London forces between molecules. S8 > P4 > Cl2 because S8 molecules have the most electrons.
• Ar: single atoms, lowest.

Period 2 shows the same pattern: Li and Be metallic, B and C (diamond, graphite) giant covalent, then N2, O2, F2 and Ne low.

Ionisation energy across a period

First IE generally increases because nuclear charge increases while shielding stays about the same (electrons enter the same shell), and atoms get smaller. Dips occur at Group 13 (new p sub-shell) and Group 16 (paired p electrons).

Down a group

First IE decreases down a group: the outer electron is further from the nucleus and more shielded, which outweighs the greater nuclear charge.

Reading graphs

On a plot of first IE against atomic number, the highest points are the noble gases and the lowest are the Group 1 metals. The repeating shape is called periodicity.

Worked example

Explain why silicon has a much higher melting point than phosphorus.

1. Silicon has a giant covalent structure: melting breaks many strong covalent bonds.
2. Phosphorus is simple molecular (P4): melting overcomes only weak London forces between molecules.

Answer: Much more energy is needed to break covalent bonds than intermolecular forces.

Check yourself

1. Which Period 3 element has the highest melting point?
   • Aluminium
   • Silicon
   • Sulfur
   • Sodium
   Show answer

   Silicon. Silicon is giant covalent.

2. Why does aluminium have a higher melting point than sodium?
   • Aluminium is a giant covalent structure
   • Aluminium has stronger London forces
   • Sodium atoms are lighter
   • Al3+ ions are smaller and more charged and there are more delocalised electrons
   Show answer

   Al3+ ions are smaller and more charged and there are more delocalised electrons. Stronger attraction between the ions and the delocalised electrons means stronger metallic bonding.

3. Why does sulfur (S8) have a higher melting point than phosphorus (P4)?
   • Sulfur forms a giant covalent structure
   • S8 molecules have more electrons, so stronger London forces
   • Sulfur has hydrogen bonding
   • Phosphorus has metallic bonding
   Show answer

   S8 molecules have more electrons, so stronger London forces. Both are simple molecular; the larger S8 molecule has more electrons.

4. Why does first IE generally increase across Period 3?
   • Atoms get larger
   • Shielding increases a lot
   • Nuclear charge increases while shielding stays about the same
   • Electrons enter new shells
   Show answer

   Nuclear charge increases while shielding stays about the same. Same shell, more protons, smaller atoms: stronger attraction.

5. Which element in Period 3 has the lowest first ionisation energy?
   • Sodium
   • Argon
   • Aluminium
   • Chlorine
   Show answer

   Sodium. Group 1 elements have the lowest first IE in their period.

6. Why does argon have the lowest melting point in Period 3?
   • It has the most electrons
   • It forms a giant lattice
   • It exists as single atoms with only very weak London forces between them
   • Its atoms are held by covalent bonds
   Show answer

   It exists as single atoms with only very weak London forces between them. Ar is monatomic, with few electrons per particle compared with S8 or P4.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.