Periodicity: Question 2

Worked solution

Step 1: Recall the overall melting-point pattern across Period 3

Going across the period, melting point rises steadily through the metals (Na $98\,^\circ\text{C}$, Mg $650\,^\circ\text{C}$, Al $660\,^\circ\text{C}$) as metallic bonding strengthens, reaches a very high peak at silicon ($1414\,^\circ\text{C}$), then falls sharply and stays low for $\text{P}_4$, $\text{S}_8$, $\text{Cl}_2$ and Ar (all well below room temperature or only slightly above it).

Step 2: Identify the structural reason for the maximum

Na, Mg and Al are giant metallic lattices: strong, but not as strong as what comes next.

Silicon is a giant covalent (macromolecular) lattice: every Si atom is joined to four neighbours by strong covalent bonds extending throughout the whole crystal, with no individual molecules present. Melting silicon means breaking a vast number of these strong bonds, which needs far more energy than disrupting a metallic lattice, so silicon has, by a large margin, the highest melting point in the period. This makes option B correct.

$\text{P}_4$, $\text{S}_8$, $\text{Cl}_2$ and Ar, by contrast, are simple molecular (or monatomic) structures. Even though strong covalent bonds do hold the atoms together within a $\text{P}_4$ or $\text{S}_8$ molecule, these bonds do not break on melting. Only the much weaker van der Waals forces between separate molecules are overcome. The number of atoms bonded together inside one molecule has no bearing on the strength of the forces between molecules, so a higher atomicity does not by itself raise the melting point.

Why the other options are wrong

• A: Aluminium’s melting point ($660\,^\circ\text{C}$) is far lower than silicon’s ($1414\,^\circ\text{C}$), so it is not the maximum, even though its metallic bonding is indeed the strongest among Na, Mg and Al.
• C: Sulfur’s melting point ($115\,^\circ\text{C}$) is one of the lowest in the period. The strong covalent bonds inside each $\text{S}_8$ molecule are irrelevant to melting, since melting only breaks the weak van der Waals forces between separate $\text{S}_8$ molecules.
• D: Phosphorus’s melting point ($44\,^\circ\text{C}$) is the lowest of all eight Period 3 elements. Compact molecular packing does not create unusually strong intermolecular forces; $\text{P}_4$ molecules are still held together only by weak van der Waals forces.

Final answer

• The Period 3 element with the highest melting point is silicon, because of its giant covalent lattice of strong covalent bonds, option B.