Melting points: why diamond survives and chlorine is a gas

The problem

Silicon and phosphorus are next-door neighbours in period 3. Silicon melts at about 1414 °C, hotter than any kitchen can reach. White phosphorus melts at about 44 °C, a hot June afternoon in Lahore. Same row, one step apart. Why?

A melting point measures how strongly the particles are held together. Silicon is one giant web of bonds. Phosphorus is a crowd of small separate molecules that barely hold hands.

Across a short period, melting and boiling points increase with the number of valence electrons up to group IVA, then decrease up to the noble gases.

Key terms

Binding electrons

Each group IA atom gives only one electron to bond with others, so melting points are low. Group IIA atoms give two, so theirs are considerably higher. Carbon, with four, has the most.

Giant covalent structures

In diamond each carbon is bonded to four other carbons in one endless network. Elements that form giant covalent structures have very high melting points.

Small molecules

From group VA onwards, the lighter elements exist as small covalent molecules, not networks. Nitrogen, oxygen and fluorine have only weak forces between molecules, so their melting and boiling points are extremely low.

Electrical conductance

Metals conduct because their outer electrons are loose and move through the lattice. Group IB, the coinage metals, conduct extraordinarily well; non-metals of groups VIA and VIIA hardly conduct at all.

Short questions with model answers

1. Q1. Explain the variation in melting points along a short period, using period 3.

   • Na 98 °C < Mg 650 °C
   • Si (IVA) 1414 °C: the peak
   • P 44 °C · S 115 °C · Cl −101 °C · Ar −189 °C

   They rise with binding electrons up to group IVA (silicon), then fall to the noble gas.

2. Q2. Why is diamond a non-conductor while graphite is fairly a good conductor?

   • Diamond: all four valence electrons are tetrahedrally bound.
   • Graphite: one of the four valence electrons is relatively free.

   Diamond has no free electrons; graphite has one free electron per carbon atom.

3. Q3. Down group IA melting points fall, but down group VIIA they rise. Explain both.

   • IA: metals made of atoms; atoms grow down the group.
   • VIIA: separate molecules; molecules grow down the group.

   In IA the bonds between atoms weaken; in VIIA the forces between molecules strengthen.

Common mistakes

• ✗ Writing that the melting points of halogens decrease down the group.

  ✓ They increase: fluorine and chlorine are gases, bromine a liquid, iodine a solid. This is a true/false item in the exercise.

• ✗ Saying diamond conducts electricity because it is carbon, like graphite.

  ✓ Same element, different structure. In diamond every valence electron is in a bond, so it is a non-conductor.

• ✗ Assuming more valence electrons always means a higher melting point, all the way across the period.

  ✓ Only up to group IVA. After that the elements form small molecules, and melting points fall sharply to the noble gas.

MCQs

1. 1. Melting and boiling points of halogens ______ down the group.

   1. (a) increase
   2. (b) decrease
   3. (c) stay the same
   4. (d) become zero
   Show answer

   (a) They increase: larger molecules are more polarizable and attract each other more strongly.

2. 2. In a short period, melting points rise up to which group?

   1. (a) IIA
   2. (b) IIIA
   3. (c) IVA
   4. (d) VIIA
   Show answer

   (c) Group IVA (carbon, silicon): the most binding electrons and giant covalent structures.

3. 3. Graphite conducts electricity because:

   1. (a) it is a metal
   2. (b) one of its four valence electrons is relatively free
   3. (c) it has no valence electrons
   4. (d) its atoms are ions
   Show answer

   (b) One valence electron per carbon is free to move; in diamond all four are locked in bonds.

4. 4. Metals with extraordinarily high electrical conductance, the coinage metals, are in group:

   1. (a) IA
   2. (b) IIA
   3. (c) IB
   4. (d) VIIA
   Show answer

   (c) Group IB. Non-metals of VIA and VIIA are at the other extreme: practically non-conductors.

5. 5. Nitrogen, oxygen and fluorine have extremely low melting points because they:

   1. (a) are metals
   2. (b) form giant covalent structures
   3. (c) exist as small molecules with weak forces between them
   4. (d) have no electrons
   Show answer

   (c) They are individual molecules with very weak intermolecular forces, which break easily.

Quick revision

• Across a short period, melting points rise with binding electrons to group IVA, then fall as elements become small molecules.
• Down IA and IIA melting points fall (bigger atoms, weaker binding); down VIIA they rise (bigger, more polarizable molecules).
• Conductance needs free electrons: metals and graphite conduct; diamond and the non-metals of VIA and VIIA do not.