Physical properties of metals and non-metals

Why are cooking pots metal and handles not?

A pot must carry heat to the food, can be beaten into shape and must not crack on the stove. A chunk of sulphur would crumble at the first hammer blow. What makes these two kinds of element so different?

Metals easily form cations and are held together by metallic bonds. They can be hammered into sheets (malleable) and drawn into wires (ductile). Their metallic bonding makes them the best conductors of heat and electricity. They are lustrous, hard and strong, and usually have high melting points, boiling points and densities; the soft alkali metals are the exception.

Non-metals generally gain electrons. They come in more colours and states. They are brittle: sulphur and phosphorus are powders that cannot be made into sheets or wires. With no free electrons, they do not conduct heat or electricity. Graphite is the exception: electrons loosely held between its layers can move.

Non-metals are dull and cannot be polished, and they are neither tough nor strong. They have low melting and boiling points (sulphur melts at 115 °C), except graphite and diamond, and lower densities than metals. The book's exceptions are worth remembering: mercury is a metal that is not a solid, and iodine is a non-metal with a shine.

Free electrons in metals explain conduction, shine and toughness; non-metals lack them (except graphite).

Key terms

Malleability

Shaping metals.

Ductility

Making wires.

Lustre

Telling metals apart.

Brittle

Describing non-metals.

Short questions with model answers

1. Q1. Why are metals generally tough and strong while non-metals are not? (Descriptive question iii, p. 128)

   • Metals
   • Non-metals

   Metallic bonding makes metals tough; weak bonds without free electrons make non-metals brittle.

2. Q2. Graphite is a non-metal. Why does it conduct electricity?

   • Rule
   • Exception

   Its mobile electrons between layers carry the current.

Common mistakes

• ✗ No non-metal can conduct electricity.

  ✓ Graphite does, thanks to mobile electrons (p. 124).

• ✗ Every metal is a solid at room temperature.

  ✓ Mercury is the exception (table, p. 125).

• ✗ All metals are hard with high melting points.

  ✓ Generally, but the soft alkali metals are exceptions (p. 124).

MCQs

1. 1. The property of being drawn into wires is called:

   1. (a) Malleability
   2. (b) Ductility
   3. (c) Lustre
   4. (d) Brittleness
   Show answer

   (b) p. 124.

2. 2. Which non-metal conducts electricity?

   1. (a) Sulphur
   2. (b) Graphite
   3. (c) Phosphorus
   4. (d) Iodine
   Show answer

   (b) Mobile electrons between layers (p. 124).

3. 3. Which metal is not a solid at room temperature?

   1. (a) Iron
   2. (b) Copper
   3. (c) Mercury
   4. (d) Zinc
   Show answer

   (c) Table, p. 125.

4. 4. Non-metals break when hammered. This is because they are:

   1. (a) Ductile
   2. (b) Brittle
   3. (c) Lustrous
   4. (d) Malleable
   Show answer

   (b) p. 124.

5. 5. Which group of metals is the exception to 'metals are hard with high melting points'?

   1. (a) Transition metals
   2. (b) Alkali metals
   3. (c) Noble gases
   4. (d) Halogens
   Show answer

   (b) Soft alkali metals (p. 124).

Quick revision

• Metals are malleable, ductile, lustrous, strong and good conductors, with generally high melting points.
• Non-metals are brittle, dull, poor conductors, with low melting points and densities.
• Exceptions: graphite conducts, mercury is liquid, iodine shines, alkali metals are soft.