Physical Properties of Non-metals

Introduction

Oxygen in the air, the carbon in a pencil, the sulfur in a volcano's yellow crust and the chlorine used to treat water are all non-metals . Although there are far fewer non-metals than metals, they make up most of the air, the oceans and living things. Their physical properties are almost the opposite of those of metals: most are dull, brittle when solid, poor conductors and easy to melt or boil.

Core explanation

States at room temperature. Non-metals show great variety. Eleven are gases at room temperature (hydrogen, nitrogen, oxygen, fluorine, chlorine and the six noble gases). One is a liquid (bromine). The rest are solids, such as carbon, phosphorus, sulfur, selenium and iodine. Compare this with metals, where every element except mercury is a solid.

Typical properties of non-metals:

Property Non-metals Metals (for comparison) --- --- --- Appearance Dull (not shiny) Lustrous Electrical conductivity Poor — insulators Good Thermal conductivity Poor Good Solids when hit Brittle — shatter or crumble Malleable Density Usually low Usually high Melting and boiling points Usually low Usually high Sound when struck Not sonorous Sonorous

For example, sulfur is a yellow solid with a density of about 2.0 g/cm³ that melts at around 115 °C and crumbles easily. Oxygen boils at −183 °C, and nitrogen at −196 °C.

Why the difference? Non-metal atoms have many valence electrons and hold them tightly. Instead of releasing electrons into a shared sea, they share pairs of electrons with each other in covalent bonds. Most non-metals exist as small molecules : O₂, N₂, Cl₂, P₄, S₈. The covalent bonds inside each molecule are strong, but the forces between molecules are weak. Very little energy is needed to separate the molecules, so melting and boiling points are low.

No free electrons. Because the electrons are locked into bonds between particular atoms, there are no delocalised electrons to carry a current. That is why non-metals are insulators. With no electron sea to hold sliding layers together, a solid non-metal breaks when stressed — it is brittle.

Giant covalent exceptions. A few non-metal solids, such as diamond (a form of carbon), are not made of small molecules but of a huge network of atoms all joined by covalent bonds. These have very high melting points, although they remain brittle and diamond is still an electrical insulator.

Step-by-step reasoning

To explain why chlorine is a gas at room temperature:

1. Chlorine exists as Cl₂ molecules held together by a strong covalent bond. 2. Between separate Cl₂ molecules there are only weak intermolecular forces. 3. Little energy is needed to overcome these weak forces. 4. So the boiling point is low (about −34 °C), and chlorine is a gas at room temperature.

Visual explanation

Picture a sample of solid sulfur as a heap of separate crown-shaped rings, each ring made of eight tightly joined atoms. The rings rest against each other only loosely, like bags of marbles piled in a cupboard: each bag is well sealed, but the pile collapses easily.

Real-world analogy

A box of paperclip chains that are each firmly linked but not attached to one another is easy to tip out and scatter. The strong links are the covalent bonds inside molecules; the loose contact between chains is the weak attraction between molecules.

Real-world example

Plastic handles on saucepans and the rubber coating on electrical plugs are made from compounds of non-metals, mostly carbon and hydrogen. They are poor conductors of heat and electricity, so they protect your hand from heat and from electric shock.

Why?

Why are solid non-metals brittle? Their atoms are held in fixed positions by directional covalent bonds or by weak forces between molecules. When the solid is struck, these bonds or attractions break rather than rearranging, and there is no electron sea to rebond the pieces, so the solid cracks.

Common misconception

"Non-metals are weak because their bonds are weak." The covalent bonds within a non-metal molecule are very strong. It is the attraction between neighbouring molecules that is weak, and that is what breaks when a non-metal melts or boils.

Worked example

Question: An element is a yellow solid that melts at 115 °C, crumbles when hit and does not light a bulb in a circuit. Is it a metal or a non-metal? Which element might it be?

Reasoning: Dull appearance, low melting point, brittleness and electrical insulation are all non-metal properties. A yellow, brittle solid that melts just above 100 °C matches sulfur.

Answer: A non-metal, most likely sulfur.

Quick check

1. Name the only non-metal that is a liquid at room temperature. Answer: Bromine.

Exam focus

Examiners often ask you to compare metals and non-metals in a table. Give matched pairs: shiny versus dull, conductor versus insulator, malleable versus brittle, high versus low melting point. Mention graphite and diamond carefully, as they are the usual exceptions.

Advanced insight

Some non-metals exist in more than one structural form, called allotropes , which can have very different physical properties. Oxygen can form O₂ and ozone, O₃. Phosphorus has a white form made of P₄ molecules that must be stored carefully because it can ignite in air, and a much more stable red form with a chain-like structure. Carbon forms diamond, graphite and fullerenes.

Summary

Non-metals are usually dull, brittle when solid, poor conductors of heat and electricity, and have low densities and low melting and boiling points. Most exist as small covalent molecules with strong bonds inside and weak forces between them. Because their electrons are held in bonds rather than being delocalised, they cannot conduct. Giant covalent solids such as diamond are high-melting exceptions.

Practice questions

1. Give three physical properties that distinguish non-metals from metals. Answer: Non-metals are dull, brittle when solid and poor electrical conductors (also low melting points and low densities). 2. Explain why oxygen has a very low boiling point. Answer: Oxygen is made of O₂ molecules with only weak forces between them, so little energy is needed to separate the molecules. 3. Why do non-metals not conduct electricity? Answer: Their electrons are held in covalent bonds between specific atoms, so there are no delocalised electrons free to carry charge. 4. Diamond is a non-metal with a very high melting point. Explain why. Answer: Diamond is a giant covalent structure, so melting requires breaking a huge number of strong covalent bonds.