Covalent bonding and giant covalent structures – GCSE Combined Science Chemistry

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Atoms of non-metals do not give electrons away. They share them. This sharing makes covalent bonds, and it gives us water, plastics, diamond and the graphite in a pencil.

It matters because the way atoms are joined decides how a substance behaves. Diamond is hard but graphite is soft, and both are only carbon. Once you see why, you can explain many properties.

By the end you will be able to describe covalent bonding and simple molecules. You will explain the properties of diamond, graphite and silicon dioxide. You will also describe graphene and fullerenes.

Covalent bonds

A covalent bond forms when two non-metal atoms share a pair of electrons. Each atom gets a full outer shell. The shared pair is attracted strongly to both nuclei, so covalent bonds are strong.

The number of bonds depends on the outer-shell electrons. In hydrogen, H₂, each atom makes one bond. In water, H₂O, oxygen makes two. In ammonia, NH₃, nitrogen makes three. In methane, CH₄, carbon makes four.

Think of two friends who each need a phone charger. They share one, so both are happy and they stay close.

Simple molecules

Simple molecules have a small number of atoms joined by covalent bonds. Examples are H₂, O₂, N₂, Cl₂, HCl, H₂O, NH₃, CH₄ and CO₂.

Inside each molecule the covalent bonds are strong. Between one molecule and the next there are only weak forces. These are called intermolecular forces.

Think of each molecule as a tight friendship group. The friends inside the group stay together, but the groups hardly cling to each other.

A dot and cross diagram shows the outer electrons of each atom, one atom as dots and the other as crosses. A shared pair is drawn where the outer shells overlap, with one dot and one cross. In H₂ there is one shared pair. In water, oxygen shares one pair with each hydrogen and has two other pairs that are not shared. In methane, carbon shares four pairs.

Every model has limits. A dot and cross diagram shows which electrons are shared but not the shape of the molecule or the sizes of the atoms. A ball and stick model shows the shape and the bonds, but the sticks are not real and there is no gap between real atoms. A two-dimensional drawing cannot show the three-dimensional shape. A space-filling model shows the size of atoms better but hides the bonds.

Some covalent molecules are very long. Many small molecules called monomers can join one after another by addition polymerisation to make a polymer. A simple polymer such as polyethene, also written poly(ethene), is a long chain made of the same repeating unit, and the atoms along the chain are held by strong covalent bonds.

Properties of simple molecules

When a simple molecular substance melts or boils, only the weak intermolecular forces are overcome. The covalent bonds stay. So little energy is needed. Melting and boiling points are low, and many are gases or liquids at room temperature.

Bigger molecules have stronger intermolecular forces, so they have higher melting and boiling points. Simple molecular substances do not conduct electricity. They have no free electrons and no ions.

Polymers have very long molecules with strong covalent bonds inside. The forces between the long chains are stronger than between small molecules. So most polymers are solid at room temperature.

Diamond and silicon dioxide

Some covalent substances are giant structures. They have a huge network of atoms joined by strong covalent bonds. Diamond is made only of carbon. Each carbon atom forms four covalent bonds to four other carbon atoms.

Diamond is very hard and has a very high melting point. Every bond is strong and many must be broken. It does not conduct electricity because it has no free electrons or ions. It is used for cutting tools.

Silicon dioxide, SiO₂, is the main compound in sand. Each silicon atom is bonded to four oxygen atoms. Like diamond, it is hard, has a very high melting point and does not conduct electricity.

Graphite

In graphite each carbon atom forms three covalent bonds. This makes flat layers of hexagons. The bars show that diamond forms four bonds and graphite forms three. The spare electron from each atom is delocalised, which means it is free to move.

The layers are held together by weak forces, so they slide over each other, like cards in a deck. Graphite is soft and slippery. It is used as a lubricant and in pencils.

The delocalised electrons let it conduct electricity, so it is used for electrodes. It still has a high melting point because the bonds in the layers are strong.

Graphene and fullerenes

Graphene is a single layer of graphite, one atom thick. It is very strong and light, and it conducts electricity. It is useful in electronics and in composites, which are materials mixed together to make something stronger.

Fullerenes are hollow molecules of carbon atoms. Buckminsterfullerene is C₆₀. Carbon nanotubes are cylinders of carbon atoms. They are very strong and conduct electricity.

They are used to deliver drugs, as catalysts, as lubricants and to strengthen materials.

Recap

A covalent bond is a shared pair of electrons between non-metal atoms.

Simple molecules have low melting and boiling points because only the weak forces between molecules are overcome.

Diamond and silicon dioxide have giant networks of strong bonds. Graphite has layers and delocalised electrons.

Graphene is one layer of graphite. Fullerenes and nanotubes are hollow carbon structures.

In an exam, name the structure, name the bonding, then link it to the property.