Chemical Bonds

A chemical bond is the force that holds two or more atoms together. Without bonds there would be no molecules, no compounds, no rocks, no plants, no animals, no people. Bonds form when atoms swap, share or transfer their outer electrons in order to reach a more stable arrangement. Different ways of doing this give rise to different types of bond. The three main ones are ionic, covalent and metallic.

  • What it doesGlues atoms togetherBy swapping or sharing electrons
  • Three main typesIonic, covalent, metallicPlus weaker forces between molecules
  • Ionic bondElectrons transferredMakes salt, minerals
  • Covalent bondElectrons sharedMakes water, sugar, DNA
  • Metallic bondSea of free electronsMakes metals shiny and conductive
  • Energy storedUp to 1,000 kJ/molReleased when bonds break in fuel

Why atoms bond

Atoms bond because their outer electron shell is happiest when full. A full outer shell is the most stable, lowest-energy arrangement. Atoms with full outer shells (the noble gases helium, neon, argon, krypton and xenon) do not bond easily; they are already content.

Other atoms have either too few or too many outer electrons. They can swap or share electrons with neighbours to fill their shells. The result is a chemical bond, and what was two unhappy atoms becomes one happy molecule.

Ionic bonds

An ionic bond is the strongest, most dramatic kind. One atom gives up one or more electrons to another atom. The donor becomes a positive ion, the receiver becomes a negative ion, and the two are pulled together by powerful electrical attraction.

The classic example is table salt (sodium chloride, NaCl).

  • Sodium has 1 electron in its outer shell. It wants to get rid of it.
  • Chlorine has 7 electrons in its outer shell. It needs one more to make 8.
  • Sodium hands its electron to chlorine. Sodium becomes a +1 ion. Chlorine becomes a -1 ion. They stick together strongly.

Ionic compounds are usually solid at room temperature, often crystals, often dissolve in water, and often conduct electricity when dissolved. Salt, chalk, baking soda and many minerals are ionic.

Covalent bonds

A covalent bond is more polite: the atoms share a pair of electrons rather than one taking from the other. Each shared pair counts as a single bond, holding the two atoms close together.

The water molecule (H2O) is held together by two covalent bonds: each hydrogen shares its single electron with the oxygen, and the oxygen contributes one of its own electrons to each pair. Result: two strong bonds, one stable molecule.

Covalent bonds form most of the molecules in living things: water, sugars, proteins, DNA, fats, vitamins. They also form gases (O2, N2, CO2) and many fuels (methane, petrol, alcohol).

Some atoms share more than one pair of electrons:

  • Single bond: 1 shared pair (like H-H in hydrogen gas).
  • Double bond: 2 shared pairs (like O=O in oxygen gas).
  • Triple bond: 3 shared pairs (like the very strong bond in nitrogen gas).
Fact The nitrogen molecules in the air (N2) are held together by a triple bond, one of the strongest bonds in nature. That is why nitrogen gas is so unreactive, even though it makes up 78 per cent of the air around you. Pulling that triple bond apart takes huge energy, which is why fixing nitrogen into fertiliser was one of the great chemistry achievements of the 20th century.

Metallic bonds

In metals, the outer electrons do not belong to any one atom. They are shared between all the atoms in a kind of "sea" that flows freely through the whole metal. This is called a metallic bond.

This explains some of the strange properties of metals:

  • Shiny: the free electrons reflect almost all the light that hits them.
  • Conducts electricity: free electrons can flow easily under an electric push.
  • Conducts heat: free electrons carry energy through the metal.
  • Bends and stretches without breaking: the metal atoms can slide past each other while the electron sea keeps everything stuck together.
Did you know? The strongest known chemical bond is the triple bond in carbon monoxide (CO). It takes about 1,072 kilojoules of energy per mole to break. That is more than the energy in a chocolate bar, packed into the gap between two atoms.

Weaker forces

Apart from the three main bond types, there are weaker attractions between molecules (rather than within them) that decide how substances behave:

  • Hydrogen bonds: weak links between molecules containing hydrogen and oxygen, nitrogen or fluorine. These hold water together and stabilise the DNA double helix.
  • Van der Waals forces: very weak short-range attractions between all atoms. These let gecko feet stick to walls and help non-stick frying pans work.
Try this Bake a cake and watch chemical bonds in action. The flour, eggs, sugar and butter start as separate molecules. As you mix them, hydrogen bonds form between the water in the eggs and the starch in the flour. Heat from the oven breaks some bonds (proteins unfold) and makes new ones (egg proteins link to form a solid network). The carbon dioxide trapped from the baking powder makes the cake rise. By the time it cools, the cake has totally new bonds and a new structure compared to its ingredients.

Breaking and making bonds

Chemical reactions are all about breaking old bonds and making new ones. Breaking a bond takes energy. Making a bond releases energy. If the new bonds release more energy than was used to break the old ones, the reaction is exothermic and gives out heat (like burning fuel). If breaking the old bonds takes more energy than the new ones release, the reaction is endothermic and absorbs heat (like cooking).

Deeper dive: why diamond and graphite are both pure carbon

One of the strangest demonstrations of how powerful chemical bonds are comes from carbon. Pure carbon atoms can join together in different ways and the results are totally different substances.

Diamond: each carbon atom is covalently bonded to four other carbons in a strong 3D network. Diamonds are the hardest natural material on Earth, do not conduct electricity, and are perfectly transparent.

Graphite (the soft stuff in pencils): each carbon atom is bonded to only three others, in flat sheets like chicken wire. The sheets slide easily over each other (which is why pencil marks rub off on paper). The leftover electrons form a sea between the sheets, so graphite conducts electricity (a rare property for a non-metal).

Graphene: a single sheet of graphite, just 1 atom thick. It was first isolated in 2004 and is one of the strongest, most conductive materials known. Engineers are racing to use it in flexible screens, super-fast computer chips and even artificial muscles.

Same element, same atoms. Different bonds, different worlds. That is the power of chemistry.

For more, see ions and what is a molecule.