How atoms bond

An atom on its own is electrically neutral: the negative charges of its electrons exactly balance the positive charges of the protons in its nucleus. Take an electron away, or add one, and the balance is lost. An atom that has lost or gained electrons, and so carries a charge, is called an ion.

Sodium has just one electron in its outer shell, and the shell beneath is full. The easiest way for sodium to reach a full outer shell is to give that one electron away. It is then one electron short, so the protons outnumber the electrons and the ion is positive. Chlorine has seven outer electrons and room for one more. It takes an extra electron, and the surplus makes its ion negative.

Bring sodium and chlorine together and each solves the other’s problem. A sodium atom hands its spare electron to a chlorine atom. Both now have full outer shells, and both are ions: the sodium positive, the chlorine negative and now called chloride. Opposite charges attract, so the two ions cling together. A bond made this way, by one atom handing electrons to another and the resulting ions attracting, is called an ionic bond.

The product is sodium chloride, ordinary table salt. Ionic bonds typically form between a metal, which gives electrons away easily, and a non-metal, which takes them. The result bears no resemblance to its ingredients. Sodium is a soft metal that reacts violently with water, and chlorine is a poisonous yellow-green gas. Their compound is something you sprinkle on food.

There is no such thing as a single molecule of salt. Each positive sodium ion attracts every negative chloride ion near it, so the ions stack in a regular pattern that repeats in every direction. This endless repeating arrangement is called a lattice, and a grain of salt is one, made of billions of billions of ions. Its cube shape is the lattice made visible.

The lattice explains how salt behaves. The attractions are strong, so salt melts only at 801 °C. Solid salt does not conduct electricity, because its ions are locked in place. Melt it or dissolve it in water and it conducts well, since the ions are then free to move. And a salt crystal shatters when struck. Shift one layer a single step and ions of the same charge sit side by side and push each other apart.

Two non-metal atoms cannot make an ionic bond with each other, because neither will give electrons up. Both need more. So they share. Each atom contributes one electron to a pair that sits between the two nuclei and counts towards both outer shells. A bond made of a shared pair of electrons is called a covalent bond.

Hydrogen shows it most simply. A hydrogen atom has one electron and needs two to fill its small shell. Two hydrogen atoms share their electrons, and each then has a full shell. The two atoms are now bound into one unit. A group of atoms held together by covalent bonds is called a molecule, and it is the smallest piece of a substance that still is that substance. A molecule of water is two hydrogen atoms, each sharing a pair of electrons with one oxygen atom.

When atoms of different elements bond, the result is a compound: a substance in which two or more elements are chemically joined. Water is a compound of hydrogen and oxygen, and salt of sodium and chlorine.

A compound is not a blend of its ingredients. It is a new substance with properties of its own. Hydrogen is a gas that burns explosively, and oxygen is the gas a fire needs in order to burn. Bond them and you get water, and it puts fire out.

A compound also has a fixed recipe. Every molecule of water has two hydrogen atoms and one oxygen atom, never any other number. The recipe is recorded in the compound’s chemical formula, which lists each element’s symbol and how many atoms of it there are. Water’s is H₂O, and salt’s is NaCl.

Some atoms need more than one extra electron, and one shared pair is not enough. Oxygen has six outer electrons and needs two more. Two oxygen atoms share two pairs between them, which is called a double bond. Nitrogen has five outer electrons and needs three more, so two nitrogen atoms share three pairs, a triple bond.

Each extra pair holds the atoms harder. The triple bond in a nitrogen molecule is one of the strongest bonds in chemistry, and it shapes the air you breathe. Air is about 78 per cent nitrogen and 21 per cent oxygen. You take in that nitrogen with every breath and breathe it straight out again, unchanged, because your body has no way to pull its two atoms apart.

Metal atoms have only a few outer electrons and hold them loosely. In a piece of metal the atoms do something neither ionic nor covalent. They all let go of their outer electrons, which drift freely through the whole piece like a sea, while the atoms sit in it in neat rows as positive ions. The attraction between the ions and the shared sea holds everything together. This is the metallic bond.

The sea explains what makes a metal a metal. Metals conduct electricity because their electrons move freely: push electrons in at one end of a wire and others flow out at the other. And metals bend instead of shattering as salt does. When a metal is hammered, its layers of atoms slide past each other, and the sea of electrons simply flows around them and keeps holding them together.

Not every blend of substances is a compound. Stir sand into water, or salt into water, and no new substance forms. The result is a mixture: two or more substances mingled but not chemically joined. Its recipe is not fixed, since you can add a little salt or a lot. And it can be separated without any chemical change. Boil seawater dry and the salt is left behind.

Many useful metals are mixtures too. Brass is copper blended with zinc, and bronze is copper with tin. Such a blend of metals is called an alloy. A sea of electrons holds its atoms together, as in any metal, but its proportions can be varied to suit the job, so it is a mixture. Water is always two hydrogen atoms to one oxygen. A fixed recipe is the mark of a compound, and a variable one the mark of a mixture.

How hard a solid is to melt shows how its particles are held. Salt melts at 801 °C and iron at 1,538 °C, because melting either one means loosening ionic or metallic bonds that run through the whole solid.

Sugar melts at about 186 °C, on an ordinary stove. Sugar is made of molecules. The covalent bonds inside each molecule are strong, but they are not what melting has to overcome. A sugar crystal is held together only by weak forces between its molecules, and a stove’s heat is enough to let them slide apart. (Sugar is not a perfectly clean case: at that heat some of its molecules also break up, which is why it browns into caramel.)

That is the pattern for substances made of separate molecules. Their melting points are low, and many, such as water and oxygen, are liquids or gases at room temperature.

Covalent bonds do not always make small molecules. In diamond, every carbon atom shares electrons with four neighbours, each of those with four more, and so on without end. The whole crystal is a single web of covalent bonds, one giant molecule.

There are no weak links in it. To scratch a diamond you would have to break the covalent bonds themselves, and those are among the strongest there are. So diamond is the hardest natural material known.

Bonding, then, explains the feel of things. Salt is brittle because of its lattice of ions. Copper bends because of its sea of electrons. Sugar melts easily because its molecules barely hold on to each other. And diamond is hard because it is bonded all the way through.