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There are 118 elements in the universe.

By itself, this is nowhere near enough to create the stunning complexity of the world and its life around us.

But most elements attach to each other, creating new building blocks called ‘molecules’.

The number of possible molecules is too many to count, but we know that there’s a minimum of 1060 different types. That’s 10 with 60 zeroes after it, a stupendous number that’s far greater than the number of grains of sand on every beach, or even of stars in the universe. This range of possible combinations is called the ‘chemical space’.

Almost every piece of every living thing, from DNA to proteins to the membrane of a dragonfly’s wing, is a molecule from this space.

How it happens is a trick of chemistry, and is one of the most important phenomena in the universe. It all comes from the push and pull of the charge of an atom’s protons and electrons (which comes from the electromagnetic force).

Electrons have a negative charge and protons have a positive charge. A normal atom has an equal number of both, giving the whole atom a neutral charge.

But while protons bind together tightly in the centre of the atom, electrons buzz around the outside in multiple layers of clouds.

The electron cloud layers of a monster-sized atom, called hassium. Protons are clustered in the centre. Image source: Technology UK

Each layer has a certain capacity of electrons that it can hold. For example, the first layer can hold two, the second layer can hold eight.

Electrons fill the lowest layer first before moving up to the next one. The periodic table is arranged around this idea; the first row has two entries, the second has eight, etc.

So far so good, but here’s the kicker. Unfilled outer layers are unstable. Atoms want to fill them up, or get rid of them. This is the basis of bonds between atoms. There are two ways this happens. They give and receive electrons (ionic bonds), or they share them (covalent bonds).

Ionic bonds

An ionic bond is an atom giving an electron to another. For instance, Sodium Fluoride (NaF) has one sodium atom and one fluorine atom. The sodium gives one electron to the fluorine. Sodium loses its incomplete outer layer, and fluorine completes its own.

But, this comes with a consequence. While they have changed their electrons, each atom still has the same amount of protons as it did before. This means that the atom is no longer neutral.

The sodium how has an overall positive charge, and the fluorine has a negative one. Because they have a charge, they are now called ‘ions’. Because of their different charges, they are attracted to each other and stick together, in an ‘ionic bond’.

Sodium (Na) giving an electron to Fluorine (F) to complete their outer layers, which creates a charged attraction.

Covalent bonds

Covalent bonds come from atoms sharing electrons. It completes the outer electron layers of both atoms at the same time, but locks them into a close relationship.

E.g. Oxygen atoms have six electrons in their outer electron layer and want to gain two more to complete it. They will often share two of their electrons with other oxygen atoms.

Two oxygen atoms sharing electrons in a covalent bond. Source: Nathaniel S at ThingLink

Putting it together

Atoms joined with ionic bonds tend to form a big lattice structure, like a crystal. When you put them in water they’ll dissolve, as water molecules interfere with the charge that holds the charge together.

Ions tend to attract each other to form a giant lattice. Image source: Get Revising UK

Atoms joined with covalent bonds tend to form independent molecules. Covalent molecules can get extremely long and complex, and are the basis of much of the chemistry that underlies all of life on our planet.

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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.