What is an atom?

The structure of these microscopic building blocks drives the chemistry of the universe

(Image source: Getty Images)

Atoms are the building blocks of everything around you, from the cells in your body to the cup of tea in your hand. Made from elementary particles and held together by fundamental forces, they drive the chemistry of the universe.

Each one has a tiny, positively charged nucleus, balanced by a cloud of negatively charged electrons. These determine the atom’s properties. Split an atomic nucleus open and you’ll find subatomic particles called protons and neutrons. The simplest atom, hydrogen, has just one proton and no neutrons. But as you move up through the periodic table the number of protons rises one by one. With each extra proton, the behaviour of the atom changes.

Protons have a positive charge, so for every extra one added the atom also gains another negative electron. These electrons cluster in fixed zones around the atomic nucleus, known as orbitals or shells. Each shell can only hold a certain number of electrons. So when a shell fills up, the next electrons have to start a new one.

Atoms are most stable when their electron shells are full, but most elements have electrons to spare. This is the driving force behind the chemistry we see around us. To complete half-filled shells, atoms form chemical bonds. They can either share their electrons to form covalent bonds, or they can steal electrons from one another to form ionic bonds. 

Covalent bonds are common inside our own bodies. Carbon atoms need four electrons to complete their outer shell, so they share with up to four other atoms, including other carbons. This makes them perfect for forming large biological molecules like DNA.

Ionic bonds are more commonly found in the ground. They happen when a non-metal steals electrons from a metal, becoming negatively charged. At the same time, the metal becomes positively charged. These ions attract one another, sticking together in repeating structures, like salt crystals.

Bonds between atoms can break and reform, allowing the elements to perform some amazing chemical tricks. They combine to produce larger compounds, decompose to form smaller ones, or swap one element for another to produce a chemical with different properties. These dynamic interactions are happening all around us, all the time.

Atom anatomy : Oxygen

(Image source: Getty Images)


The atomic nucleus at the centre of an oxygen atom contains eight protons and eight neutrons


Colour coded as yellow, these subatomic particles have a mass of 1 and a charge of 0.


Colour coded as red, These subatomic particles have a mass of 1 and a charge of +1.


An oxygen atom contains eight electrons, arranged into two orbital shells.


Situated within the two shells, these subatomic particles have a mass of almost zero and a charge of -1.

Unpaired electron

Two electrons in the outer shell don’t have a partner. This allows oxygen atoms to form chemical bonds.

This article was originally published in How It Works issue 126, written by Laura Mears

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