Atoms and Molecules

The hidden building blocks of everything

Pick up any object near you — a pencil, a glass of water, your own hand. Now imagine cutting it in half, then in half again, and again, and again, getting smaller and smaller. Could you keep doing that forever? More than two thousand years ago, Greek thinkers asked exactly this question. One of them, Democritus, suggested that eventually you would reach a piece so small it could not be cut any further. He called it atomos, meaning "uncuttable." From that ancient idea comes our word atom.

He was remarkably close to the truth. Everything around you — every solid, liquid and gas, every living and non-living thing — is built from atoms. They are unimaginably tiny: a single drop of water contains more atoms than there are stars in the entire visible universe. Understanding atoms and how they join together is the foundation of chemistry, and it explains why the world contains such an astonishing variety of materials.

Inside an atom

For a long time atoms were thought to be the smallest things that exist. We now know they are made of even smaller parts. Every atom has two regions: a dense centre called the nucleus, and a surrounding cloud of moving particles.

In the nucleus sit two kinds of particle:

• Protons, which carry a positive electric charge.
• Neutrons, which carry no charge at all (they are neutral).

Whizzing around the nucleus are tiny, light particles called:

• Electrons, which carry a negative electric charge.

A useful way to picture it is like a tiny solar system: the nucleus is like the Sun at the centre, and the electrons move around it like planets — though in reality electrons move in fuzzy clouds rather than neat circles. Atoms are mostly empty space; if the nucleus were the size of a marble, the nearest electrons would be hundreds of metres away.

Normally an atom has equal numbers of protons and electrons, so the positive and negative charges balance and the atom overall is neutral.

What makes one element different from another

Here is the single most important fact about atoms: the number of protons decides what kind of atom it is. This number is called the atomic number.

An atom with 1 proton is hydrogen — the lightest, simplest element. An atom with 6 protons is always carbon, the element at the heart of all living things. An atom with 8 protons is oxygen, the gas you need to breathe. Change the number of protons and you change the element entirely.

A substance made of just one kind of atom is called an element. Gold, oxygen, iron, helium and carbon are all elements. So far scientists have identified more than 110 elements, and they are organised in one of the most powerful charts in all of science.

The periodic table

The periodic table is a grid that arranges every known element in order of atomic number, from hydrogen (1) onward. It was first arranged into a meaningful pattern by Dmitri Mendeleev in 1869, and it is so well designed that he could predict elements that had not even been discovered yet.

The table is not just a list — its layout reveals patterns. Elements in the same vertical column (called a group) behave in similar ways. For example, the metals in the first column react strongly with water, while the gases in the last column barely react with anything. The periodic table lets chemists predict how an element will behave just from where it sits.

When atoms join: molecules and compounds

Atoms rarely sit alone. They tend to bond together, sharing or swapping their outer electrons to become more stable. When two or more atoms join, they form a molecule.

Some molecules are made of atoms of the same element. The oxygen in the air, for example, usually travels as O₂ — two oxygen atoms bonded together. But things get really interesting when different elements bond. A substance made of two or more different elements joined chemically is called a compound.

The most famous example is water. A water molecule is written H₂O: two hydrogen atoms bonded to one oxygen atom. On their own, hydrogen and oxygen are both gases — and hydrogen is highly flammable. Yet bond them in this exact way and you get a liquid that puts out fires. This is the magic of chemistry: bonding atoms creates a brand-new substance with brand-new properties.

An even more striking example is ordinary table salt, the compound sodium chloride (NaCl). Sodium on its own is a soft metal so reactive it bursts into flame in water. Chlorine on its own is a poisonous green gas. Yet bond a sodium atom to a chlorine atom and you get harmless, edible salt that you sprinkle on your food. The new compound bears no resemblance to the dangerous elements it came from.

It is important not to confuse a compound with a mixture. In a compound, atoms are chemically joined and a new substance forms. In a mixture, substances are simply mixed and keep their own properties. You can explore that difference in Mixtures and Solutions.

Why this matters

Almost every change you see — wood burning, iron rusting, bread baking, your body digesting food — is atoms breaking old bonds and forming new ones. The carbon atoms that cycle through plants, animals and the air are the same atoms rearranging into different molecules, a journey traced in The Carbon Cycle. Medicines, fuels, plastics and the food you eat all exist because chemists understand how atoms bond. Atoms are the alphabet of the universe, and molecules are the words they spell.

Try this — build molecule models

You do not need a laboratory to model molecules. Gather some small balls of modelling clay (or marshmallows or gumdrops) in two colours and some toothpicks. Let one colour be oxygen and another be hydrogen. Now build a water molecule: push two hydrogen balls onto one oxygen ball using toothpicks as bonds — that is H₂O. Next try carbon dioxide, CO₂: one carbon (use a third colour) with two oxygen atoms attached. Notice that the same atoms can be arranged in different ways to make different molecules. Take a photo of your models and label each one. This is exactly how chemists picture the invisible architecture of matter — never eating the clay version, of course.

From the tiniest atom to the grandest mountain, everything is built from these astonishing particles. Learn how they bond, and you begin to understand how the entire material world is put together.