Atoms, molecules and the structure of the atom

Cut a piece of chalk in half, then in half again, and again. Is there an end? The Indian philosopher Kanada and the Greek thinker Democritus both argued, long ago, that there must be a smallest particle that cannot be divided further. Today we call it the atom.

Atoms and elements

An atom is the smallest particle of an element that can take part in a chemical reaction. An element is a pure substance made of only one kind of atom: hydrogen, oxygen, carbon, iron, gold. In 1808 John Dalton put forward his atomic theory: all matter is made of atoms, atoms of one element are alike, and atoms combine in simple whole-number ratios to form compounds. Atoms are unimaginably small; millions would fit across the full stop at the end of this sentence.

Each element has a symbol of one or two letters: H for hydrogen, O for oxygen, C for carbon. Some come from Latin names: Na for sodium (natrium), Fe for iron (ferrum), Au for gold (aurum).

Molecules and compounds

Atoms usually do not exist alone. They join to form molecules. A molecule is the smallest particle of a substance that can exist independently. Two oxygen atoms form a molecule of oxygen gas, O₂. When atoms of different elements combine in a fixed ratio, the substance is a compound. A molecule of water, H₂O, has two atoms of hydrogen and one of oxygen. Carbon dioxide is CO₂, and common salt is NaCl.

A chemical formula tells us which elements are present and how many atoms of each.

Inside the atom

Dalton thought the atom was solid and indivisible. Later experiments showed that it has smaller parts:

Protons and neutrons have nearly the same mass. An electron is about 1,800 times lighter than a proton.

Models of the atom

• Thomson imagined the atom as a ball of positive charge with electrons stuck in it, like seeds in a watermelon.
• Rutherford fired alpha particles at a thin gold foil. Most went straight through, but a few bounced back. He concluded that the atom is mostly empty space, with all its positive charge and nearly all its mass packed into a tiny centre, the nucleus.
• Bohr proposed that electrons move around the nucleus only in certain fixed paths called shells or energy levels, named K, L, M, N. The shells can hold at most 2, 8, 18 and 32 electrons, following the rule 2n².

Atomic number and mass number

The atomic number (Z) is the number of protons in an atom. It decides which element it is: every atom with 6 protons is carbon. An atom is neutral, so it has as many electrons as protons.

The mass number (A) is the total number of protons and neutrons. So the number of neutrons = A − Z. Sodium has Z = 11 and A = 23: it has 11 protons, 11 electrons and 12 neutrons, and its electrons are arranged as 2, 8, 1.

The electrons in the outermost shell are the valence electrons. They decide how an atom combines with others. Atoms of the same element with different numbers of neutrons are called isotopes, for example carbon-12 and carbon-14.