Atoms and Molecules

Everything around you is made of atoms: tiny particles too small to see, even with the most powerful normal microscopes. Atoms join together in different combinations to form molecules, which then build up into all the substances we see and touch. A glass of water is built from trillions of water molecules. A breath of air contains trillions of nitrogen and oxygen molecules. Even you are made of atoms: approximately 10 to the power of 27 atoms (a thousand trillion trillion) arranged into the cells of your body. Atoms are the alphabet from which everything in the universe is spelled out.

• What an atom isSmallest unit of an elementCannot be split chemically
• Three main partsProton, neutron, electron
• Atom sizeApproximately 0.1 nanometres1 millionth of 1 millimetre
• Atoms in your bodyRoughly 10^27A thousand trillion trillion
• Elements118 knownEach is a different kind of atom
• MoleculeTwo or more atoms bondedLike H2O for water

What is an atom?

An atom is the smallest unit of a chemical element that still has the properties of that element. A single atom of gold is the smallest amount of gold you can have; split it into smaller parts and it stops being gold. Atoms are incredibly small (about 0.1 nanometres across), and a teaspoon of water contains roughly 10 to the power of 23 molecules: more than the number of grains of sand on every beach on Earth.

The three subatomic particles

Each atom is made of three smaller particles.

• Protons: in the nucleus (centre). Have a positive electric charge. The number of protons defines which element the atom is (1 proton = hydrogen, 6 protons = carbon, 26 protons = iron).
• Neutrons: also in the nucleus. No electric charge. The number of neutrons can vary, producing different "isotopes" of the same element.
• Electrons: tiny particles orbiting the nucleus in a fuzzy cloud. Have a negative electric charge. Almost all the chemistry of an atom (how it reacts with other atoms) comes from its electrons.

What is a molecule?

A molecule is a group of two or more atoms held together by chemical bonds. Molecules can be very simple (water, H2O, is just three atoms) or extraordinarily complex (a single DNA molecule in your cells can contain billions of atoms). When chemists describe a molecule, they use a chemical formula listing the atoms it contains:

• H2O: water (2 hydrogens + 1 oxygen).
• CO2: carbon dioxide (1 carbon + 2 oxygens).
• NaCl: salt (1 sodium + 1 chlorine).
• C6H12O6: glucose, the sugar that powers your cells (6 carbons + 12 hydrogens + 6 oxygens).

Fact Atoms are almost entirely empty space. The tiny dense nucleus is roughly 1/100,000th the diameter of the whole atom. If an atom were the size of a football stadium, the nucleus would be the size of a marble in the middle. The electron cloud spread around it is mostly nothing. The "solid" feeling of touching anything is just the electrons of your skin repelling the electrons of whatever you are touching.

Why elements behave differently

The number of protons in an atom's nucleus determines which element it is. The arrangement of electrons around it determines how the atom reacts with other atoms. Atoms with full outer electron shells (like the noble gases) are stable and rarely react. Atoms with just one or two outer electrons (like sodium) easily give them away. Atoms missing just one or two outer electrons (like chlorine) eagerly grab them. When opposite types meet, they form ionic bonds: sodium and chlorine become salt (NaCl).

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