Introduction to Atoms

The building blocks of everything

Every object around you — this screen, the air you breathe, your own body — is built from atoms. An atom is the smallest unit of an element that still behaves like that element. They are unimaginably small: about 0.1 nanometres across, so tiny that a single grain of sand contains more atoms than there are stars you could ever count.

For a long time people thought atoms were solid, indivisible balls. We now know they have an inner structure, and understanding it explains chemistry, electricity, and much of physics. Electricity itself depends on these particles — see electricity basics.

Inside an atom

An atom has two regions: a central nucleus and a surrounding cloud of electrons.

The nucleus is tiny but holds almost all the atom's mass. It contains two kinds of particle:

• Protons — positive charge (+1), found in the nucleus.
• Neutrons — no charge (0), also in the nucleus.

Whizzing around the nucleus, in regions called energy levels or shells, are the electrons:

• Electrons — negative charge (−1), with almost no mass.

Particle	Charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	~1/1836	Energy levels

In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges cancel out.

A remarkable fact: the atom is mostly empty space. If the nucleus were the size of a marble, the nearest electrons would be hundreds of metres away.

Atomic number and mass number

Two numbers identify any atom.

• Atomic number (Z) — the number of protons. This defines the element. Every hydrogen atom has 1 proton; every carbon atom has 6. Change the number of protons and you change the element entirely.
• Mass number (A) — the total number of protons + neutrons in the nucleus.

So the number of neutrons is simply:

neutrons = mass number − atomic number

For example, a typical carbon atom has atomic number 6 and mass number 12, so it has 6 protons and 12 − 6 = 6 neutrons.

Isotopes and ions

Two important variations exist.

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. Carbon-12 and carbon-14 are both carbon — each has 6 protons — but carbon-14 has two extra neutrons. Isotopes behave the same chemically but differ in mass, and some are radioactive, which makes carbon-14 useful for dating ancient remains.

Ions form when an atom gains or loses electrons, upsetting the balance of charge:

• Lose electrons → more protons than electrons → a positive ion (cation).
• Gain electrons → more electrons than protons → a negative ion (anion).

Ions are central to how electric current flows and how chemical bonds form.

How our model changed

Science updates its picture of the atom as new evidence appears.

1. Dalton (1803) — atoms are tiny, solid, indivisible spheres.
2. Thomson (1897) — discovered electrons; proposed the "plum pudding" model, electrons dotted through a positive blob.
3. Rutherford (1911) — his gold-foil experiment showed a tiny, dense, positive nucleus at the centre.
4. Bohr (1913) — electrons orbit the nucleus in fixed energy levels.
5. Modern model — electrons exist in fuzzy regions of probability, not neat orbits.

This is science working as it should: models are revised when experiments demand it.

Worked example: reading an atom

A sodium atom has an atomic number of 11 and a mass number of 23. Describe its particles.

Protons = atomic number = 11 Electrons (neutral atom) = 11 Neutrons = mass number − atomic number = 23 − 11 = 12

Now suppose this sodium atom loses one electron. It now has 11 protons but only 10 electrons, giving an overall charge of +1 — a sodium ion, written Na⁺.

Try it yourself! 🔬

Model static charge moving electrons.

1. Blow up a balloon and tie it. Rub it on a wool jumper or your hair for about 20 seconds.
2. Rubbing transfers electrons onto the balloon, giving it a negative charge.
3. Hold the balloon near a thin stream of water from a tap, or near small pieces of torn paper.
4. Watch the water bend or the paper jump toward the balloon, pulled by the charge.

You have just moved electrons from one object to another — the same particles that orbit every atom.