An atom has a tiny, positively charged nucleus made of protons and neutrons. Electrons move around the nucleus at set distances called energy levels.

• proton: charge +1, relative mass 1, in the nucleus
• neutron: charge 0, relative mass 1, in the nucleus
• electron: charge −1, very small mass (about 1/2000), around the nucleus

An atom has the same number of electrons as protons, so it has no overall charge.

Sizes. The radius of an atom is about 1 × 10⁻¹⁰ m. The radius of the nucleus is about 10 000 times smaller, about 1 × 10⁻¹⁴ m, yet almost all the mass is there. Most of an atom is empty space.

1. A sodium atom has a radius of 0.18 nm, and 1 nm = 1 × 10⁻⁹ m
2. 0.18 × 10⁻⁹ m = 1.8 × 10⁻¹⁰ m
3. nucleus: 1.8 × 10⁻¹⁰ ÷ 10 000 = 1.8 × 10⁻¹⁴ m

Dividing by 10 000 is dividing by 10⁴: take 4 from the power. 10⁻¹⁰ becomes 10⁻¹⁴, not 10⁻⁶.

A nuclide is written with two numbers: the mass number on top (protons + neutrons) and the atomic number underneath (protons).

¹⁴₆C   carbon-14: 6 protons, 14 − 6 = 8 neutrons, 6 electrons in the atom

1. protons = atomic number (bottom)
2. neutrons = mass number − atomic number
3. electrons in a neutral atom = protons

The atomic number decides the element. Isotopes of an element have the same number of protons but different numbers of neutrons: carbon-12 and carbon-14 both have 6 protons, with 6 and 8 neutrons.

If an atom loses an outer electron it becomes a positive ion; if it gains one it becomes a negative ion. The nucleus does not change.

The top number is not the number of neutrons. Subtract the bottom number from it first.

Each new experiment either backed up the model or forced a change:

• Tiny spheres: atoms were once thought to be solid spheres that could not be divided.
• Plum pudding: after the electron was discovered, the atom was pictured as a ball of positive charge with electrons spread through it.
• Nuclear model: in the alpha particle scattering experiment, alpha particles were fired at thin gold foil. Most went straight through (mostly empty space); some were deflected (a positive centre); a very few bounced back (the mass and charge are packed into a tiny nucleus). The plum pudding model could not explain this.
• Bohr: electrons orbit at fixed distances, in energy levels. His calculations matched what experiments showed.
• Protons and the neutron: the positive charge was found to come from protons. About 20 years after the nuclear model, James Chadwick showed the nucleus also contains neutrons.

Electrons can move between energy levels: absorbing electromagnetic radiation lifts one to a higher level, and dropping back emits radiation.