Atoms are very small, with a radius of about 0.1 nm (1 × 10-10 m). The nucleus is much smaller still: its radius is less than 1/10 000 of the radius of the atom (about 1 × 10-14 m). If an atom were 100 m wide, like a football pitch, its nucleus would be less than 1 cm across. Almost all of the mass of an atom is in the nucleus. A proton and a neutron each have a relative mass of 1, and an electron has a very small relative mass.
The sum of the protons and neutrons in an atom is its mass number. Atoms of the same element can have different numbers of neutrons. These atoms are called isotopes of that element. They have the same atomic number but different mass numbers. In the symbol for an atom, the mass number is written at the top left and the atomic number at the bottom left of the chemical symbol.
To find the particles in an atom or ion, use the numbers. The number of protons is the atomic number, the number of neutrons is mass number minus atomic number, and a neutral atom has as many electrons as protons. A sodium atom has atomic number 11 and mass number 23, so it has 11 protons, 12 neutrons and 11 electrons. A sodium ion, Na+, has lost one electron, so it has 10 electrons. A chlorine atom with atomic number 17 and mass number 35 has 18 neutrons.