3.3.1 The central part of an atom

The central part of an atom, which makes up most of its mass, is called the nucleus; this is surrounded by an ‘electron cloud’, which largely determines how the atom reacts with other atoms or molecules. The nucleus of an atom is made up of building blocks called protons and neutrons. The number of protons determines what element the atom actually is. An atom with one proton is hydrogen, and an atom with eight protons is oxygen.

However, the number of neutrons in the nucleus of an atom can vary. Oxygen exists in its natural state with eight protons and either eight, nine or ten neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. The most abundant oxygen isotope, with eight protons and eight neutrons, is called oxygen-16 (8 protons + 8 neutrons), the oxygen isotope which has eight protons and nine neutrons is oxygen-17 (8 protons + 9 neutrons), and the oxygen isotope which has eight protons and ten neutrons is called oxygen-18 (8 protons + 10 neutrons).