Nucleus

The nucleus is the small, dense, positively charged centre of an atom, containing the protons and neutrons. Malay: nukleus · Chinese: 原子核.

The nucleus is positively charged because it contains the protons, each with a charge of +1, while the neutrons add mass but no charge. Since a proton and a neutron each have a relative mass of 1 and the electrons are almost massless, nearly all of the atom’s mass is packed into the nucleus. The nucleus is extremely tiny compared with the whole atom, and the electrons move around it in shells at relatively great distances.

Example. In an oxygen atom with proton number 8 and nucleon number 16, the nucleus contains 8 protons and 8 neutrons, while the 8 electrons occupy the shells outside it. Rutherford’s alpha-particle scattering (the gold-foil experiment) in the development of the atomic model showed that the atom is mostly empty space with this dense, positive nucleus at the centre.

Confusion to avoid. Do not confuse the nucleus with a nucleon or with the proton number. The nucleus is the whole central region; a nucleon is a single particle inside it (a proton or a neutron); the proton number is just the count of protons. Remember also that electrons are not in the nucleus, they are in the shells around it, which is why they do not count towards the nucleon number.

In the exam, questions on the nucleus often give you the proton number and the nucleon number of an atom and ask for the number of protons, neutrons and electrons. The safe method is: protons equal the proton number, neutrons equal the nucleon number minus the proton number, and electrons equal the protons in a neutral atom. Keep the nucleons (protons and neutrons) inside the nucleus and the electrons outside it in your diagram.

Understanding the nucleus is what makes proton number, nucleon number and isotopes meaningful in the Matter and the Atomic Structure chapter, since all three describe what is inside it. When our teachers introduce atomic structure, they always start from a clearly labelled nucleus so the counting rules that follow make sense.