| English | Nucleus |
|---|---|
| Bahasa Melayu | Nukleus |
| 中文 | 原子核 |
Definition
The small, dense, positively charged centre of an atom, made up of protons and neutrons.
In exam terms
The nucleus is the small, dense, positively charged centre of an atom, made up of protons and neutrons (together called nucleons). Almost all the mass of an atom is concentrated in the nucleus, while its diameter is tiny compared with the whole atom.
The number of protons is the proton number Z and the total number of nucleons is the nucleon number A.
A nuclide is written as ᴬ𝗓X, for example ²³⁵₉₂U. A correct sentence: "The nucleus of a helium atom contains 2 protons and 2 neutrons, so its nucleon number is 4." There is no special SI unit; masses are often given in atomic mass units (u).
Do not confuse it with…
Do not confuse the nucleus with the whole atom. The atom includes the electrons orbiting outside, which carry the negative charge and take up almost all the volume; the nucleus is only the positive core.
Also distinguish proton number (Z) from nucleon number (A): number of neutrons = A − Z.
In Paper 2 you may be asked to state what a nucleus is made of, determine the number of neutrons from A and Z, or explain why the nucleus is positively charged.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)