Atomic number, mass number and isotopes
Atoms contain protons, neutrons and electrons. The electrons are arranged in shells around the nucleus. The periodic table is a chart of all the elements arranged in increasing atomic number.
Structure of the atom
The nuclear model
Atoms contain three sub-atomic particles called protons, neutrons and electrons.
The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in shells around the nucleus.
Properties of sub-atomic particles
| Particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | | -1 |
The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall.
Atoms can lose or gain electrons. When they do, they form charged particles called ions:
- if an atom loses one or more electrons, it becomes a positively charged ion
- if an atom gains one or more electrons, it becomes a negatively charged ion