Atoms
- An atom has a tiny central nucleus containing protons and neutrons, with electrons in shells around it.
- Atoms have no overall charge because they have equal numbers of protons and electrons.
- The radius of an atom is about 0.1 nm (1 × 10⁻¹⁰ m). The nucleus is less than 1/10 000 of that size.
Protons, neutrons and electrons
- Proton: relative mass 1, relative charge +1.
- Neutron: relative mass 1, relative charge 0.
- Electron: very small relative mass (about 1/2000), relative charge −1.
Atomic number and mass number
- Atomic number = the number of protons. Every atom of an element has the same atomic number.
- Mass number = the number of protons + neutrons.
- Number of neutrons = mass number − atomic number.
Isotopes
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
- For example, chlorine-35 and chlorine-37 both have 17 protons.
- Relative atomic mass is the average mass of an element's isotopes, taking their abundance into account.
Elements, compounds and mixtures
- An element contains only one type of atom.
- A compound contains two or more elements chemically combined in fixed proportions.
- A mixture contains substances that are not chemically joined. They can be separated by physical methods such as filtration or distillation.
Key terms
- Atom
- The smallest part of an element that can exist.
- Nucleus
- The centre of an atom, containing protons and neutrons.
- Proton
- A particle in the nucleus with relative mass 1 and charge +1.
- Neutron
- A particle in the nucleus with relative mass 1 and no charge.
- Electron
- A tiny particle with charge −1, found in shells around the nucleus.
- Atomic number
- The number of protons in an atom.
- Mass number
- The total number of protons and neutrons in an atom.
- Isotope
- Atoms of the same element with different numbers of neutrons.
- Element
- A substance made of only one type of atom.
- Compound
- Two or more elements chemically combined in fixed proportions.