Revision notes · Chemistry
All matter is made of atoms. An atom has a small, dense nucleus containing protons (positive, mass 1) and neutrons (no charge, mass 1), surrounded by electrons (negative, negligible mass) orbiting in energy levels (shells).
Atomic number = number of protons (defines the element)
Mass number = number of protons + neutrons
Number of neutrons = mass number − atomic number
When representing an element, the mass number and atomic number are often written alongside the element's symbol (nuclear notation), with the mass number at the top left and the atomic number at the bottom left. For example, carbon-12 can be written as 126C.
In a neutral atom, the number of electrons equals the number of protons.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties but different masses.