Atomic toolkit

Atoms contain protons and neutrons in a tiny central nucleus, with electrons around it. A proton has relative charge +1 and relative mass 1; a neutron has charge 0 and relative mass 1; an electron has charge −1 and a very small relative mass (about 1/1836). The atomic number, Z, is the number of protons. The mass number, A, is the total number of protons and neutrons. In a neutral atom, the number of electrons equals the number of protons. Isotopes are atoms of the same element with different numbers of neutrons. An ion forms when an atom gains or loses electrons.