1. The elements
An element consists of several identical atoms and is a substance that can not be further decomposed by chemical means. The mass of elements is determined only by the number of protons and neutrons, since the electron mass is negligible. Hydrogen with one proton and no neutron has the mass number 1, the next heavier element, helium, has the mass number 4 (2 protons + 2 neutrons).
The elements are usually named with the initials of their Latin or Greek names (from latin hydrogenium: hydrogen H, from greek lithos: lithium Li).