1. The Elements
An element consists of many identical atoms (meaning atoms with the same number of protons, i.e. the same atomic number) and is a substance that cannot be broken down further by chemical means. The mass of an element is determined solely by the number of protons and neutrons, since the mass of the electrons is negligibly small. Hydrogen, with one proton and no neutrons, has a mass number of 1; the next heavier element, helium, has a mass number of 4 (2 protons + 2 neutrons). The mass number gives the number of particles in the nucleus. Multiplying it by the atomic mass unit u = 1.660·10-27 kg gives approximately the mass of an atom – for helium, 6.64·10-27 kg.
Chemical elements are usually named after the initial letters of their Latin or Greek names (hydrogen [H] from Latin hydrogenium, lithium [Li] from Greek lithos).