An atom is the basic unit of matter. It is the smallest particle of an element that retains the chemical properties of that element. Atoms consist of a nucleus, which contains positively charged protons and uncharged neutrons, and negatively charged electrons that orbit the nucleus. The number of protons in the nucleus determines the element’s atomic number, while the total number of protons and neutrons determines its atomic mass. Atoms combine to form molecules and compounds through chemical reactions.