atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons. atomic Having to do with atoms, the smallest possible unit that makes up a chemical element. atomic number The number of protons in an atomic nucleus, which determines the type of atom and how it behaves. average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group. chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H 2 O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds. electron A negatively charged particle, usually