← Periodic Table of Elements Export Options Alphabetize Word-Def Delimiter Tab Comma Custom Def-Word Delimiter New Line Semicolon Custom Data Copy and paste the text below. It is read-only. Select All atomic mass The average mass of one atom of an element. atomic number The number of protons in the nucleus of an atom. periodic table A chart of the elements showing the repeating pattern of their properties. chemical symbol A one- or two-letter representation of an element. element A substance that cannot be broken down into other substances by chemical or physical means.