| atomic mass | number of protons and neutrons in the nucleus of an atom |
| periodic table | a chart that organizes information about all of the known elements according to their properties. |
| atomic number of an element | the number of protons contained in the nucleus of each of its atoms |
| period | each horizontal row in the periodic table - elements in each horizontal row increase in atomic number, moving across the row from left to right |
| group | a vertical column in the periodic table that is a chemical family |