| Atom | an extremely small particle of matter that retains its identity during chemical reactions. |
| Atomic mass unit (amu) | a mass unit equal to exactly one-twelfth the mass of a carbon-12 atom. |
| Atomic number (Z) | the number of protons in the nucleus. |
| Atomic weight | the average atomic mass for a naturally occurring element, expressed in atomic mass units. |
| Avogadro's number | the number of atoms in a 12-g sample of carbon-12, equal to 6.02 x 1023 to three significant figures. |
| Chemical change | a change in which one or more kinds of matter are transformed into a new kind |
| Electron | a very light particle that carries a unit negative charge and exists in the region around the positively charge nucleus. |
| Isotopes | atoms whose nuclei have the same atomic umber but different mass numbers |
| Mass number (A) | the total number of protons and neutrons in the nucleus. |
| Mole (mol) | the quantity of a given substance that contains as many molecules or formula units as the number of atoms in exactly 12 g of carbon- 12. The amount of substance containing Avogadro's number of molecules or formula units |
| Neutron | a particle found in the nucleus of an atom; it has a mass almost identical to that of the proton but has no electric charge. |
| Nucleus | the central core of an atom; it has most of the mass of the atom and one or more units of positive charge |
| Proton | a particle found in the nucleus of the atom; it has a positive electric charge equal in magnitude, but opposite in sign, to that of the electron and a mass 1836 times that of the electron |