| A | B |
|---|
| nucleus | the center of an atom made up of protons and neutrons |
| proton | positively charged subatomic particle in the neucleus of an atom |
| neutron | a neutral subatomic particle in the nucleus of an atom |
| electron | a tiny negatively charged subatomic particle moving around outside the nucleus of an atom |
| energy level | any of the possible energies an electron may have in an atom |
| orbital | a region in an atom where there is a high probability of finding electrons |
| valence electrons | an electron in the outermost energy level of an atom |
| atomic number | the number of protons in the nucleus of an atom |
| mass number | the total number of protons and neutrons in the nucleus of an atom |
| average atomic mass | the weighted average of the masses of all naturally occurring isotopes of an element |
