| A | B |
|---|
| nucleus | contains protons and neutrons |
| atomic number | number of protons in nucleus |
| mass number | number of protons and neutrons in nucleus |
| electrons | travel around outside of nucleus |
| isotope | atom with different number of neutrons |
| 2 in 1st level, 8 in 2nd, 18 in 3rd | maximum number of electrons |
| dot diagrams | represent the valence electrons |
| groups or families | vertical columns (# of valence electrons) |
| periods | horizontal rows (# of energy levels) |
| Bohr model | protons/neutrons (nucleus), electrons (energy levels) |
