| A | B |
|---|
| atom | smallest particle and is divided to become the same substance |
| electron | negative charged atomic particle |
| nucleus | tiny dense charged region of the center of the atom |
| electron cloud | a region inside an atom where electrons are found |
| proton | (+) charged subatomic particle of the nucleus |
| atomic mass unit | the SI unit for the masses of particles in atoms |
| atomic number | number of atoms in the nucleus of an atom |
| isotope | atom that has the same number of protons but different neutrons |
| mass number | total number of protons and neutrons in an atom |
| atomic mass | weight average of the masses of all naturally occurring isotopes |
| Dalton | proposed the atomic theory |
| Democritus | argued that atoms were the smallest particle of matter |
| Rutherford | discovered that atom contains a small, dense (+) charged center |
| Bohr | suggested that electrons moved around the nucleus at certain fixed distances |
