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| 323142804 | Democritus | coined the term "atom", which is Greek for indivisible | 0 | |
| 323142805 | law of conservation of mass | mass is neither created nor destroyed during ordinary chemical reactions or physical changes | 1 | |
| 323142806 | law of definite proportions | a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the compound | 2 | |
| 323142807 | law of multiple proportions | if 2 or more different compounds are composed of the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers | 3 | |
| 323142808 | John Dalton | 1. all matter is composed of atoms 2. atoms of a given element are identical 3. atoms can't be subdivided, created, or destroyed 4. atoms of different elements combine in whole-number ratios to form chemical compounds 5. atoms are combined, separated, or rearranged in chemical reactions | 4 | |
| 323142809 | Robert Millikan | studied charge of electrons by using an atomizer from a perfume bottle to spray tiny drops of oil in a special chamber | 5 | |
| 323142810 | Joseph John Thompson | discovered electrons with a cathode ray tube; cathode ray was deflected by a negatively charged magnet, so cathode rays are composed of negatively charged particles called electrons | 6 | |
| 323142811 | Ernest Rutherford | discovered nucleus with gold foil experiment; shot alpha particles through gold foil, and a few deflected back | 7 | |
| 323142812 | James Chadwick | discovered neutrons using calculations | 8 | |
| 323142813 | atomic number | number of protons | 9 | |
| 323142814 | mass number | number of protons and neutrons | 10 | |
| 323142815 | isotopes | elements with different numbers of neutrons, like carbon-14 | 11 | |
| 323142816 | nuclear forces | short-range proton-neutron, proton-proton, and neutron-neutron forces that hold nuclear particles together | 12 | |
| 323142817 | average atomic mass | weighted average of atomic masses of natural occurring isotopes in an element | 13 | |
| 323142818 | molar mass | mass of 1 mole of pure substance | 14 | |
| 323142819 | Avogadro's number | 6.022x10^23; number of particles in exactly 1 mole of a pure substance | 15 | |
| 334325632 | 1 amu | 1/12 the mass of carbon-12 | 16 | |
| 334325633 | 1 mole | amount of substance that contains as many particles as there are atoms in 12 g of carbon-12 | 17 |
