| A | B |
|---|
| Marie Curie | worked on the radioactivity of atoms and discovered radium and polonium |
| Dmitri Mendeleev | developed a table of the elements based on periodic properties, leaving gaps for as yet undiscovered elements. |
| William Ramsay | discovered the noble gases. |
| James Chadwick | proposed the existence of the neutral proton or neutron |
| Lise Meitner | proposed that uranium could undergo fission. |
| Niels Bohr | proposed that electrons circled the nucleus in fixed orbits of a particular energy. |
| John Dalton | proposed that all matter was made of atoms which were identical for a particular element and that atoms were not created or destroyed during reactions. |
| Democritus | proposed that matter was made of indivisible units called atoms. |
| Frederick Soddy | proposed the existence of isotopes. |
| Ernest Rutherford | atoms have a small dense cnetral region of positive charge (nucleus) which is surrounded by a large volume occupied by electrons. |
| Glenn Seaborg | discovered a number of the trans-uranium elements |
| Robert Boyle | defined elements as pure substances of only one type of atom. |
| Humphrey Davy | was first to extract Na, K, Ba, Ca, Sr, Mg, and B among others. |
| Joseph Priestly | was first to isolate elemental oxygen. |
| Jons Berzelius | devised a system of using letters as symbols for elements and calculated the relative atomic masses of many elements. |
| Antoine Lavoisier | proved that oxygen was involved in combustion reactions. |
