| Answer | Crossword Clue |
|---|---|
| OSMIUM | Densest natural element |
| OSMIUM | Densest stable element |
| URANIUM | Heaviest natural element |
| ETHANE | Element in natural gas. |
| protactinium | The chemical element of atomic number 91, a radioactive metal of the actinide series, occurring in small amounts as a product of the natural decay of uranium |
