| Answer | Crossword Clue |
|---|---|
| molecules | the smallest physical unit of a chemical element or compound |
| molecule | the smallest physical unit of an element |
| molecules | MOLECULE, the smallest physical unit of an element |
| MOLECULE | Smallest physical unit of an element |
| allotropy | The existence of two or more different physical forms of a chemical element |
| isotope | any of two or more species of atoms of a chemical element with the same atomic number and nearly identical chemical behavior but with differing atomic mass or mass number and different physical properties |
