| 1 | It goes after oxygen because it is small. | |
| 2 | A molecule of water consists of two atoms of hydrogen and one atom of oxygen. | |
| 3 | Two atoms of hydrogen combine with one atom of oxygen to form a molecule of water. | |
| 4 | A compound having five atoms of oxygen combined with another element or radical. | |
| 5 | In the stratosphere, sunlight breaks apart an oxygen molecule (O2), producing two oxygen (O) atoms. |