| An atom is mostly ____ with a tiny dense nucleus. |
alloy |
| The ____ of pure water is 7. |
atomic number |
| Copper, gold, iron, lead, mercury, and silver are ____ elements. |
metal |
| The air we breathe is about 78% ____ and 21% oxygen. |
mixture |
| An ____ is a mixture of a metal and one or more other elements. |
neutron |
| A ____ is either homogeneous or heterogeneous. |
nitrogen |
| A ____ is a subatomic particle with no electric charge. |
pH |
| An element's ____ is the number of its protons. |
space |
