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