Reactant molecules are any substance that goes into a reaction. When forming product molecules, the reactant molecules combine and create a new molecule (i.e. the combination of methane and oxygen can make carbon dioxide and water).
The atomic number in an element is usually how many protons the element has. For example, Hydrogen has a 1 on top of the H (on the periodic table), therefore, Hydrogen has 1 proton. Oxygen has an 8 on top of the O (on the periodic table) so therefore, Oxygen has 8 protons.
I cannot see your question to help you... sorry
I would say physical, because a physical change is affecting the form of a chemical substance, but not it's chemical makeup.
