2H2O2
1. Number of hydrogen atoms in chemical formula 1.

А  There are two peroxide molecules on the reactant side.

The statement is correct because the left hand side is the reactant side. We can read the reaction as;

  2 molecules of hydrogen peroxide decomposed  to given 1 molecule of oxygen gas and 2 molecules of water .

D Mass must be conserved so there are an equal number of molecules in the reactants and products.

E Mass must be conserved so there are an equal number of atoms in the reactants and products.

Since the chemical equation is balanced, choices D and E are correct. The reaction will have equal number of molecules and atoms on both sides of the expression.

Explanation:

it's c. elements are placed in rows based on how may valencies electrons have.

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