# What are the elements or compounds that enter into a chemical reaction?

$\text{The reactants..........}$
For $A + B \rightarrow C + D$, $A$ and $B$ are the $\text{reactants}$, and $C$ and $D$ are the $\text{products}$. Given the particle nature of chemical reactivity, the MASS of $C$ and $D$ will be PRECISELY EQUAL to the MASS of $A$ and $B$.