# Can a chemical change alter the total number of atoms?

Nov 11, 2015

#### Explanation:

The Law of Conservation of Mass and Energy ("matter can neither be created nor destroyed") clearly states that a chemical change cannot alter the number of atoms in a given reaction. The atoms can only rearrange so that it can yield a new molecule/compound but the number of atoms should stay the same.

This is the reason why in stoichiometry, or balancing chemical equations, we do not change the subscripts of the chemical formulas to balance but we use coefficients instead.

For example,

$\textcolor{g r e e n}{2} {H}_{2}$ + ${O}_{2}$ $\rightarrow$ $\textcolor{g r e e n}{2} {H}_{2} O$ (color (blue) "correct")

${H}_{2}$ + ${O}_{2}$ $\rightarrow$ ${H}_{2} {O}_{\textcolor{g r e e n}{2}}$ (color (red) "incorrect")

Above we have two equations, supposedly, for water. Notice that the first one is correct because, not only does it gives a balance number of $H$ and $O$ atoms in both sides of the equation, but also the chemical formula for water is written correctly (two $H$ atoms + one $O$ atom).

Now take a look at the second equation. Essentially it is also "balanced" as the number of atoms on the left matches the number of the atoms on the right, however, the chemical formula for "water" is now wrong (${H}_{2} {O}_{2}$ is the chemical formula for hydrogen peroxide).

So in summary, for chemical reactions, the number of atoms on the reactants side MUST be the same as the numbers on the products side.