All matter is made up of tiny particles called atoms that are invisible to the naked eye. In chemical reactions atoms are rearranged to produce new substances. 

One way of representing a chemical reaction is to use a word or symbol equation. An equation. for example, might be written as:

A + B = C + D 

where A and B are reactants and C and D are products. 

Sometimes the equation for the same reaction might be written as:

A + B ↓ C + D

[INSERT RR_CI_01_ima]

In a chemical reaction atoms are rearranged but are not created or destroyed. 

What law contains the idea that atoms cannot be made or destroyed?

The Law of Conservation of Mass.

Writing equations is discussed in greater detail in the Quantitative chemistry course (Chemical equations).