Simplify the following text for a Grade 9 native English-speaking person.
Original: The atomic mass is the total mass of protons , neutrons and electrons in a single atom ( when the atom is motionless ) .
Simplified: Atomic mass ( not to be confused with atomic weight ) is a term for the mass of a single atom of a chemical element . It includes the weights of the 3 subatomic particles that make up an atom : protons , electrons , and neutrons .