An atom is the smallest unit of an element that keeps its chemical properties. The structures that atoms are composed of that give them their properties are called subatomic particles.
The types of subatomic particles are protons, neutrons, and electrons.
Protons are contained in the centermost part of the atom called the nucleus. Protons have a relative charge of +1. The mass of a proton is 1 atomic mass unit (amu).
Neutrons are contained in the nucleus of the atom. Neutrons have a relative charge of 0. The mass of a neutron is 1 atomic mass unit (amu).
Electrons are arranged around the nucleus and have a relative charge of -1. The mass of a electron is 0.0005atomic mass units (amu). For most purposes the mass of an electron is estimated to be 0.
If you add the masses of protons and neutrons in an atom, you can find the atom's atomic mass.
If a carbon atom contains 6 protons and 6 neutrons it has an atomic mass of 12.