The two subatomic particles that make up the atomic weight of an atom are protons and neutrons.

Here's why:

* Protons: Each proton has a mass of approximately 1 atomic mass unit (amu).

* Neutrons: Each neutron also has a mass of approximately 1 amu.

* Electrons: Electrons have a significantly smaller mass (almost negligible compared to protons and neutrons) and therefore don't contribute much to the atomic weight.

Atomic weight is essentially the average mass of an atom of a specific element, taking into account the different isotopes of that element. Since protons and neutrons are the heaviest particles in an atom, their number primarily determines the atomic weight.