Final answer:
Atomic mass is calculated by counting the number of protons and neutrons in an atom's nucleus, as electrons contribute negligibly to the atom's mass.
Step-by-step explanation:
When calculating atomic mass, the two subatomic particles included are protons and neutrons. These particles are collectively known as nucleons, which are found inside the nucleus of an atom. Both protons and neutrons have a mass of approximately 1 atomic mass unit (amu), which is defined as one-twelfth the mass of a carbon-12 atom. Electrons, although part of the atom, are so light (approximately 1/1800 of an amu) that their mass is generally ignored when calculating the atomic mass of an element. Thus, to find the atomic mass, you count the number of protons and neutrons in an atom's nucleus.