1. Protons and neutrons: These are located in the nucleus of the atom and are collectively called nucleons.

- Protons have a positive charge and contribute to the atom's atomic number.

- Neutrons have no charge and contribute to the atom's mass number.

2. Electrons: These are negatively charged particles that orbit the nucleus. They are much lighter than protons and neutrons.

- While electrons contribute to the atom's overall mass, their contribution is so small that it is often considered negligible.

Here's a breakdown of the contributions:

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

* Electrons: Each electron has a mass of approximately 0.00054858 amu.

Therefore, the majority of an atom's mass comes from the protons and neutrons within its nucleus.

Important Note: The actual mass of an atom is slightly less than the sum of the masses of its individual protons, neutrons, and electrons. This difference in mass is known as the "mass defect" and is a result of the energy binding the nucleus together.