The most of the mass of the atom can be found in

The mass of an atom is almost entirely concentrated in its nucleus, which contains protons and neutrons [1]. Protons and neutrons each have a mass of approximately 1 atomic mass unit (amu), whereas electrons are significantly lighter, weighing only about 1/1800th of an amu. Consequently, electrons contribute negligibly to the overall atomic mass, often being disregarded in mass number calculations. While the nucleus occupies less than 0.01% of the atom's total volume, it typically accounts for more than 99.9% of its mass. The electron cloud, which defines the atom's volume and chemical properties, contains the electrons but lacks the density of the central nucleus. Therefore, the nucleus is the primary site of an atom's mass.

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Glossary > p. 100