Question

Which are the fundamental particles whose masses are mainly responsible for the mass of an atom?

Answer 1

The smallest unit of ordinary matter that makes up a chemical element is an atom. Atoms that are neutral or ionised make up every solid, liquid, gas, and plasma. Atoms are very tiny, measuring around 100 picometers in diameter. Due to quantum phenomena, it is impossible to correctly anticipate their behaviour using traditional physics—as if they were tennis balls, for example.

Complete answer:
Every atom is made up of a nucleus and one or more electrons attached to it. One or more protons and a number of neutrons make up the nucleus. Only one kind of hydrogen, the most prevalent, lacks neutrons. The nucleus contains more than 99.94 percent of an atom's mass. The protons have a positive electric charge, whereas the electrons have a negative charge and the neutrons have none. The atom is electrically neutral if the number of protons and electrons is equal. When an atom contains more or fewer electrons than protons, it has a negative or positive overall charge, and these atoms are known as ions.
The protons and neutrons that make up an atom account for the vast bulk of its mass. The mass number refers to the total number of these particles (also known as "nucleons") in an atom. Because it conveys a count, it is a positive integer and dimensionless (rather than having a mass dimension). "Carbon-12," which has 12 nucleons, is an example of how a mass number is used (six protons and six neutrons). Protons and neutrons are basic particles whose masses account for the majority of an atom's mass.
The mass of electrons is insignificant.
Hence neutrons and protons are correct.

Note:
The relative isotopic mass and the atomic mass of an isotope relate to a specific isotope of an element. Because most substances are not isotopically pure, the elemental atomic mass, which is the average (mean) atomic mass of an element, weighted by the quantity of the isotopes, is a handy measurement. The weighted mean relative isotopic mass of a (typical naturally-occurring) combination of isotopes is the dimensionless (standard) atomic weight.