The total mass of an atom is not exactly equal to the sum of the masses of its fundamental particles. While the masses of protons, neutrons, and electrons contribute to the overall mass of an atom, there are other factors that need to be considered.
Protons and neutrons, which are found in the nucleus of an atom, contribute significantly to its mass. These particles have a relatively large mass compared to electrons. Electrons, on the other hand, have a much smaller mass and orbit around the nucleus in regions known as electron shells.
However, it's important to note that the mass of an atom is not solely determined by the masses of its constituent particles. The concept of atomic mass takes into account the average mass of all the isotopes of an element and their relative abundances. Isotopes are atoms of the same element that have different numbers of neutrons. Since different isotopes have different masses, the atomic mass of an element is a weighted average of the masses of its isotopes.
Additionally, there is a small difference between the measured mass of an atom and the sum of the masses of its constituent particles due to the concept of mass defect. Mass defect refers to the difference between the mass of a nucleus and the sum of the masses of its individual protons and neutrons. This difference arises from the conversion of mass to energy during the formation of the nucleus. According to Einstein's mass-energy equivalence principle (E = mc²), the binding energy of the nucleus contributes to a slight decrease in its overall mass.
Therefore, while the masses of the fundamental particles do contribute to the mass of an atom, other factors such as isotopic composition and mass defect must be considered to determine the actual mass of an atom.