Write the names of three elementary particles which constitute an atom. Which of these particles determine the mass of an atom? Justify your answer with an example.

(N/A) The three elementary particles that constitute an atom are $Electrons$,$Protons$,and $Neutrons$.
The mass of an atom is determined by the $Protons$ and $Neutrons$ present in the nucleus.
Justification:
Consider a Sodium $(Na)$ atom.
Number of $Protons = 11$
Number of $Neutrons = 12$
Number of $Electrons = 11$
Mass Number = (Number of $Protons$) + (Number of $Neutrons$)
Mass Number = $11 + 12 = 23$
Since the mass of $Electrons$ is negligible compared to $Protons$ and $Neutrons$,the atomic mass is primarily determined by the sum of $Protons$ and $Neutrons$.