Question

What two particles account for almost all of the mass of an atom?

Answer 1

Hint : An atom comprises a nucleus and the revolving electrons around it. The nucleus of an atom has protons and neutrons, where the number of protons is equal to the atomic number of an element or atom.

Complete Step By Step Answer:
Protons, neutrons, and electrons make up an atom's primary structure. Nucleons are made up of protons and neutrons that make up the nucleus of an electron, while electrons reside outside the nucleus of an atom. There is an equivalent number of protons and electrons in a neutral atom.
Many explanations have been suggested to describe the composition of the atom.
Even after Dalton's and Thomson's theories, Rutherford proposed the atomic structure, which claimed that since an atom's form is spherical, the nucleus contains the majority of its mass and storage. The electrons surround the nucleus in a circular orbit.
These three particles were named "subatomic particles" by him. The proton is positively charged, with a mass of $1.672 \times {10^{ - 24}}$, while neutrons are neutral, with a mass of $1.674 \times {10^{ - 24}}$. Electrons have a mass of $9.1 \times {10^{ - 31}}$ , making them $1800$ times lighter than protons. As a result, since electrons have negligible mass, they are ignored when measuring an atom's mass.
Therefore, we can conclude that the two particles which account for almost all of the mass of an atom are protons and neutrons.

Note :
Atoms that are electrically neutral but lack electrons will occur. So an atom can’t have any electrons so it has protons by definition and does have electrons to be neutral. An ion, such as a hydrogen ion, may exist (we can call it a proton).