Give reasons:
$(a)$ Mass number of an atom excludes the mass of an electron.
$(b)$ The nucleus of an atom is charged.
$(c)$ The alpha particle scattering experiment was possible by using gold foil only and not by foil of any other metal.

(N/A) Electrons have negligible mass; therefore,the mass of an atom is practically determined by the protons and neutrons present in the nucleus.
$(b)$ The nucleus is positively charged because it consists of positively charged protons.
$(c)$ The gold foil was used because it is highly malleable and could be made extremely thin,approximately $1000$ atoms thick,which was necessary for the alpha particles to pass through with minimal interference.