CBSE Class 9 Answered
When Rutherford performed his alpha-particles scattering experiment, neutrons were not discovered. Helium atom has 2 electrons and 2 protons. So, the mass of helium was expected to be twice the mass of hydrogen atom which has one electron and 1 proton. But actually mass of helium atom is four times that of hydrogen atom. It was found that atoms of all elements (except hydrogen) were at least twice as heavy as could be explained by the number of protons they had. To explain this it was predicted that other particles with no charge but mass equal to that of proton must be present in all atoms except hydrogen. This prediction was later proved to be correct with discovery of neutrons.