Explanation: Alpha rays are fast moving streams of positively charged particles which are shown to be helium nuclei represented by 4He2, having a mass number of 4, i.e., having 2 protons and 2 neutrons. Mass number of an element is the sum of the number of protons and neutrons in the nucleus of an atom. A helium atom has 2 protons and 2 neutrons in its atom.
