Question

Rutherford assumed that helium atoms contained two protons and two electrons. What evidence led him to propose that they also contained neutrons?

Answer 1

Helium atoms have atomic mass of 4u.However,on considering the mass of only electrons and protons in helium atom,the mass concluded was half of the actual mass.Since,electrons have negligible mass.Thus,he concluded that the atoms must also have some other particle with some mass but no charge,which would have affected the neutral state of the atom.   Moreover,he assumed that the presence of just protons in the nucleus would brek it up due to repulsion between them.