* J.J. Thomson (1897): He is credited with the discovery of the electron itself through his cathode ray tube experiments. By measuring the deflection of the rays in electric and magnetic fields, he calculated the charge-to-mass ratio (e/m) of the electron. This demonstrated that the electron was much lighter than the hydrogen atom, which was the smallest atom known at the time.
* Robert Millikan (1909): He conducted the oil drop experiment, which determined the charge of a single electron. Combining this with the e/m ratio, scientists could then calculate the mass of the electron, finding it to be incredibly small.
* Ernest Rutherford (1911): His gold foil experiment established the nucleus as the dense, positively charged core of an atom, with electrons orbiting around it. This model further emphasized the relative emptiness of the atom, with electrons occupying a vast space compared to the nucleus.
Therefore, while Thomson's discovery of the electron and its charge-to-mass ratio was crucial, it was a combination of work by Thomson, Millikan, and Rutherford that led to the understanding that the electron's mass is very small compared to the size of an atom.
