Question

1. For each of the following scientist that contributed to the early modern atomic theory,
   write a paragraph on their contribution and how their contribution changed Dalton’s atomic theory: Rutherford, Thomson, Millikan, Chadwick

2. For each of the following experiments, write a paragraph on how it contributed to the
understanding of the structure of the atom: Rutherford Cathode Ray Tube, Millikan Oil
Drop Experiment, Rutherford Gold Foil Experiment, and Chadwick Neutron Experiment.

Answer 1

The basic points put forward by Dalton about the atom was that it is indivisible. Each one of the personalities mentioned contributed in disproving this.

JJ Thomson performed the cathode ray tube experiment which led to the discovery of electron and somewhat disproving Dalton's atomic theory. Based on his observations he put forward the plum pudding model of an atom.

Ernest Rutherford and his two students, Hans Gieger and Ernest Marsden, performed the famous gold foil experiment to study the anomaly presented by the plum pudding model. Most of the alpha particles passed through the foil unhindered and thus leading to the discovery of nucleus. Based on this experiment he put forward the nuclear model of an atom. But this model also had certain discrepancies which remained unexplained until Neils Bohr came up with his Quantum mechanical model of hydrogen atom.

Robert Milikan, on the other hand, set out to determine the charge on the electron and its mass, at a time when the very existence of electron was highly questioned. He performed, at that point of time, probably the greatest experiment, the famous Oil Drop experiment and was successful in determining the charge to a great precision.

It was always a mystery as to why the actual mass number A of an atom is greater than atomic number. An experimental breakthrough came in 1930 with the observation by Bothe and Becker that bombardment of beryllium with alpha particles from a radioactive source produced neutral radiation which was penetrating but non-ionizing. James Chadwick was able to prove that these particles couldn't be photons. And by series of other experiments, he was able to predict the mass of the particle which agreed to great extent to the values of neutron we know today.