Gurkirat Kamboj ( James Chadwick )
Andrew Calonge ( William Crookes )
Kevin Montenegro ( J.J. Thomson and Ernest Rutherford )
Ethan Kawough ( Robert Millikan )
William Mendez ( Niels Bohr ) The History of Atomic Structure
William Crookes
William Crookes was a British scientist most famously known for his experiment involving “Crookes’ Tubes”. Born in London, England, Crookes attended the Royal
College of Chemistry, becoming a chemist and physicist. After discovering the excess element Thallium from sulfuric acid production, Crookes set out to weigh this new element in a vacuum environment as to ensure accuracy in measurement. Observation of the element’s erratic reactions to a hot or cold state of being would be described by
Crookes as “repulsion from radiation”. William was then inspired by this information to construct a radiometer which spun in correlation to reactions between negatively charged objects and the contents of the vacuum tubes he created. This helped to determine the presence of mass for the contents of the tubes. Crookes’ later experiments with cathode/anode tubes containing low pressure gasses would be the most important. After observing a flash from the tubes’ reflective inner coatings in low light conditions, the discovery of a cathode ray was made. Radiation from the cathode of the tube formed a straight beam and had reflected off of the opposing end’s reflective coating, producing heat and phosphorescence. This tube experiment later became

known as “Crookes’ Tubes” and the cathode ray experiment would help to define an even smaller set of negatively charged particles known today as electrons.
­The diagram below depicts a model similar to that of Crookes’ in which phosphorescence created by the cathode ray(s) are visible on the anode or opposite end. J.J. Thomson
Thomson is an English physicist who ultimately disproved the idea of an indivisible atom. In his experiments, Thomson placed cathode rays into a vacuum tube making a cathode ray tube, which shoots a beam of electrons through the tube. Regardless of the
Gases within the vacuum tube or the material used for the cathode, the ray is unaffected so the particles must be part of all matter. However, when a magnet is placed on the vacuum tube, the ray is deflected, suggesting the particles must be charged. When using a magnet with known polarities on each end, the bean was attracted to the positive polarity, revealing that the particles are negatively charged. when he measured the charge­to­mass ratio of the particles, it was much less than that of a hydrogen atom, the smallest known atom. (Dingrando) This discovery proved that atoms could be divided further, debunking the indivisible belief from the 1700s. When

trying to explain why if atoms are made up of electrons, why is matter neutral, and what accounts for the rest of the mass of an atom, Thomson proposed a new model: the
Plum­pudding model. In this model, he proposed that in an atom, there a spherically shaped atom with a uniformly distributed positive charge, and the electrons are spread out equally throughout atom. Though inaccurate, Thomson’s model was the first to show an atom as divisible into smaller parts. (Joseph) This graph shows Thomson’s
Cathode Ray Tube, and how he deduced the existence of Elections.

Robert Millikan
Robert Millikan was an American physicist is best known for measuring a charge of an electron, this is which he won the Nobel Prize in Physics in 1923. One of Robert
Millikan’s most famous experiments, oil drop experiment, Robert used an atomizer to spray fine mist of oil droplets in an chamber to suspend the oil droplets in mid­air. In the chamber there was two electric charged plates, first plate was postively charged with a pinhole for the oil droplets to fall through once the droplets go through the pinhole an x­ray source was used ionize the gas molecules to make the oil droplets