By: Nadia Konovalchik, Kayla Jiang, CJ Randazzo, and Micah Cary
Atoms in the context of history
The Greek philosophers Aristotle and Democritus disagreed on the existence of atoms, or what Democritus called "Atomos". Democritus believed that atoms were the smallest things in existence, and that they were indivisible, which is the translation of "Atomos" in Greek.
Aristotle rejected this theory by Democritus and instead insisted that all things made in the universe are a different combination and variation of Earth, Wind, Fire, and Water.
John Dalton also believed that atoms were indivisible, so his model of the atom is simply just the atom itself as what he believed to be the smallest particle to exist.
John Dalton was an english chemist who finally placed atoms as a fundamental chemical object after the concept was ignored for 2,000 years. Dalton had four main theories.
For the most part these theories are still true today.
JJ Thomson began experimenting with cathode ray tubes. He observed that when he added a high voltage into two electrodes at one end of the tube, a beam of particles flowed from the cathode to the anode. The cathode is the negatively- charged electrode, while the anode is the positively charged electrode. He then placed two oppositely charged particles around the cathode ray. Upon observing that the ray went towards the positively charged plate and away from the negatively charged plate he determined that the ray was composed of negatively charged particles which are now known as electrons.
Cathode Ray Tubes experiment
Thomson then created a magnetic field by placing two magnets on either side of the tube. He observed that the magnetic field he created deflected the cathode ray. This deflection led him to determine that the mass of the cathode ray particles were each way smaller than an atom. Based on that information, he concluded that there were even smaller particles that made up atoms.
Based on his two experiments, JJ Thomson concluded that atoms had even smaller particles inside of them, and he also concluded that there were negatively charged particles in atoms. Since atoms are neutrally charged, Thomson determined that the negatively charged particles he called electrons, must be floating around in a soup of positively charged matter. This led to his illlustration of the Plum Pudding Model which has electrons floating around in positively charged matter. It is named the Plum Pudding Model due to its resemblance to to English dessert, Plum Pudding.
Plum Pudding Model
Electrons floating in positively charged matter that cancels out to create a neutral power.
Alpha particles from a radioactive material raced down tube and hit atoms on a gold foil target. If the atoms were in the formation of Thomson's Plum Pudding model then the alpa particles would be deflected only a little bit or not at all as they hit the electrons. Rutherford measured the angles of deflection and then determined the mass of what the alpha particles hit. He reasoned that the plum pudding model did not make sense with these deflections and determined that the atom had a nucleus that contained little of the atom's volume but nearly all of its mass.
Rutherford and his team learned about the electron from Thomson. They deduced that the remaining part of the atom had to be much heavier and have a positive charge.
Ernest's Gold Foil Experiment
Rutherford's model depicts the dense, positively charged nucleus and electrons in orbit around it. Because so few molecules deflected in Rutherford's experiment, he reasoned that the nucleus was very small, and the atom was mostly made up of empty space.