Early Atomic Theory: Dalton, Thomson, Rutherford and Millikan

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  • 0:06 History of Atomic Theory
  • 0:25 The Early Greeks
  • 1:01 John Dalton's Atoms
  • 1:49 Thomson's Discovery of…
  • 3:05 Rutherford's Nucleus
  • 4:39 Millikan's Charge of…
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Lesson Transcript
Instructor: Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

Imagine firing a bullet at a piece of tissue paper and having it bounce back at you! You would probably be just as surprised as Rutherford when he discovered the nucleus. In this lesson, we are going to travel back in time and discuss some of the major discoveries in the history of the atom.

History of Atomic Theory

Picture an atom. What does it look like? Most likely it will resemble something like this: a fairly large nucleus surrounded by orbiting electrons whizzing around the nucleus. This image is a popular icon of the atom, but it only vaguely represents our current model of what the atom looks like.

The Early Greeks

J.J. Thomson theorized that electrons were surrounded by a positively charged material.
Electrons Surrounded by Positive Material

First, we are going to travel back a little over 2,000 years ago to the times of Aristotle and Democritus. The Greek philosopher Aristotle believed that matter could be divided infinitely without changing its properties. Democritus disagreed. He thought that matter could only be divided until you got to the smallest particle (which he called the atom, coming from the Greek word atomos, meaning indivisible). So, who was right? Aristotle was very convincing and did many experiments using the scientific method, so more people believed him.

John Dalton and Atoms

It wasn't until around 2,000 years later, in the early 1800s, when John Dalton came along and disproved Aristotle. Dalton went on to say that matter is made up of tiny particles, called atoms, that cannot be divided into smaller pieces and cannot be destroyed. He also stated that all atoms of the same element will be exactly the same and that atoms of different elements can combine to form compounds. The really awesome thing about Dalton's model of the atom is that he came up with it without ever seeing the atom! He had no concept of protons, neutrons or electrons. His model was created solely on experiments that were macroscopic, or seen with the unaided eye.

Thomson and the Discovery of Electrons

A diagram of the Rutherford alpha particle experiment
Rutherford Experiment Diagram

Now, let's fast-forward to the late 1800s when J.J. Thomson discovered the electron. Thomson used what was called a cathode ray tube, or an electron gun. You've probably seen a cathode ray tube without even knowing it! They are the bulky electronic part of old television sets. Thomson used the cathode ray tube with a magnet and discovered that the green beam it produced was made up of negatively charged material. He performed many experiments and found that the mass of one of these particles was almost 2,000 times lighter than a hydrogen atom. From this he decided that these particles must have come from somewhere within the atom and that Dalton was incorrect in stating that atoms cannot be divided into smaller pieces. Thomson went one step further and determined that these negatively charged electrons needed something positive to balance them out. So, he determined that they were surrounded by positively-charged material. This became known as the 'plum pudding' model of the atom. The negatively charged plums were surrounded by positively charged pudding.

Rutherford and the Nucleus

A few years later, Ernest Rutherford , one of Thomson's students, did some tests on Thomson's plum pudding model. The members of his lab fired a beam of positively charged particles called alpha particles at a very thin sheet of gold foil. (Later on you will learn that alpha particles are really just the nuclei of helium atoms.) Because these alpha particles had so much mass, he fully expected that all of the alpha particles would go right through the gold foil. This is because, if Thomson were correct about the plum pudding model of the atom, the alpha particles would just go through the positively charged matter and hit the detecting screen on the other side.

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