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Brief History of the atom model.

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Brief History of the atom model.


The roots of atomic theory can be traced back over 2000 years ago to the ancient Greeks. The Greeks were responsible for many advancements in civilization, but one of the things that drove them was the question of origin. Who are we? What are we made of? Where did we come from?

These questions were not limited to humans, but to all matter in the world. Around 400 BC a philosopher named Democritus proposed a theory of composition of matter. Democritus proposed that all matter was composed of tiny indivisible particles.

His reasoning was simple. A piece of wood could be divided into two equal pieces. Those two pieces could then be divided again to form four pieces. Those four pieces could be divided to form eight, and then eight to form sixteen, etc. Eventually, Democritus reasoned, a point would be reached where the wood could not be divided any further. This point represented the building blocks of all matter.


In fact, the word "atom" is based on the Greek word "atomos":

  • "tomos": to cut
  • "a": not
  • "atomos": uncuttable

This Greek influence lives with us today, even if Democritus' name remains relatively unknown. Other more famous philosophers are remembered for their theories of the structure of matter. Perhaps the one that is most famous is Aristotle's theory, which said that matter was composed of four base elements: fire, water, earth, and air.

Why did Aristotle's theory take hold, and Democritus was forgotten?

...read more.


J.J. Thompson

The atomic theory proposed by Dalton in the early 1800's held true until the end of that century. At that point scientists began finding characteristics about the atom that pointed to a more complicated picture than the one painted by Dalton years ago.

The first discovery that altered our picture of the atom actually came from physics, not chemistry. Physicists were working with cathode ray tubes (see diagram below). A cathode ray tube consists of a glass tube with most of the air taken out. When the tube is hooked up to a voltage source, electricity flows from the cathode to the anode, and the remaining gas in the tube glows.


from Chemistry: Visualizing Matter (c.1996: Holt, Reinhart and Winston)

It was observed that when a small paddle wheel (like the ones in old water powered mills) was placed in the beam going between the anode and cathode, the wheel rotated. Reseachers assumed that the electricity must be composed of tiny particles that would be able to rotate the wheel.

Finally in 1897 J.J. Thompson found that the beam going between the anode and cathode could be deflected by bringing a magnet close to the cathode ray tube. The deflection that Thompson observed showed that the beam must have been made up of negatively charged particles. These particles were called electrons. Later experimentation showed the charge and mass of these tiny particles.

...read more.


Here is where things get interesting. A German scientist named Werner Heisenberg made an important observation. Hiesenberg proposed that it is impossible to know both the exact position and exact velocity of an electron at the same time. Huh?

Let's examine this a little closer. When we look at an object like a computer or a car we are seeing the light waves that are reflected from that object. In other words, to see an object a photon of light must hit the object and reflect back to our eyes. These collisions between a large car and a photon of energy has a negligable effect on the car.

When a photon of light collides with an electron results in a large effect on the electron because of its size. If we were able to pinpoint an electron's position, we cannot know its velocity because the collision with light has changed it (like two billiard balls colliding). If we pinpoint an electron's velocity we cannot know its position because it changes too quickly. This concept is called Heisenberg's Uncertainty Principle.

Because of this uncertainty describing the electrons in terms of specific orbits is impossible. Instead we can only give a possible area the electrons could be located in at any specific time. These possible locations are called electron clouds. Instead of orbits, each different energy level can abe described as a different cloud.

We are going to stop our discussion of the development of the atom will stop here. The shapes of the clouds and the quantities of energy associated with each level are not important to us at this level of chemistry.

...read more.

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