Atomic Theory Experiments

Niels Bohr

Studied under Thomson and Rutherford and built upon the work of Planck, Hertz, Einstein, and others 1913: predicted that electrons are only found in specific circular paths, or orbits, around the nucleus Electrons can only move from one orbit to another, but not in between, much like a ladder

Dalton's Atomic Theory

1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can chemically combine in simple whole-number ratios to form compounds. (Law of Multiple Proportions) 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

The Oil Drop Experiment

1909 • With an atomizer, sprayed oil droplets into box with two charged plates • Used X-rays to give droplets one or more electrons • As droplets fell between plates, he measured how different charges on plates change the rate of fall

The Gold Foil Experiment

Done by Rutherford • Surrounded thin piece of gold foil with phosphorescent screen • Rutherford "shot" positively charged alpha (α) particles at the foil • He looked at where the screen would "glow" in order to see where the alpha particles were going after they traveled through the foil.

Democritus

Greek philosopher, ~400 B.C.E. • Is there a limit to how many times a substance can be divided, and still be that substance? • Believed everything is made of "atoms" - Greek atomos, meaning indivisible - These "atoms" are indivisible and indestructible • No experimental evidence

Atom

The smallest part of an element that is still considered to be that element

Millikan's Conclusions

• Credited with experimentally determining the charge of an electron as 1.60 x 10-19 Coulombs • Mathematically, he used Thomson's charge-to-mass ratio to determine the mass as 9.108 x 10-28 g

Dalton

• Dalton used experimental methods to transform Democritus' ideas into a scientific theory - Studied the ratios in which elements combine in chemical reactions - His ideas are summarized in Dalton's Atomic Theory

Cathode Ray Tube Experiment

• Empty (vacuum) tube connected to a high voltage source of electricity • When connected, a beam (ray) would originate from the cathode • This cathode ray would glow green when it struck a special phosphorescent coating

J.J. Thomson

• Famous cathode ray tube experiment in 1807

James Chadwick

• Frederic and Irene JoliotCurie bombarded beryllium with alpha particles, forming a powerful beam • 1932: Chadwick noted that the beam was not deflected by electric or magnetic fields, thus he determined the beam had no charge • He predicted that this beam was composed of neutrally charged neutrons

What did Rutherford expect?

• If Thomson's plum pudding model of the atom were true, Rutherford expected the positively charged particles to go straight through the atom.

What did Rutherford observe?

• In fact, most particles did shoot straight through • However, a very small number of particles were deflected or even shot back in the other direction • Since alpha particles are positive, and like charges repel, this must mean they "hit" something with a positive charge • Since most particles went straight through, however, the positive charge must be very small and dense • "It was almost as incredible as if you fired a 15-inch bullet at a piece of tissue paper and it came back and hit you."

How did Niels Bohr do it?

• Run electricity through gaseous elements, thus exciting the electrons to a higher energy level • Electrons "fall" back to their ground state and release the absorbed energy in the form of waves

What did Rutherford conclude?

• Rutherford's Gold Foil Experiment helps him to discover: - The atom is mostly empty space - The nucleus - The positively charged proton

So, what is the cathode ray/beam made of?

• The beam was attracted to the positively charged plate, and deflected from the negatively charged plate • Since opposite charges attract, that means the beam must be negatively charged • What is it made of? Corpuscles Electrons

What was so surprising about what observed in the cathode ray experiment?

• The beam was deflected (moved) by electric or magnetic fields. - Only particles with charges are affected by electric/magnetic fields. - The atom is supposedly the smallest unit of matter, with no charge. • So, this mysterious beam must be made up of charged particles, not atoms.

When the droplet is not moving in the Oil Drop Experiment...

• The forces on the droplet are balanced • This allowed Millikan to determine the total charge on each droplet • Millikan realized the charge of each droplet was a whole-number multiple of the same value (the charge of one electron)

"Plum Pudding" Model

• Thomson assumed the electrons were spread throughout the atom like chocolate chips in a cookie • Since the atom was neutral overall, he assumed the entire atom was positive (to balance out the negative electrons)