History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come...

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History of Atomic Theory

Transcript of History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come...

Page 1: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

History of Atomic Theory

Page 2: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Democritus (c. 450 BCE)

• Was commonly held that “something” could come from “nothing”

• Response to this by atomists like Democritus: matter is made up of tiny indivisible particles (atomos)

• Atoms were thought of as retaining the identity of the object they made up.

Page 3: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Why is this model (idea) good?-Starts to get at conservation of matter

What things does this model fail to explain/ explain incorrectly?-Atoms are divisible

-We know atoms are not simply miniature versions of the objects they make up

Page 4: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Antoine Lavoisier

• Measured the masses of substances before and after a chemical reaction and found that they were always equal.

Conservation of Matter:• Matter can neither be created nor destroyed,

only transformed.

Page 5: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Joseph Proust

Law of Constant Composition• Same compounds always contained the same

proportions of elements no matter the size of the sample. (i.e. The mass of H2O is always 88.9% Oxygen and 11.1% Hydrogen.)

Page 6: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

John Dalton (1803)

Postulates:1) Each element is composed of extremely small, indivisible particles called atoms.2) All atoms of a given element are identical.3) Atoms are neither created nor destroyed in any chemical reaction.4) A given compound always has the same relative numbers and kinds of atoms.

Page 7: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Why is this model (idea) good?-Takes into account empirical data (i.e. Lavoisier,

Proust)

What things does this model fail to explain/ explain incorrectly?-Atoms are divisible

-Atoms are more or less identical, except they may have different weights (isotopes)

Page 8: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

The Idea of Electric Charge

• Charge is a physical property of matter, like mass.

• The charge of a particle is defined in relation to how it behaves in an electric field.

• Essentially… like charges will repel while opposite charges attract (sign convention is arbitrary).

Page 9: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Electrons

• Scientists knew about electrical currents

• A cathode ray tube (think TV, “the tube”) was one device used for studying electrical currents.

-One plate called anode (+) and one plate called cathode (-)

By the 1900s, scientists knew several things about the behavior of these rays:1) The ray could spin a small paddle wheel (particles!)2) Putting a magnet close to the beam would bend it in the direction

expected for negatively charged particles (- charge!)

Page 10: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Thomson Activity

Page 11: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Cathode Ray Tube

• https://www.youtube.com/watch?v=GzMh4q-2HjM

Page 12: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

JJ. Thomson (1896)

• Carefully measured the angle at which the beam could be deflected

• CONCLUSIONS:– Atoms were not indivisible balls, instead they had

some substructure (electrons!)– Determined charge-to-mass ratio of electrons

Charge = C = 1.76 x 108 C/g mass g

Page 13: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Tue 10/20

• Please turn your Chp. 3 outlines into the box

• Clear everything off your desk for the reading quiz

Page 14: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Millikan (1909)

• Successfully measured the charge on a single electron. Charge = 1.60 x 10-19 C

• Thus: 1.60 x 10-19 C = 9.11 x 10-28 g 1.76 x 108 C/g

CONCLUSION:-Electron is an extremely light particle!

only about 1/2000 the mass of H

Page 15: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:
Page 16: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Protons

• Thomson had established that atoms contained negatively charge particles (electrons).

• Atoms were known, however, to be electrically neutral.

• There must be an equal number of positively charged particles… but how are they arranged?

Page 17: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Rutherford Articles + Outline

• Please read thoroughly through the 3 articles on Rutherford and his experiment.

• Fill out your outline sheet with as much detail as possible.

• Take any extra notes on Rutherford that you think may be helpful in answering the prompt.

Page 18: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Rutherford (1909)Observations Conclusions

Most alpha particles passed directly through gold foil

The atom IS mostly empty space

Some alpha particles were scattered at large angles, and 1 in 20,000 were sent directly back to the source

All of the atom’s positive charge, and most of its mass was centered in a small core (nucleus)

Page 19: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

Gold Foil Experiment

• https://www.youtube.com/watch?v=5pZj0u_XMbc

Page 20: History of Atomic Theory. Democritus (c. 450 BCE) Was commonly held that “something” could come from “nothing” Response to this by atomists like Democritus:

MODELS (so far)

DALTON THOMSON RUTHERFORD


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