History of Atomic Theory

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.

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

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.

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.)

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.

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)

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).

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!)

Thomson Activity

Cathode Ray Tube

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

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

Tue 10/20

• Please turn your Chp. 3 outlines into the box

• Clear everything off your desk for the reading quiz

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

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?

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.

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)

Gold Foil Experiment

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

MODELS (so far)

DALTON THOMSON RUTHERFORD