Atomic Theory

Catalyst (5 min)

1. What does the atomic number of each atom represent?

2. What does the mass number of each atom represent?

3. How many protons and neutrons are in the nuclei of the following elements?

a. phosphorus (P)b. cadmium (Cd)c. lead (Pb)

Let’s Take a Trip Through Time!

``Nothing exists except atoms and empty space; everything else is opinion.'' Democritus

Democritus460 – 370 B.C.

• the first to propose that matter was not infinitely divisible

• Believed that matter was made up of tiny indivisible particles called atomos

• Everything is composed of small atoms moving in a void

• Ideas rejected by leading philosophers because they did not believe in empty space

First Concept of an Atom

John Dalton1766-1844

• English School Teacher• Introduced his ideas in

1803• Used experiments that

studied the ratio that elements combine in chemical reactions to transform Democritus’s ideas on atoms into a scientific theory

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 physically mix together or chemically combine to form compounds

4. In a chemical reaction, atoms are separated, combined or rerranged. Atoms of one element are never changed into atoms of another element.

5. Atoms cannot be created, divided into smaller particles or destroyed.

Dalton’s Model

But if the scientists couldn’t SEE the atom, how did they even know it existed?!

``To the electron: may it never be of any use!''J.J. Thomson

J.J. Thomson1856-1940

• Hypothesized that the structure of an atom was like ‘plum pudding’

• Discovered the electron in the Cathode Ray Experiment

• Discovered that this subatomic particle has a negative charge

• Discovered isotopes 1913

Cathode Ray Tube Experiment

• Identified the first subatomic particle, the electron• Determined the charge to mass ratio of the electron• Mass of the electron is MUCH less then the mass of a hydrogen atom• Meant there are particles smaller than atoms!

Video!

Cathode Ray Experiment

Altering the gas in the tube and the material used for the cathode have no effect

Proves: the ray’s negative particles (electrons) are found in all forms of matter

Cathode Ray is deflected toward the positively charged plate by an

electric field

Proves: the particles in the ray must have a negative charge

Plum Pudding Model

“Fullness of knowledge always means some understanding of the depths of our ignorance; and that is always conducive to humility and reverence. “

Robert Millikan

Robert A. Millikan1868-1953

• Conduced oil drop experiment to determine the mass and quantity of an electrons charge

“You should never bet against anything in science at odds of more than about 1012 to 1.”Ernest Rutherford

Ernest Rutherford1871-1937

• Student of J.J. Thomson• Conducted the gold foil

experiment • Discovered that an atom consists

of mostly empty space through which the electrons move

• Concluded there is a tiny dense region called the nucleus which is located in the center of the atoms and contain all its mass

• Electrons are held in the atom by attraction with positively charged protons.

Gold Foil Experiment

• Shot alpha particles at a thin piece of gold foil• Most of particles passed through foil which

indicated that atoms are composed of mostly empty space

• Alpha particle sometimes bounced back which indicated they were deflected from the nucleus

Video!

“If an atom were expanded to the size of a cathedral, the nucleus would be about the size of a fly - but a fly many thousands of times heavier than the cathedral.”

“This is almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”Ernest Rutherford

Rutherford Model

But what keeps the electrons from spiraling into the nucleus?

``An expert is a person who has made all the mistakes that can be made in a very narrow field.''Niels Bohr

Niels Bohr1885-1962• Developed theory that

electrons orbited the nucleus in neat, planet-like orbits

• Planetary Model 1913• Nucleus surrounded by orbiting

electrons at different energy levels

• Electrons have definite orbits

• Utilized Planck’s Quantum Energy theory

• Worked on the Manhattan Project (US atomic bomb)

Bohr Model for Nitrogen

How can the protons bundle inside the nucleus without blowing the atom apart?

James Chadwick1932

showed that the nucleus contained a neutral particle called the neutron.

Quantum Model (learn next class)

Developed from wave equation by Erwin Schrodinger (1926)

Fill in the chart using your notes (5 min)

Name Democritus Dalton Thomson Rutherford Bohr

Time Frame

Key Points

Model

Who Did It? (5 min)

Six scientists were sitting around a table discussing their contributions to Atomic Theory. Use the clues to determine where they were sitting and what contribution they made.

Subatomic Particles Table SummaryIn your notes fill in the table comparing the relative charge, mass and location of each of the subatomic particles (3 min)

400 BC 1803 1904 1910 1913 1926

Niels Bohr

J J Thomson

Democratus

John Dalton

Schroedinger / Heisenberg

Ernest Rutherford

Atomic Theory

In your notes, make a timeline graph showing the development of modern atomic theory (5 min)

You Do:

Complete the following book problems on a separate sheet of paper: p.104 #1, 4p.105 #13, 15p.122 #39, 62

Closing

How did the concept of the atom change over time and what key experiments and scientists led to our current understanding of the atom?

Homework

1. Describe the location of the electrons in Thompson’s ‘plum pudding’ model of the atom

2. In the Rutherford atomic model, which subatomic particles are located in the nucleus?

3. Describe Thomson’s and Millikan’s contributions to atomic theory.