HISTORY OF THE ATOMHISTORY OF THE ATOM

AristotleAristotle

• • 400 BC400 BC

- Claimed that there was no smallest part - Claimed that there was no smallest part of of matter matter

- Thought different substances were made - Thought different substances were made up of proportions of fire, air, earth, and up of proportions of fire, air, earth, and water. water.

DemocritusDemocritus

• • 400 BC400 BC

- - Matter could be subdivided into smaller Matter could be subdivided into smaller and smaller particles until a basic part of and smaller particles until a basic part of matter could not be divided further matter could not be divided further

- He called these particles - He called these particles atomsatoms, which , which means “indivisible” in Greek. means “indivisible” in Greek.

John DaltonJohn Dalton1766-18441766-1844

Dalton’s Atomic TheoryDalton’s Atomic Theory1. All matter is made of atoms. Atoms are indivisible 1. All matter is made of atoms. Atoms are indivisible

and indestructible. and indestructible.

2. All atoms of a given element are identical in mass 2. All atoms of a given element are identical in mass and propertiesand properties

3. Compounds are formed by a combination of two or 3. Compounds are formed by a combination of two or more different kinds of atoms.more different kinds of atoms.

4. A chemical reaction is a rearrangement of atoms4. A chemical reaction is a rearrangement of atoms

Sir William CrookesSir William Crookes1832-19191832-1919

• 1870’s- Studied the effects of magnetic and electric

fields on the cathode ray using a cathode ray tube (CRT)

• • He discovered cathode rays had the following He discovered cathode rays had the following properties:properties:

1. Travel in straight lines from the cathode1. Travel in straight lines from the cathode

2. Cause glass to glow2. Cause glass to glow

3. Transmit a negative charge to objects they 3. Transmit a negative charge to objects they strike strike

4. Are deflected by electric fields and magnets to 4. Are deflected by electric fields and magnets to suggest a negative charge suggest a negative charge

5. Cause pinwheels in their path to spin 5. Cause pinwheels in their path to spin indicating they have mass indicating they have mass

What is a CRT?What is a CRT?1. Consists of a gas-filled glass tube with 2 metal plates1. Consists of a gas-filled glass tube with 2 metal plates

a. 1 negatively charged (the a. 1 negatively charged (the cathode) and the other ) and the other positively charged (the positively charged (the anode)

2.2. When a very large voltage is placed across the electrodes When a very large voltage is placed across the electrodes

a. the neutral gas inside the tube will ionize into a a. the neutral gas inside the tube will ionize into a plasma

b. The current will flow as electrons travel from the b. The current will flow as electrons travel from the cathode (-) to the anode (+) called a cathode ray. cathode (-) to the anode (+) called a cathode ray.

J.J. ThomsonJ.J. Thomson1856-19401856-1940

• • 18971897

- Discovered the electron using the CRT - Discovered the electron using the CRT

• • Found that the cathode ray was attracted Found that the cathode ray was attracted to to a positively charged metal plate a positively charged metal plate

• • Cathode ray must be negative because Cathode ray must be negative because opposites attract. opposites attract.

Thomson’s Plum Pudding ModelThomson’s Plum Pudding Model

• • Atoms are neutrally charged (0)Atoms are neutrally charged (0)

- there must be a positive charge in the atom, too - there must be a positive charge in the atom, too if there are electrons (-) if there are electrons (-)

• • Made a speculation at what an atom might look like: Made a speculation at what an atom might look like:

- a cloud of positive electricity with “electrons” - a cloud of positive electricity with “electrons” stuck in the cloud (chocolate chip cookie!) stuck in the cloud (chocolate chip cookie!)

R.A. MillikanR.A. Millikan(1868-1953)(1868-1953)

• Oil drop experiment measured the charge of an electron

- Electron’s charge = 1.602 x 10-19 coulombs

- Electron’s mass = 9.11 x 10-28 grams

Ernest RutherfordErnest Rutherford 1871-19371871-1937

• • Used a Used a gold foil experimentgold foil experiment to determine to determine

- the tiny center of the atom is dense and positively - the tiny center of the atom is dense and positively charged charged

- Discovered:

• the + nucleus

• The proton (+)

• the nucleus has almost all of the mass of an atom

• most of the atom is empty space most of the atom is empty space

• assumed electrons were outside the nucleus and assumed electrons were outside the nucleus and create most of the volume of the atom create most of the volume of the atom

The Gold Foil ExperimentThe Gold Foil Experiment

"It was as incredible as if you fired a 15-inch "It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came shell at a piece of tissue paper and it came back and hit you."back and hit you."

Niels BohrNiels Bohr1885-19621885-1962

Bohr expanded upon Rutherford’s theory by proposing that

1. electrons travel only in certain successively larger orbits.

2. the outer orbits could hold more electrons than the inner ones, and that these outer

3. the orbits determine the atom's chemical properties

The Bohr ModelThe Bohr Model

James ChadwickJames Chadwick1891-19741891-1974

• 1932: Discovered

- the neutron in the nucleus (with the + proton)

- the neutron has mass, but no (0) charge

Louis de BroglieLouis de Broglie1892-19871892-1987

Discovered:Discovered:

1. All particles, including light & electrons 1. All particles, including light & electrons show both wave behavior & particle show both wave behavior & particle behavior behavior

2. Electrons do not travel in any specific pattern 2. Electrons do not travel in any specific pattern

- The exact location cannot be determined- The exact location cannot be determined

- There is a high probability of - There is a high probability of finding electrons in regions finding electrons in regions (orbitals) (orbitals)

Werner HeisenbergWerner Heisenberg1901-19761901-1976

• Determined the “Uncertainty Principle”

- Any effort to locate an electron will cause the electron to move, therefore, it is impossible to locate electrons