History of Atomic Theories(The Changing Views of Atomic Structure)

Empedocles(Greek Philosopher, 450 B.C.)

Everything that existed was thought to be composed of four elements.

(i) Water (ii) Fire

(iii) Air (iv) Earth

Democritus(Greek Philosopher, 400 B.C.)

All matter is made of tiny hard indestructible particles which are indivisible (i.e. can not be broken down). The Greek word for “indivisible” is atomos, therefore all matter is made up of atoms.

Aristotle(Greek Philosopher, 350 B.C.)

Believed in Empedocles’ “four element” model.-Despite the more recent “atomic model” proposed by Democritus, Aristotle was so influential that the “four element” model was accepted for almost 2000 years.

Note: The Greek philosophers did not test their ideas with experiments. They were thought of as great “thinkers”, but not scientists.

Alchemists(Philosophers, Mystics, Magicians, Chemists, 500-1600 A.D.)

Performed experiments and devised chemical symbols for substances that we now call elements and compounds.

They also invented many lab tools that are still used today…

John Dalton(English Scientist, 1800’s)

Using experiments, he discovered that

1.Atoms are tiny indestructible particles that cannot be broken down

2. Atoms combine with other atoms to form molecules (e.g. carbon + oxygen carbon dioxide)3. Atoms of an element are identical (e.g. H2 gas are all H atoms)

4. Atoms do not lose their identity during chemical reactions (i.e. cannot be created and/or destroyed)5. Molecules of a compound are identical (e.g. water = H2O)

Dalton developed a 6 part theory based on these experiments which explains1. The Law of Conservation of Mass

The total mass of the reactants = the total mass of the products2. The Law of Constant

Composition A compound always contains

the same elements in the same proportions by mass.

Model: Billiard Balls

Theory Model Analogy

Indivisible atoms

(Featureless Sphere)

(Billiard Balls)

J.J. Thompson(Physicist, 1904)

Thompson discovered electrical particles can be lost or gained from elements. He named these particles electrons. He also believed that atoms were made of positively charged matter with negatively charged electrons scattered throughout.

Model: The Raisin Bun Model

Theory Model Analogy Electrons embedded within a positive sphere Net charge of zero

(Uniform Charge

Distribution)

(Raisin Bun)

Hantaro Nagaoka(Japanese Scientist, 1904)

He modeled the atom as a large positive sphere surrounded by a ring of negative electrons.

Model: The Saturn ModelTheory Model Analogy

Positive sphere with ring of electrons

(Ring of Flat Electrons)

(Saturn)

Ernest Rutherford(Nuclear Physicist, 1911)

At McGill University (in Montreal), Rutherford designed the “Gold Foil Experiment” to test Thompson’s and Nagaoka’s models

The Gold Foil Experiment Positive alpha particles were fired at a thin sheet of gold foil

Rutherford predicted that the alpha particles would pass directly through the metal foil, untouched.

The Gold Foil Experiment

The Results…

The Gold Foil Experiment

Most alpha particles passed through in a direct path, however, some deflected and bounced back.

“Its like shooting a bullet at a piece of

tissue paper and having the bullet bounce off”

The Conclusion…

The Gold Foil Experiment

Atoms are mostly empty space with a tiny central nucleus which contains almost all of the total mass of the atom and is positively charged. He called these positively charged particles protons. Protons have a mass nearly 200x greater than electrons. The nucleus is surrounded mostly by empty space, containing rapidly moving negative charges called electrons.

Model: The Nuclear Model

Theory Model Analogy Small positive nucleus surrounded by negative electrons

(Nuclear Model)

(Beehive)

James Chadwick(Nuclear Physicist, 1932)

Chadwick discovered particles in the nucleus of an atom possessing no electrical charge (neutral). He called them neutrons. The neutrons make up the remaining mass of the nucleus.

Neils Bohr(Danish Physicist, 1920)

Suggested electrons moved around the nucleus in a definite “orbit” arranged in “shells”.

Model: The Planetary Model

Theory Model Analogy

Explains periodic law Electrons are quantized in energy levels

(Planetary Model)

(Planets Orbiting

Around the Sun)

Neutrons + Protons are in the nucleus

Electrons in orbits around the nucleus

Neutrons = Zero Charge

Protons = Positive Charge

Electrons = Negative Charge

HOMEWORK Read pg. 82 – 85

Answer # 1 – 6, 9 on pg. 85

Read pg. 90 – 92 Answer # 1 – 3 on pg.

93

The End