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

(Greek Philosopher, 450 B.C.) Empedocles Empedocles (Greek Philosopher, 450 B.C.)

Everything that existed was thought to be composed of four elements. Empedocles (i) Water (ii) Fire (iii) Air (iv) Earth

(Greek Philosopher, 400 B.C.) Democritus Democritus (Greek Philosopher, 400 B.C.)

Democritus 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.

(Greek Philosopher, 350 B.C.) Aristotle Aristotle (Greek Philosopher, 350 B.C.)

Believed in Empedocles’ “four element” model 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. Aristotle

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

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

Alchemists 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…

(English Scientist, 1800’s) Dalton John Dalton (English Scientist, 1800’s)

Using experiments, he discovered that 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) Atoms Molecule 3. Atoms of an element are identical (e.g. H2 gas are all H atoms) Dalton 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 explains The Law of Conservation of Mass The total mass of the reactants = the total mass of the products Dalton 2nd law: any pure substance has a fixed composition in terms of the chemical elements, regardless of where located (earth, moon) (ex. Iron oxide located in Australia is SAME as iron oxide found in Zanzibar) The Law of Constant Composition A compound always contains the same elements in the same proportions by mass.

Dalton Model: Billiard Balls Theory Model Analogy (Featureless Sphere) Indivisible atoms (Featureless Sphere) (Billiard Balls) Dalton

Thompson J.J. Thompson (Physicist, 1904)

Thompson 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) Thompson

Nagaoka Hantaro Nagaoka Hantaro Nagaoka (Japanese Scientist, 1904)

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

Model: The Saturn Model Theory Model Analogy Positive sphere with ring of electrons (Ring of Flat Electrons) (Saturn) Nagaoka

Rutherford Ernest Rutherford (Nuclear Physicist, 1911)

Rutherford 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. Rutherford

The Gold Foil Experiment Rutherford

Rutherford The Gold Foil Experiment The Results… Most alpha particles passed through in a direct path, however, some deflected and bounced back. Rutherford

“Its like shooting a bullet at a piece of tissue paper and having the bullet bounce off” Rutherford

An atom is mostly empty space !!! The Gold Foil Experiment An atom is mostly empty space !!! The Conclusion… 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. Rutherford 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) Rutherford

Chadwick James Chadwick (Nuclear Physicist, 1932)

Chadwick 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.

Bohr Neils Bohr (Danish Physicist, 1920)

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

Model: The Planetary Model Theory Model Analogy Explains periodic law Electrons are quantized in energy levels (Planetary Model) (Planets Orbiting Around the Sun) Bohr

Summary Neutrons + Protons are in the nucleus Electrons in orbits around the nucleus Summary Protons = Positive Charge Electrons = Negative Charge Neutrons = Zero Charge

HOMEWORK Read pg. 82 – 85 Read pg. 90 – 92 Answer # 1 – 6, 9 on pg. 85

The End The End