1. Atomic Theory
Chapter 4

2. Atomic Theory Science is based off of observations.
A Scientific Law is a summary of what is seen in observations.
A Scientific Theory is an explanation of why these observations are occurring.
Both laws and theories are tested by using them to predict what would happen in certain situations.

3. Atomic Theory
-Explains why all matter acts as it does because it is composed of tiny particles called atoms, the basic building blocks of all matter
First theorized by Democritus ~400 BC in ancient Greece.
He thought that matter could NOT be divided infinitely. You had to reach a smallest possible piece. He named this piece an atom, which meant indivisible or can’t be cut.

4. More Democritus
He and his followers, atomists, said atoms were small hard particles all made of the same material, but different sizes and shapes.
They were also always moving and capable of joining together.
Since this was ancient Greece, He and the atomists had no way of ever proving his theories and few people believed it.

5. Law of Conservation of Mass
A chemical reaction does not change the mass of the matter present.
The mass of the reactants is equal to the mass of the products.
This is true even if a different phase of matter is created.
This was shown by Antoinne Lavoisier. He also defined elements as things that can not be broken down, and compounds as things made up of elements.

6. Law of definite proportions
All pure compounds have exactly the same proportions by mass of elements regardless of size
Water is always 2 hydrogen atoms to every 1 oxygen atom
By mass that is 2 (g) H : 16 (g) O

7. Dalton’s Model (1803)
people started to accept the idea of atoms because of his experiments
He worked with gases and found that they acted as though they were made of solid microscopic particles
all elements are made of atoms (indivisible and indestructible)
atoms of the same element are exactly alike
atoms of different elements are different
compounds are formed by joining two or more elements

8. John Dalton

9. Thomson’s Cathode Ray

10. Thomson’s Model (1897)
found negative particles could come from neutral elements
atom is made of smaller things (+ & -), and is divisible
successfully separated negative particles (electrons) but could not separate the positive particle (protons)
“plum pudding model” negative particles floating in a positively charged gel like material

11. Plum Pudding Model- Thomson
Positive Gel
Negative
Particles

12. Sir J. J. Thomson
The sir means
he was knighted

13. Rutherford’s Model (1911)
fired protons at a sheet of gold foil most went through unaffected, some bounced away
there is a small dense area of positive particles at the center of the atom- the nucleus
electrons are scattered near the outside of the atom with mostly empty space between the nucleus and the electrons

14. Gold Foil Experiment
Gold foil
Radioactive source

15. Rutherford’s Model Empty Space nucleus (small dense positive area)
electrons

16. Ernest Rutherford

17. Bohr Model (1913) electrons move in definite orbits around the nucleus
these orbits or energy levels are located at certain distances from the nucleus

18. Bohr’s Model
nucleus
Electrons

19. Neils Bohr

20. Wave Model (present day)
based on complex math equations
orbits are more complex than originally thought
de Broglie stated that electrons (particles) have wave properties, and he viewed these as standing waves, like those produce when a guitar string is plucked (classical physics.)
Schrodinger assumed that the electron in Hydrogen behaves as a standing wave.

21. Wave Model (continued)
When Schrodinger’s equation is analyzed, many solutions are found.
Each solution represents an atomic orbital.
An atomic orbital is the most probable location for finding an electron.

22. What is an orbital?
It is not a Bohr orbit (not moving in a circular path.)
How is the electron moving?
We don’t know!
There is a fundamental limitation to just how precisely we can know both the position and momentum of a particle at a given time

23. This is kind of how we assume an electron travels

24. Heisenberg Uncertainty Principle
The more accurately we know the particle’s position, the less accurately we can know it momentum and vice versa.
We can’t know the exact motion of the electron around the nucleus.
The area that an electron orbits is called an “electron cloud”

25. Louis de Broglie Erwin Schrodinger

26. Werner Heisenberg