1. Introduction to Atoms
Chapter 11

2. Development of the Atomic Theory
Atoms are small particles that are made up of even smaller particles, which determine the properties of the atom.
The Beginning of the Atomic Theory
Around 440 BCE, Greek philosopher Democritus first proposed the idea of a particle that could not be cut any smaller. He called this particle atom, from the Greek word atomos, which means indivisible.
Aristotle, also a Greek philosopher, disagreed with Democritus. He did not believe that such an indivisible particle existed.
It turned out that Democritus was right. Atoms do exist.
An atom is defined as the smallest particle into which an element can be divided and still be the same substance.

3. Dalton’s Atom Theory Based on Experiments
John Dalton, a British Chemist and teacher, first proposed his atomic theory in 1803
Dalton’s Atomic Theory
All substances are made of atoms.
Atoms are small particles that cannot be created, destroyed, or divided.
Atoms of the same element are exactly alike
Atoms join with other atoms to make new substances.

4. Thomson’s Discovery of Electrons
In 1897, J.J. Thomson, a British scientist, showed that part of Dalton’s Atomic Theory was incorrect.
Thomson showed that there were even smaller particles inside the atom, disproving Dalton’s idea that atoms could not be divided into smaller parts.
Thomson actually had discovered the presence of a negative particle (electrons) outside the atom. This particle was called an electron.
Thomson believed that electrons were distributed through the whole atom. (Plum pudding Model)

5. Rutherford’s Atomic Shooting Gallery
In 1909, Ernest Rutheford, a student of Thomson’s, further revised the atomic theory.
Rutherford proposed that the atom was mostly empty space with a tiny part made of highly dense matter.
He believed that in the center of the atom was an extremely dense, discovered positively charged center which he called the nucleus.

6. Where are the Electrons?
In 1913, a Danish scientist, Niels Bohr proposed that electrons move around the nucleus in set paths that he called energy levels.
Bohr believed that electrons traveled in set paths and had the ability to change energy levels. This was known as the Bohr Model.

7. Started on Levels, Now into the Clouds
Austrian physicist Erwin Schrodinger and German physicist Werner Heisenberg showed one aspect of the Bohr Model was incorrect. They proved that electrons do not travel in set, predictable paths.
The current atomic theory states that the electron paths are not definite, but there are regions within the atom where the electrons are likely to be.
These likely regions are known as the electron cloud. The model is known as the electron cloud model.
The now know that electrons move so quickly within the cloud that it is like they are everywhere at once.