1. Atomic Theory
A Brief History

2. The Greek Model
Democritus concluded that matter couldn’t be divided into smaller and smaller pieces forever. Eventually the smallest piece would be obtained. This piece would be indivisible.

3. The word atom comes from the Greek word atomos meaning
Democritus – 400 B.C.
The word atom comes from
the Greek word atomos meaning
indivisible or uncuttable

4. Dalton Model – Early 1800’s There are 4 basic ideas of
Dalton’s Atomic Theory

5. All elements are composed of atoms that cannot be divided.
Dalton’s Theory
All elements are composed of atoms that cannot be divided.

6. Dalton’s Theory
All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses.

7. Dalton’s theory
An atom of one element cannot be changed into an atom of a different element. Atoms cannot be created or destroyed in any chemical change, only rearranged.

8. Dalton’s Theory
Every compound is composed of atoms of different elements, combined in a specific ratio.

9. J. J. Thomson
Discovered that the atom is divisible and negatively charged particles he called rays.
Today they are known as electrons. He also reasoned that positive charges existed to make the atom neutral, but he couldn’t find them.

10. Plum Pudding Model The pudding represents the
sphere of positive electricity and
the bits of plum scattered in the
pudding are the electrons.
I like to call it the "blueberry muffin"
model. All those round little
blueberries surrounded by the
bread of the muffin. Ummmm, good.
Some butter on top of a muffin
hot from the oven and some nice,
COLD milk. Oh my.

11. Ernst Rutherford
Rutherford concluded that atoms were mostly made of empty space and proposed that the atom has a small, dense, positively charged center called the nucleus. He thought the electrons were moving around the nucleus in the empty space.

12. Gold Foil Experiment

13. Gold Foil Experiment
Rutherford’s research team aimed a beam of positively charged particles at a thin sheet of gold foil. They predicted that, if Thomson’s model were correct, the charged particles would pass right through the foil in a straight line. The gold atoms would not have enough positive charge in any one region to strongly repel the charged particles.

14. Gold Foil Experiment
Rutherford’s team observed that most of the particles passed through the foil undisturbed, as expected. But, to their surprise, a few particles were deflected strongly. Since like charges repel each other, Rutherford inferred that an atom’s positive charge must be clustered in a tiny region in its center, called the nucleus. Any particle that was deflected strongly had been repelled by a gold atom’s nucleus.

15. Bohr Model Niels Bohr – 1913 Showed that electrons
could only have specific
amounts of energy leading
them to move in certain orbits
Like layers of an onion.

16. Modern Day Model
Electrons do not have a specific path around the nucleus and it’s impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.
Also called the Cloud Model

17. Modern Atomic Model
“According to the modern atomic model, an atom has a positively charged nucleus surrounded by a large region in which there are enough electrons to make the atom neutral.”