1. Section 1- Discovering the parts of the atom
Chapter 7
Section 1- Discovering the parts of the atom

2. Early Ideas about matter
Greek Philosophers
Aristotle ( BC)
Matter made up of four Elements
Fire, air, water, earth
Democritus ( BC)
Matter made up of small, solid objects that cannot
Be created, destroyed or divided
Called them atomos (English- atoms)

3. Democritus He had no way to test his ideas
His ideas did not conform to popular belief
Therefore, he was not believed
Aristotle was believed

4. John Dalton English school teacher and scientist- late 1700’s
Revised the idea of the atom
Proposed the atomic theory
All matter is made up of atoms that cannot be divided, created, or destroyed
During chemical reaction, atoms of one element cannot be converted into elements of another element
Atoms combine in specific ratios
Atoms of one element are identical to each other but different from atoms of another element

5. Current theory
Matter is made of atoms with empty space between and within them
An atom is the smallest piece of an element that still represents the atom
Atoms are very small
7.5 trillion carbon atoms could fit into a period at the end of a sentence
Can be viewed with Scanning Tunneling microscope (STM)
First observed in 1981

6. J.J. Thomson English Scientist ( 1856-1940) Studied cathode tubes
In his experiment- placed two plates on opposite sides of the cathode ray tube
One plate was + charged while the other was – charged.
The rays bent towards the + charged plate
He concluded the cathode ray tube was – charged.

7. Thomson continued
He conducted further experiments and found the particles had mass
He proposed that these negatively charged particles came from the metal atoms in the electrode
He called these particles electrons
A particle with a – 1 charge
He also proposed that atoms must contain + charged particles

8. Thomson cont. His model contained both positive and negative charges.
An atom was a sphere with positive charges spread throughout
Negative charges were mixed with the positive charges
Like chocolate chips in cookie dough

9. Rutherford Ernest Rutherford ( 1871- 1937) Student of Thomson
“Famous Gold Foil Experiment”
He shot alpha particles which dense and positively charged at gold foil
Most of particles bounced off the gold foil
1 In 10,000 hit the foil and bounced straight back

10. Rutherford’s Atomic Model
Since most of the alpha particles went straight through then the nucleus is mostly empty space
Those that bounced and did not go in a straight direction hit something positive
His Model says
Most of the mass of an atom and the positive charge is in the center of an atom called the nucleus
Protons are positively charged particles

11. Discovering Neutrons James Chadwick ( 1891-1974)
He determined that the nucleus also contained neutrons which are particles with no charge.

12. Bohr Model Niels Bohr ( 1885-1962)
There were limitations of Rutherford’s model
He could not explain what the electrons were doing
Bohr proposed that they move in circular levels or orbits around the nucleus

13. Modern Atomic Model
Electrons form an electron cloud around the nucleus
An electron cloud is an area around an atomic nucleus where an electron is most likely to be located

14. Quarks
Protons and neutrons are made of smaller particles known as quarks
6 types of quarks
Up, down, charm, strange, top, bottom