1. Atomic Theory
Science 9

2. Objectives By the end of the lesson you should be able to:
Describe how 4 men developed the atomic theory
Draw and describe an atom

3. Historical Understanding
Chinese: world was based on 5 elements (earth, water, fire, metal and wood)
Greeks: argued that matter could be endlessly divided until reaching the atomos – Aristotle did not agree; therefore, nobody challenged the idea again for 2000 yrs!
Alchemists in the Middle Ages tried to turn common metals into gold - nobody succeeded!

4. Your Turn Each person in the class will be assigned a number
1. John Dalton
2. J.J. Thomson
3. Ernest Rutherford
4. Niels Bohr
You need to read the section of the text book on your assigned scientist and take notes
Pick the most important pieces of information to share with others in the class.
Give numbers 1-4 and students researched assigned person
Groups all number 4 together to share what they learned
Make new groups with numbers 1-4 so they can share with eachother (jig saw)

5. Dalton’s Atomic Theory (Late 18th Century)
All matter is made of small particles
Atoms cannot be created, destroyed or divided into smaller particles.
All atoms of the same element are identical in mass and size but different in mass and size from atoms of different elements.
Compounds are made when atoms of different elements combine together

6. John (J.J.) Thomson (Late 18th Century)
Discovered that currents in gas discharge tubes (like the fluorescent lights in this room) were made of negatively charged particles.
Named these negatively charged particles electrons
Found that all substances could produce these charges
Proposed the “raisin bun model” (also called the “plumb pudding model”)

7. Ernest Rutherford (Late 18th Century)
Discovered that there is a lot of space within and between atoms
Reasoned that there must be a positively charged nucleus with electrons moving around outside the nucleus
Later discovered that there must be a neutral particles in the nucleus as well

8. Niels Bohr (Late 18th Century)
Discovered that gasses could be made to glow by forcing an electric current through them (NEON LIGHTS!!!)
Reasoned electrons orbit the nucleus at different levels
higher levels have more energy
When an electron escapes from its orbit, it emits light

9. After Niels Bohr’s contribution, the atomic model as we know it was complete. An atom is said to be the smallest unit of matter that retains the properties of an element (e.g. gold atoms are different from lead atoms are different from chlorine atoms).
However, the atom is made of three subatomic particles: protons, neutrons and electrons.

10. The Components of an Atom
Name
Symbol
Relative Mass*
Electric Charge
Location in the atom
Proton p+ 1 +1
Nucleus
Neutron n0
~1.008
Electron e-
~0.0005 -1
Electron cloud
*Relative mass is the mass of the particle in relation to the mass of 1 proton.