1. Atom Models
From Democritus to Bohr’s

2. Learning Goals
Explain how different atomic models evolved as a result of experimental evidence.
Describe the characteristics of neutrons, protons and electrons (charge, location and relative mass)

4. Questions to ask...
How did we find out that gold was gold and silver was something different?
How did we find out what part of a particle makes a substance unique?
How can models of the atom help explain matter?

5. Democritus (400 BCE)
- Greek philosopher
- All matter can be divided into smaller and smaller pieces
- He called the last piece that “cannot be cut” an ATOM

6. Democritus’ Atomic Theory
atoms are different sizes
atoms are in constant motion
atoms are separated by empty spaces

7. Democritus' Atomic Theory

8. Aristotle (~340 BCE) Rejected the idea of the atom
All matter is made up of 4 substances
Earth, Water, Fire and Air

9. - each atom of one material is the same - atoms cannot be divided
John Dalton (1807)
Billiard ball model
- each atom of one material is the same
- atoms cannot be divided

10. Dalton’s Atomic Model
All mater is made up of tiny, indivisible particles called atoms
all atoms of an element are identical
atoms of different elements are different
atoms are rearranged to form new substances in chemical reactions, but they are never created or destroyed

11. J.J. Thomson (1897)
Discovered electrons
- negatively charged particles
“Plum Pudding” model
- negative electrons embedded in a positive atom
- electrons are evenly distributed

12. Ernest Rutherford (1909) Discovered the nucleus is very small
contains most the mass
surrounded by a cloud of negatively charged electrons
most of the atom is empty space

13. Rutherford’s Gold Foil Experiment
See page 230

14. Niels Bohr (1913) Bohr refined Rutherford's model:
- Electrons orbit the nucleus (like planets)
- Electrons cannot be between orbits / they can jump between orbits
- Each orbit can hold a maximum number of electrons

15. James Chadwick (1932) Student of E. Rutherford
The nucleus contains 2 particles
Positive Protons (discovered by Rutherford in 1920)
Neutral NEUTRONS
A neutral atom contains the same number of protons and electrons

16. Type of Subatomic Particles
Proton
Neutron
Electron
Charge + O -
Location
in nucleus
orbiting nucleus
Relative Mass 1
1/2000

17. The Bohr-Rutherford Model is what we will use (for the first 20 elements).

18. Have we met our Learning Goals?
Can YOU explain how different atomic models evolved as a result of experimental evidence?
Can YOU describe the characteristics of neutrons, protons and electrons (charge, location and relative mass)?

19. Group Task
1. Create a TIMELINE of the history of the atom. OR
2. Create a mini poster of the 1 you think is MOST important