1. Theories of the Atom

2. What is a theory?
A concise, predictive and broadly applicable explanation for a wide range of phenomena (Scientific observations)
It is not a “hunch”
They are constantly evolving

3. 450 BCE: Earth, Water, Air and Fire
Idea supported by Greek philosopher Aristotle
All matter is made of four basic substances: earth, water, air and fire
The substances have specific qualities: dry, wet, cold and hot
This was accepted for almost 2000 years

4. 400 BCE: An Indivisible Particle - Atom
Democritus develops the idea of atoms
suggested that matter was made up of tiny particles called: ATOMOS
Greek for indivisible

5. 1807: Billiard Ball Model John Dalton, English Scientist and teacher
working with gases, reconsidered Democritus’ theory that particles are indivisible
suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them ATOMS

6. Dalton’s Atomic Model:
1. All matter is made up of atoms, which are particles too small to see 2. Each element has its own kind of atom, with its own particular mass
3. Compounds are created when atoms of different elements link to form molecules 4. Atoms cannot be created, destroyed, or subdivided in chemical changes

7. Unlike charges attract
Problem with Dalton’s Theory:
Unable to explain the
Electrical nature of matter:
Like charges repel
Unlike charges attract

8. Used cathode ray tube to conduct experiments Discovered the electron
1898: Plum Pudding Model
JJ Thompson
Used cathode ray tube to conduct experiments
Discovered the electron
idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a sphere of positive charge to balance the electron's charge 8

9. 1910: Nucleus and Proton Ernest Rutherford
- designed an experiment using RADIUM (this element spits out positive ALPHA particles)
- he placed a piece of gold foil in front of the beam, surrounded by a screen to detect the path of the particles
- they found that although most of them passed through, about 1 in 10,000 hit 9

10. Gold Foil Experiment
gold foil
helium nuclei
helium nuclei 10

11. Rutherford’s Model
Centre of atom, called nucleus, has a positive charge
Nucleus occupies small space, but contains most of the atom’s mass
Cloud of negatively charged electrons surround the nucleus
Most of the atom is empty space
Credited for discovering the proton 11

12. 1932: Planetary Model - James Chadwick, Rutherford’s student
Nucleus contains another particle which
has NO charge (neutral) called a:
NEUTRON
Atom is empty space with tiny, dense nucleus
Nucleus contains protons and neutrons
Electrons circle rapidly in the space around the nucleus
A neutral atom has the same number of protons and electrons

13. 1913: Electron Orbits
refined Rutherford's idea by adding that the electrons were in orbits
Each orbit is only able to contain a set number of electrons. 13

14. Bohr-Rutherford Model
Explains properties of first 20 elements
Electrons orbit the nucleus
Each electron in an orbit has a definite amount of energy
The farther an electron is from the nucleus, the greater the energy
Electrons cannot be between orbits but they can jump to and from different orbits
Each orbit can hold a maximum number of electrons

15. Bohr-Rutherford Model