1. Atomic Theory Lesson Objectives
Understand the experimental design and conclusions used in the development of modern atomic theory including
Dalton’s Postulates
Thomson’s discovery of electron properties
Rutherford’s nuclear atom
Bohr’s nuclear atom

2. A brief look at the history of the atom.
Atomic Structure
A brief look at the history of the atom.

3. Democritus of Abdera
Began the discussion of the possible existence of ATOMS!
ATOM: from the Greek meaning “cannot be cut”.
But, he had no empirical evidence, so for centuries people believed Aristotle.

4. Democritus' Ideas
Matter is composed of empty space through which atoms move.
Atoms are solid, homogeneous, indestructible, and indivisible.
Diff. kinds of atoms have different sizes and shapes
The differing properties of matter are due to the size, shape, and movement of atoms.
Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves.

5. The Aristotelian Approach to Knowledge
The scientific method is still another 2000 yrs. away.
They believed one should separate the mind from the body (emotions) in order to understand anything. Attaining knowledge by pure reason, or Rationalism.
What’s the purpose of knowledge??
To lead you to the truth.

6. The Empirical Approach
Come to knowledge by relying on our senses the beginning of experimentation.
So, compare Rationalism to Empiricism.
What’s better??
Rational – problems with our reason. Plato’s Academy.
Empirical – problems with trusting our senses.

7. Development of Atomic Theory
First credible atomic theory introduced by John Dalton in 1808
Dalton studied the ratio in which elements combine when reacted with each other
Based theory on experimental results gathered by other scientists (Lavosier’s conservation of mass and Proust’s law of definite proportions)

8. Dalton’s Experimental Theory
Dalton’s Postulates
Elements are made of small and indivisible particles called atoms
Atoms of a given element are identical to one another but differ from atoms of other elements
Chemical reactions occur when atoms are combined, separated, or rearranged in simplified whole-number ratios
Atoms are neither created nor destroyed in chemical reactions
Shortcomings of Dalton’s Postulates
Atoms of an element can have masses that differ slightly
Atoms can be divided into subatomic particles

9. Atoms and Electricity
Scientists noticed that some substances decomposed when exposed to electrical current
Elements are held together by electrical forces
Scientists discovered the cathode ray
Cathode Ray Tube

10. Discovery of the Electron
Rays in a cathode tube were attracted to a metal plate that had a positive charge
Cathode ray – stream of negatively charged particles
Thomson calculated the electric charge to mass ratio of the particles in cathode rays
Mass of these particles was much less than the atom with the least mass
Electron (e-) – negatively charged subatomic particle
Cathode Ray Tube

11. Thomson’s Atomic Model
Thomson formulated the “Plum Pudding” model
Atoms are made of a positively charged mass and electrons are scattered throughout the mass, like plums mixed in a pudding
Plum Pudding Model
Failed to correctly establish the structure of the atom

12. Rutherford’s Gold Foil Experiment
Bombarded a thin foil of gold with alpha particles
Alpha particles have a +2 charge
Discovery of nucleus
Nucleus – positively charged, dense center of an atom

13. By en:User:Cburnett [GFDL]
Rutherford’s Nuclear Atom
Rutherford estimated that diameter of the nucleus was only 1/10000 of the diameter of the atom
Nucleus accounts for almost all of an atom’s mass
Nucleus contains protons and neutrons
Proton – positively charged subatomic particle (p+)
Neutron – neutrally charged subatomic particle (n0) - J. Chadwick
Rutherford’s nuclear atom showed a positive nucleus orbited by electrons
By en:User:Cburnett [GFDL]

14. Bohr’s Nuclear Atom
Following discovery of the nucleus, Bohr proposed his own atomic theory
Tried to explain why the emission spectrum of the hydrogen atom included only some light frequencies
Proposed a diagram that explained electron movement and energy relations
Hydrogen gas discharge tube
Silt
Prism
Detecting Screen

15. Bohr’s Nuclear Atom Proposed that electrons
Are arranged in circular orbits around the nucleus
Do not lose energy as they orbit
In each orbit have fixed amount of energy
In an orbit closest to the nucleus are in their lowest energy state
Gain and lose energy in certain, fixed amounts called quanta
May receive energy and move to a higher energy orbit
Return back to lower energy orbit when they release energy
Photon – particle of electromagnetic radiation that possesses a quantum of energy
Energy of each photon corresponds to a certain frequency e-
Photon

16. Short Comings of Bohr’s Nuclear Atom
Did not explain emission spectrums of other elements
Did not explain chemical properties of atoms
Further experimentation indicated that electrons don’t revolve in circular orbits
Movement of electrons is much more complex