Development of Atomic Theory The Scientific Method in Action.

Development of Atomic Theory

The Scientific Method in Action

Democritus vs. Aristotle

Matter is composed of discrete particles. Democritus named these “atoms”.

Matter is continuous. You can break it into smaller and smaller particles indefinitly.

John Dalton - Early 1800’s Conservation of

Matter Mass is conserved in a

reaction. The amount you begin with is equal to the amount you end up with.

John Dalton - Early 1800’s Conservation of Matter Law of Definite

Proportions Types of Matter combine in

given ratios. For example:

H2 + O2 -------> H2O

100g + 793g ----> 893g

50g + 793g ----> 445gWater

John Dalton - Early 1800’s Conservation of Matter Law of Definite

Proportions Law of Multiple

ProportionsMatter combines in Whole Number Ratios

C + O2 -------> CO2

2C + O2 --------> CO

Dalton’s TheoryThe “Pool Ball” Model

All matter is made up of atoms. Atoms are tiny, indivisible, indestructible,

fundamental particles. Atoms cannot be created or destroyed. Atoms of a particular element are alike. Atoms of different elements are different. A chemical change involves the union or

separation of individual atoms.

Problems with Pool Ball Model Doesn’t explain bonding. Doesn’t explain ions

Michael Farraday demonstrated that some matter can be charged or can carry a charge.

Michael Farraday When certain substances are

dissolved in water, they conduct electricity.

Certain compounds decompose into their elements when current is passed through them. Atoms of some elements are then attracted to the negative electrode, others to the positive electrode.

J. J. Thompson’s “Plum Pudding” Modeln Cathode Ray Tube Experimentsn A charge was applied across a tube filled with

various gases or a vacuum. Tiny charges flowed out of the cathode (-) and traveled towards the anode(+). These particles were negatively charged. Where were they coming from?

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J.J.Thompson’s “Plum Pudding” Model

n Tiny, negatively charged particles which Thompson called “corpuscles” and are now known as electrons were imbedded in a positive mass, like raisins in a plum pudding.

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Rutherford’s Scattering Experiment

n Rutherford shot alpha particles (+ charged, 7000x the mass of an electron) at gold foil.

n Prediction - They would go straight through.n Results - The particles were scattered, sometimes at wide angles.

Rutherford wrote - “-----Scattered! It was as if you fired

a 15 inch shell at a piece of tissue paper, and the shell bounced back and hit you.”

Rutherford’s Planetary Model Something small

and massive inside the atom.

Rutherford called it the “nucleus”

Nucleus contains positive protons

Negative electrons orbit around

The Neutron Discovered by

James Chadwick in 1932

About the same mass as a proton

No charge (neutral)

Proton Neutron

Electron(-)

+

Line Spectra of Elements Gas discharge

tubes filled with gas of one element produce light.

Light contains discrete lines

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Bohr’s Planetary Model Electrons exist in

quantized orbitals Certain

wavelengths of light result when electrons jump or fall from one level to another.

Quantum Mechanic Model

Electrons can be written as an equation

Solving equation gives orbitals

These orbitals are areas where electrons can exist

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