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The History of Atomic Theory. A piece by piece progression through the structure of the atom. What is Atomic Theory. Atomic theory states that all matter is composed of discrete units called atoms The Greek word atomos means “indivisible”

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the history of atomic theory

The History of Atomic Theory

A piece by piece progression through the structure of the atom

what is atomic theory
What is Atomic Theory
  • Atomic theory states that all matter is composed of discrete units called atoms
    • The Greek word atomos means “indivisible”
    • Before this, people believed that matter could be endlessly subdivided
      • An idea of Aristotle
  • Began as a philosophical idea
    • Shared by Hindu philosopher Kanada (600 BC) and Greek philosopher Democritus (500 BC)
    • No experiments, just ideas

I hope no one asks me to prove this with data…

Any piece of matter can be broken down into smaller pieces.

Therefore, matter is continuous, not discrete.


Matter is made of atoms

In between these atoms is empty space

Therefore, matter is discrete, not continuous.

the first understanding
The first understanding…
  • Early philosophers believed that atoms were all made of the same material but had different shapes and sizes
    • For example, liquids had smooth atoms while solids were made of rough ones
  • Islamic philosophers of the Islamic Golden age (c. 1000 AD) first propsed that smaller particles than atoms might exist
    • Still, no experiments
the birth of the modern theory
The birth of the modern theory
  • 1789 – Antoine Lavoisier formulates the law of conservation of mass, stating that in a chemical reaction matter is not destroyed
  • 1799 - Joseph Louis Proust proved the law of definite proportions, whereby small portions of compounds have the same relative composition as large portions
    • 1 gram of CO2 has twice as many atoms of Oxygen as atoms of Carbon. So does 50 grams, and so does 3400 grams.
john dalton
John Dalton
  • Early 1800’s, Englishman John Dalton builds off these two laws and proposes that:
    • each element is composed of a single type of indivisible atom, so atoms of the same element are identical
    • Atoms of different elements are different
    • Different atoms can combine in whole-number ratios to form compounds
    • Chemical reactions don’t change atoms, they just change how these atoms are arranged
an atom divided
An atom divided
  • 1897 – J. J. Thomson performs an experiment that proves that part of an atom must have an electric charge (negative)
  • The cathode ray tube experiment used a positively-charged plate to deflect a cathode ray (a beam of electrons)
    • Since the positive plate would have to attract negative particles, Thomson knew that negative particles existed. He called them “corpuscles”
    • To balance things out, he knew there must be positive particles as well
the cathode ray tube experiment
The Cathode Ray Tube Experiment

+ Positive field

- Negative field

thomson model
Thomson Model
  • So, Thomson proposes that atoms are not the smallest particle possible. Positive and negative subatomic particles do exist.
  • His model is called the “plum pudding model”



Proper understanding

of electrons

Plum Pudding… yummy…

let s get nuclear
Let’s get NUCLEAR!
  • 1909 – a student of Thomson’s named Ernest Rutherford proposed a new model, which he based on someone else’s experiment – the famous “gold foil experiment”
  • Rutherford claimed that atoms were mostly empty space, and that the positive charge of an atom was located at the center in a dense nucleus.
a simpler view
A simpler view
  • Thomson’s plum pudding model would have predicted this (all particles pass through undeflected)
  • The gold foil experiment showed that some positive particles were deflected (obviously by a positive nucleus)
rutherford model
Rutherford Model
  • A dense, positive nucleus with negative electrons orbiting in the space around it.
  • Often called the “planetary model”

To visualize, the atom is spread out like this:

If the whole atom were pirate stadium, the nucleus would be your class ring sitting on the 50-yard line

this is bohring
This is Bohring…
  • Niels Bohr worked for Thomson and Rutherford, and proposed that electrons travel in orbits that are discretely separated
    • Came up with the idea of energy levels
    • Electrons had to be in one energy level or the next one, not anywhere between
    • When electrons moved down in energy, they released a photon of light
      • The color of this light was related to the energy of the photon, or it’s wavelength
      • Based his findings on experiments with light
bohr s model
Bohr’s model
  • Notice:


the modern understanding
The Modern Understanding
  • Some fellows by the names of Schrodinger, Planck, and Heisenberg did a bunch of quantum physics type experiments that expanded on Bohr’s “orbits”
  • The modern understanding changes these 2-D orbits to 3-D “orbitals,” and we know the probabilities that electrons are in the orbitals at a certain time
    • Hard to measure because electrons are so small
    • Heisenberg uncertainty principle

Now, we know that protons, neutrons, and electrons are actually composed of even smaller particles. But that’s another course…


Do Mean Teachers Mosly read science books

Dalton, Mendeleev, Thomson, Mosley, Rutherford, Schrodinger, Bohr