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” 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

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

Therefore, matter is continuous, not discrete.

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

Matter is made of atoms

In between these atoms is empty space

Therefore, matter is discrete, not continuous.

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

►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►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

Dalton’s model

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

+ Positive field

- Negative field

A more 3-D depiction of Thomson’s Experiment

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

Thomson’s

understanding

Plum Pudding… yummy…

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.

Gold Foil Experiment

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

►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…►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

►Notice: 2-Dimensional

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

Orbitals

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