Transcript Slide 1

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
► Began
idea of Aristotle
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
Thomson’s
of electrons
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