The Story of the Atom
A history of atomic theory over many years
Democritus
Many years ago, between 460BC and 370BC the Greek philosophers wondered
what we were made of.
Leucippus and Democritus came up with the atomic theory, stating that "the
universe is composed of two elements, the atoms and the void in which they exist
and move."
Democritus' theory:
1. Matter is composed of empty space through which atoms move
2. Atoms are solid, homogeneous, indestructible & indivisible.
3. Different kinds of atoms have different sizes and shapes
4. The different sizes and shapes cause the different properties of matter
5. Changes in matter are due to changes in groupings of atoms, not because the atom it
self changes.
Democritus thought we were all made of small solid particles that cannot be divided.
He said that these were called "atomos" meaning indivisible. Without any of the
scientific tools, there was no evidence to back this up, but the term "atom" and the
original thought of what an atom is came from Democritus.
At the same time Aristotle's theory was that all matter consists of 4 elements, fire, water, air & earth, and that
the combination of these elements gives each material its qualities. This theory went on to influence the
alchemists for about 2,000 years.
Alchemy
For the next few thousand years many discoveries were made. Alchemy began around
300BC and continued until the 1700's when the science of chemistry began. The alchemists
developed many of the tools we use for science today in order to pursue their dreams of
finding one of two things:
One goal of the alchemists was to turn "base metals" the inexpensive metals, into gold.
The other goal was to find eternal life through creation of the "Philosophers stone" or
the "elixir of life". Although neither of these tasks were possible, the experimentation
of the alchemists lead its way into the science we now know as chemistry. Alchemists
used experiment and observations to learn about the world around them.
In the 18th century we began to practice modern chemistry. One main difference was that Chemistry was
quantitative, meaning that everything was measured and could be studied that way. Alchemy was primarily
Qualitative, looking merely at the physical outcome and physical properties, without significant measurement.
Realizing that "air" was made of "gasses" was a huge turning point, because it meant that Aristotle was wrong
about air being an element itself. The ability to measure everything that went into and out of an experiment
was a significant change in science, leading to the Quantitative science of chemistry, where the temperature
and mass of all substances was recorded during reactions.
You will learn about some of the key chemists in the transition from Alchemy to
Chemistry throughout the school year. Robert Boyle objected to the Aristotelian
elements. Antoine Lavoisier began "the chemical revolution" with his law of
conservation of Mass, stating that no matter can be created or destroyed. It is
then, in 1773, that Chemistry was truly born.
Robert Boyle
Antoine Lavoisier
Dalton
In 1803, John Dalton wanted to condense the information that was known about
the atom. Through many quantitative experments, he was able to discover mass
ratios of elements involved in reactions, leading to further development of the
ideas that Democritus had two thousand years earlier.
He ended up with 5 statements that make up Dalton's atomic theory.
DALTON'S ATOMIC THEORY: 1803
1. All matter is made of extremely tiny particles called atoms
2. All atoms of a given element are identical in mass and properties, and they are
different from any other element
3. Atoms cannot be created, divided into smaller particles or destroyed.
4. Atoms combine in simple whole number ratios to form compounds
5. In chemical reactions, atoms are separated, combined or rearranged.
Although we have found some exceptions for the above rules in the last 200+ years (isotopes, nuclear
reactions & subparticles), these are key ideas that were used as the foundation of modern chemistry.
Dalton considered the atom to be a sphere, and that the atom was the smallest possible particle.
JJ. Thompson
Scientists like to study the unknown. If you have ever been shocked when you touched a friend or a piece of
metal and wondered what just happened, you are not unlike JJ Thompson and other early scientists who
decided to study this phenomenon.
What they did was take a glass tube and pump most of the
air out of it. They then passed electricity through the tube.
They found that the electricity formed rays from the cathode
to the anode across the tube, giving it the name Cathode Ray
Tube or CRT.
By the end of the 1800's they had found that these "rays"
were actually a stream of negatively charged particles.
In the 1890's JJ Thompson decided to find out how much mass each particle had when compared to it's
charge.
The amazing thing was that JJ Thompson found out that the mass of one of these particles was smaller than
the Hydrogen atom... but at the time it was thought that the Hydrogen atom was the smallest possible particle!
This meant that this particle, called the electron, was smaller than an atom, and was part of what made up an
atom. JJ Thompson had disproved Dalton and found that Atoms were divisible into smaller subatomic
particles, because the electron was one of them!
BEFORE JJ Thompson:
DISCOVERY: 1890
After JJ Thompson
Electrons are a sub atomic particle
Atoms are the
smallest particle
Plumb Pudding Model: Atoms
are made of negative electrons
in a positive sphere.
Ernest Rutherford
Rutherford wanted to find out what happens when you pass alpha particles (emitted from a radioactive
source) were passed through an atom. Because gold could be made very very thin without breaking, he
decided a very thin piece of gold foil would be the perfect way to do his experiment. If the radioactive source
was in a small box with one hole, the alpha particles would make a stream.
Rutherford thought the atom looked like JJ Thompson's plumb pudding model, and so he
thought that most of these positive alpha particles would go right through, but some would pass
close enough to electrons for them to shift their path slightly.
When he began his experiment and some of the particles bounced backwards he
said "it is like shooting a missile at a piece of paper and having it bounce back at you" because
the actual results were so unexpected. The alpha particle was very large and the only way for it
to bounce back is if there was something with greater density inside the atom.
Based on these findings, the atom could not look like JJ Thompson had modeled it.
Rutherford considered the data in order to come up with a new atomic model. The only way this could
have happened is if there was a tiny dense region he called the nucleus in the center of the atom. He
concluded it contained all of the positive charge and most of its mass. He said that the positively
charged particles in the nucleus were protons. The rest of the atom, about 99.999% of its volume, was
empty space!
Before Rutherford:
Plumb Pudding Model: Atoms
are made of negative electrons
in a positive sphere.
Discoveries 1911-1920
Dense nucleus contains
positive protons
The atom is 99.99%
empty space
After Rutherford:
Rutherford's Model includes a central
nucleus made of protons and electrons
moving through empty space
Niels Bohr
When Rutherford discovered the nucleus, it was known that a new model of the atom was needed. Bohr
created his idea of the atomic model in 1915, and it can also be called the "planetary model" of the atom. The
electrons orbit the nucleus like planets orbiting the sun. No actual image of this model can be "to scale"
because if the atom was 2 football fields in diameter, the nucleus would be the size of a nickel. This means hat
no model of the atom is actually to scale.
The key to the Bohr model is that the orbits are quantized, meaning
they are at specific distances from the nucleus, with no orbitals (no
electrons) in between. Each orbital has an energy level. If the
electrons are in the lowest available orbital they are in the ground
state, but they can move up to higher levels if they are excited by
energy.
Each element has different distances between the nucleus and orbitals,
giving that element specific properties.
You will study this more in depth when you find out more about
electron configuration, but the organization of electrons into these
"orbitals" is the key to many properties of each type of atom.
Before Bohr
Rutherford's Model includes a central
nucleus made of protons and electrons
moving through empty space
Discovery: 1915
Bohr merged the ideas of
Rutherford and Plank to build
a model of the atom explaining
the behavior of electrons
After Bohr
The Bohr model shows electrons orbiting
around the nucleus at quantized levels
James Chadwick
Chadwick worked in Rutherford's lab, where they bombarded all kinds of things with alpha particles, studying
the atomic nuclei.
He set up an experiment where he shot alpha particles at beryllium
foil, which caused an uncharged radiation to be emitted. This
radiation hit the wax, which bumped the protons off of the wax.
Using charges he was able to discover that the radiation from the
beryllium was not charged, however it had mass. This was the
discovery of a neutral particle with mass that Chadwick called the
neutron.
Chadwick worked with Hans Geiger (of the Geiger counter) until he
was imprisoned in Germany during WWI, then moved to America
and eventually went on to be part of the Manhattan project,
developing the world's first atomic weapons.
Before Chadwick
Discovery: 1932
After Chadwick
The atom contains
neutral particles
with large mass in
the nucleus. These
are called neutrons.
The Bohr model shows electrons orbiting
around the nucleus at quantized levels
The nucleus now has protons AND
neutrons.
Schrodinger
The Bohr model continues to be a helpful way to imagine the distribution of
electrons and some of the behaviors of electrons, but Schrodinger set out to
include more information about the orbitals and the dual wave-particle nature of
the electron. His model is based on probability, because we cannot know where
an electron is, but rather where it might be.
Schrodinger uses quantum
numbers as coordinates to describe
the size, shape and orientation in
space of the orbitals. His model is
known as the Quantum Mechanical
Model of the atom. The probable area of finding an electron is
represented by an electron cloud in the shape of the orbital.
Each electron pair's probability is represented by an orbital
cloud and these orbitals are all layered over one another to
make the atom:
Before Schrodinger
The Bohr model shows
electrons orbiting the nucleus
Discovery: 1926
After Schrodinger
The electrons
behave based on
probability and can
be thought of as a
particle or a wave
The current model
of the atom is
known to have
orbital clouds
representing
electrons probable
locations.
This booklet was created by Jewyl Clarke using Glencoe's Matter and Change as well as the following
references
http://the-history-of-the-atom.wikispaces.com/
http://antoine.frostburg.edu/chem/senese/101/atoms/dalton-postulates.shtml
http://rudar.ruc.dk/bitstream/1800/3961/1/The%20Birth%20of%20Modern%20Chemistry.pdf
http://csep10.phys.utk.edu/astr162/lect/light/bohr.html
http://www.nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-bio.html
http://www.nndb.com/people/728/000099431/
https://sites.google.com/site/chadwickexperiment/home
http://wekasyah88.blogspot.com/2012/06/normal-0-false-false-false-in-x-none-ar.html
http://chemed.chem.purdue.edu/genchem/history/schrodinger.html
http://www.abcte.org/files/previews/chemistry/s1_p6.html