The Story Behind Atomic Theory
Introduction
Meet Democritus
Chemistry is the science of matter. You should already be familiar
with concepts like substances, elements, compounds, molecules,
and other such terms.
Ever wondered though, what are all these things made of?
Let’s use Lego® as an example. I have a nice house made of
Lego® pieces. Now, if I break down this house, I’ll have walls and
windows and a door. I can break down these further, and even
further, until I get an individual, smallest piece of Lego® that still
retains the properties of the house (in this case, colour and
texture).
Does this work in nature? And if so, who thought of it?
Atomic theory is a theory that attempts to answer that question.
It states that all matter is composed out of extremely small
particles called atoms. An atom is the smallest particle that
retains the properties of an element (so a carbon atom would still
have the properties of carbon, but if you break the atom down
these properties will disappear). This theory, although it sounds
simple, is in reality extremely complex. It eliminated the notion
that matter like water would never stop being water no matter
how small a quantity of it you have. Although it’s still only a
theory, nowadays, it is widely accepted and supported by
virtually everyone.
And like most theories, the atomic theory has great story and
history behind it.
Like most modern science, the atomic theory started in Ancient
Greece. The guy who thought it all up was known as Democritus,
a materialist philosopher, in the 5th century BC. He thought very
hard about matter and the universe, and came up with an idea.
Democritus stated that all matter is made up of units that move
around in a void. Democritus believed that these units are
indivisible and unchangeable. In fact, he decided to call them
atomos, which means uncuttable in Greek. Although these
atomos are all made up of the same matter, their shape, size, and
orientation in space explains the existence of different
substances and materials.
While most of Democritus’ ideas aren’t acceptable today (atoms
are divisible into smaller units, and they are in constant change),
the revolutionary thought that matter is composed of tiny
particles moving in a void is accepted today as the basis for
modern atomic theory.
For hundreds of years after this, nobody cared, and atomic
theory wasn’t explored. People either accepted the notion that
matter can be divided into infinitely small amounts without a
change in properties, or that there’s something that makes up
matter, but since nobody could really check, it made no
difference.
Adapted from http://chemistry.learnhub.com/lessons
Ms. Withrow 2008
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The Story Behind Atomic Theory
John Dalton’s Atomic Theory
There are a few important rules that came about in the late
1700s that were crucial to Dalton’s thoughts. First was the Law of
Conservation of Mass. This law, formulated by Lavoisier, states
that during a chemical reaction, the total mass of the reactants
equals the total mass of the products. This suggested that matter
is indestructible.
The second law was called the Law of Definite Proportions
proposed by Proust. The law states that whatever quantity of
substance you take, the proportions of the masses of elements
composing this substance will always remain equal. For example,
the ratio of oxygen to carbon in a tiny vile of carbon dioxide will
equal the ratio of O to C in a large container.
In an attempt to explain how and why elements would combine with
one another in fixed ratios and sometimes also in multiples of those
ratios, Dalton formulated his atomic theory.
Here are the main points of Dalton’s atomic theory:
 All elements are made of tiny particles, called atoms.
 All atoms of one element are identical, though atoms of
different elements are different, specifically in terms of
mass.
 Atoms can’t be created, destroyed, or subdivided.
 Atoms of one element combine with atoms of another
element to form compounds. The molecules of these
compounds always have the same proportions of
elemental atoms.
It’s worth noting that this theory isn’t totally accepted today- lots
of improvements were made over the years. For example, we
now know that atoms can be subdivided into protons, electrons,
and neutrons.
Once this theory was introduced, things started to move a lot
more rapidly. Everyone seemed to be researching this or that,
trying to find out something worthy and get into the history (and
science) books.
JJ Thomson’s Discoveries
By the 1850s, scientists began to realize that the atom was made
up of subatomic particles that were thought to be positively and
negatively charged. During this time scientists began to study
electrical discharge through cathode-ray tubes (think neon
signs). These were partially evacuated tubes in which a current
passes through forming a beam of electrons. This beam moves
from the cathode (positive end) to the anode (negative end). The
electrons cannot be seen, but it was found that certain materials
fluoresce (give off light) when energized.
JJ Thompson observed that when a magnetic or electric field are
placed near the electron beam, they influence the direction of
flow – opposite charges attract each other and like charges will
repel. He also found that the magnetic field forces the beam to
bend, depending on its orientation.
From this Thompson concluded that: (1) cathode rays consist of
beams of particles and (2) the particles have a negative charge.
Adapted from http://chemistry.learnhub.com/lessons
Ms. Withrow 2008
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The Story Behind Atomic Theory
However, this wasn’t all. Thomson understood that all matter
was inherently neutral, so there must be a counter to this
negative charge, a positive charge. But where to put it? He
suggested that the negative charges were balanced by a positive
umbrella-charge as shown in the picture below.
From this, Thomson described the atom as having an umbrellacharge, also known as a positively charged electron cloud, with
negatively charged electrons flowing freely throughout. This
model is often referred to as the “plum pudding model” or the
“chocolate chip cookie model” because of its resemblance to
these things. The dough in the cookie represents the positive
umbrella-charge while the chocolate chips represent the
negatively charged electrons.
Ernest Rutherford “The Father of Nuclear Physics”
Rutherford, who was Thomson’s student, is widely known for the
Gold Foil Experiment. This experiment was simple – you take a
really thin foil of gold, a few atoms thick, and bombard it with
positively charged alpha particles. According to the cookie model
proposed by Thomson, the particles should pass through
undisturbed.
Above is a picture of the Gold Foil experiment. The real results of
this were shocking- while most particles passed through
undisturbed, a few were heavily deflected. In 1911, Rutherford
finally published a solution to this odd result- the cookie model is
wrong. Atoms aren’t made of electrons with positive dough
around them.
Based on this work, and contributions from other scientists,
Rutherford developed a new model of the atom. This model
introduced the nucleus – a centre where the majority of the
atom’s mass and charge is located. His nucleus contains protons
and neutrons and has an overall positive charge. The remaining
area of the atom was an electron cloud, negatively charged,
making up the majority of the atoms volume.
This model of the atom is pretty much
what we use today: a nucleus with
orbiting electrons around it. It gets a
little more complex than this, but we
will leave that for another time.
Adapted from http://chemistry.learnhub.com/lessons
Ms. Withrow 2008
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