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The History of Atomic
Theory
The atom
We know the atom is made up of 3 particles...
What are they?
3 Subatomic Particles
Protons = Positively charged
Neutrons = Neutrally charged (no charge)
Electrons = Negatively charged
The atom
Protons and
Neutrons are in the
nucleus… so what
charge does the
nucleus have?
The atom
Electrons orbit
around the nucleus in
different ENERGY
LEVELS
Atomic Models
This is the Bohr model of the atom, this is actually
NOT how the atom looks but it’s pretty close, so we
can use it for now…
Models of the Atom
Over the last 2400 years, the model of the
atom has gone through 7 major changes
You need to know:
 Who came up with each model
 What it looks like
 How they came up with it
Democritus
In 400 B.C. Greek Philosopher
Democritus came up with a theory:
 Matter could not be divided into
smaller and smaller pieces forever
 Eventually the smallest possible
piece would be obtained
 This piece would be indivisible
Atomos
He named the smallest piece of matter “atomos,”
meaning “not to be cut.” We now know these as
atoms.
To Democritus, atoms were small, hard particles
that were all made of the same material but were
different shapes and sizes.
Atoms were infinite in number, always moving
and capable of joining together.
This theory was ignored and
forgotten for more than 2000
years!
Why?
Aristotle and Plato believed
all matter was made up of
earth, fire, air and water.
They were more well known
so their ideas were believed
to be correct for 2000 years.
Dalton’s Model
In the early 1800s, John
Dalton performed
experiments that eventually
led to the acceptance of the
idea of atoms.
Dalton’s Theory
He deduced that all elements are composed of atoms.
Atoms are indivisible and indestructible particles.
Atoms of the same element are exactly alike while
atoms of different elements are different.
This theory became one of the foundations of modern
chemistry
Thomson’s Plum Pudding Model
In 1897, the English scientist J.J.
Thomson provided the first hint that
an atom is made of even smaller
particles.
He proposed a model of the atom
that is called the “Plum Pudding”
model.
Thomson Model
Atoms were made from a
positively charged
substance with negatively
charged electrons scattered
about, like raisins in a
pudding.
Thomson Model
Thomson was doing an experiment- running an
electric current through a gas.
As the current passed through the gas, it gave off rays
of negatively charged particles. This surprised
Thomson, because the gas had no charge
Where did the negative
charge come from?
Thomson Model
Thomson concluded that the
negative charges came from
within the atom.
Where did
they come
from?
Thomson found the electron!
Since the gas had no charge, he
thought that there must also be
positively charged particles in
the atom, but he could never find
them.
Rutherford’s Gold Foil Experiment
In 1908, Ernest Rutherford was doing an experiment
involving firing a stream of positively charged particles
at a thin sheet of gold foil
Most of the particles passed through the foil but some
of the particles bounced off the gold foil.
He knew that positive charges repel positive charges
so there must be a positive charge in the atom
Rutherford
He concluded that
MOST of the atom is
empty space.
Rutherford concluded that
an atom had a small,
positively charged center,
he called it the nucleus.
The nucleus is tiny
compared to the atom as a
whole.
Bohr Model
In 1913, Danish scientist
Niels Bohr proposed an
improvement to the model.
In the Bohr model, each
electron is placed in a
specific energy level.
Bohr Model
In the Bohr model, electrons
move in definite orbits
around the nucleus, like the
planets circling the sun.
These orbits, or energy
levels, are located at certain
distances from the nucleus.
The Wave Model
The current atomic model is known as the
“wave model”
While many things added up to form the model,
Schrodinger was the biggest contributor
The Wave Model
The most current atomic
model is based on the
principles of wave mechanics.
According to the theory of
wave mechanics, electrons do
not orbit the atom like planets
around the sun.
It is impossible to determine
the EXACT location of an
electron.
The Wave Model
Instead we can know the
general location of an
electron is based on how
much energy the electron
has.
According to the wave
model, the atom has a small
positively charged nucleus
surrounded by a large
electron cloud
Electron Cloud
The electron cloud is a space in which electrons are
likely to be found.
Depending on how much energy the electron has,
the electron is locked into a certain area in the cloud.
Electrons with the lowest energy are found in the
energy level closest to the nucleus
Electrons with the highest energy are found in
the outermost energy levels, farther from the
nucleus.
What does the periodic table tell us
about the structure of the atom?
Atomic Number = the number of protons
Atomic Number also = the number of electrons
Atomic Mass = Number of Protons + Number of
Neutrons
Neutrons = Atomic Mass – Atomic Number