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The History of Atomic Theory
TEKS 3B, 3C and 5A
Atomic Models

This model of the
atom is the “Bohr
model”. In this model,
the nucleus is orbited
by electrons, which
are in different energy
levels.

A model uses familiar ideas to
explain unfamiliar facts
observed in nature.

A model can be changed as
new information is collected.
Atomic Models through Time


The atomic model has
changed throughout the
centuries, starting in 400
BC, when it looked like a
billiard ball →
To the modern model
theory that recognizes
sub-particles of quarks,
hadrons, gluons and
more.
Critical Understanding
It is crucial to
understand that
atomic theory has
changed over time
as more has been
learned.
 It is not stagnant
and will continue to
evolve as we learn!

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Let’s start WAY
back in time with
the ancient Greeks
Who are these men?
In this lesson, we’ll learn
about the men whose quests
for knowledge about the
fundamental nature of the
universe helped define our
views.
Men of Atomic Theories:
400 BC
Democritus

This is the Greek philosopher
Democritus who began the
search for a description of
matter more than 2400 years
ago.
 He asked: Could matter be
divided into smaller and
smaller pieces forever, or
was there a limit to the
number of times a piece of
matter could be divided?
Atomos

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His 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.
He named the smallest
piece of matter “atomos,”
meaning “not to be cut.”
Atomos


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?

The eminent
philosophers of
the time,
Aristotle and
Plato, had a
more respected,
(and ultimately
wrong) theory.
Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter.
Their ideas held sway because of their
eminence as philosophers. The atomos idea
was buried for approximately 2000 years.
Dalton’s Model

In the early 1800s,
the English
Chemist John
Dalton performed a
number of
experiments that
eventually led to
the acceptance of
the idea of atoms.
Dalton’s Theory
The main points of Dalton's atomic theory are:
 Everything is composed of atoms, which are the indivisible
building blocks of matter and cannot be destroyed.
 All atoms of an element are identical.
 The atoms of a given element are different from those of
any other element.
 Atoms of one element can combine with atoms of other
elements to form compounds.
 Atoms cannot be created, divided into smaller particles, nor
destroyed in the chemical process. A chemical reaction
simply changes the way atoms are grouped together.
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.
Thomson Model
He proposed a model
of the atom that is
sometimes called the
“Plum Pudding”
model.
 Atoms were made
from a positively
charged substance
with negatively
charged electrons
scattered about, like
raisins in a pudding.

Thomson Model
Thomson studied
the passage of an
electric current
through a gas.
 As the current
passed through the
gas, it gave off
rays of negatively
charged particles.

Thomson Model
 This
surprised
Thomson,
because the
atoms of the gas
were uncharged.
Where had the
negative charges
come from?
Where did
they come
from?
Thomson concluded that the
negative charges came from within
the atom.
A particle smaller than an atom had
to exist.
The atom had “parts”!
Thomson called the negatively
charged “corpuscles,” today known
as electrons.
Since the gas was known to be
neutral, having no charge, he
reasoned that there must be
positively charged particles in the
atom.
But he could never find them.
Ernest Rutherford - 1909




Also conducted experiments to
study the atom
He found that atoms have a
nucleus – a small dense center
That center was surrounded by
electrons
His work also concluded that
within the nucleus were smaller
particles, protons
Rutherford’s Gold Foil
Experiment

Rutherford’s
experiment involved
firing a stream of tiny
positively charged
particles at a thin
sheet of gold foil
(2000 atoms thick)
Most of the positively
charged “bullets” passed
right through the gold
atoms in the sheet of gold
foil without changing
course at all.
 Some of the positively
charged “bullets,”
however, did bounce away
from the gold sheet as if
they had hit something
solid. He knew that
positive charges repel
positive charges.


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This could only mean that the gold atoms in the
sheet were mostly open space. Atoms were not
a pudding filled with a positively charged
material.
Rutherford concluded that an atom had a small,
dense, positively charged center that repelled
his positively charged “bullets.”
He called the center of the atom the “nucleus”
The nucleus is tiny compared to the atom as a
whole.
Rutherford’s conclusion

Rutherford reasoned
that all of an atom’s
positively charged
particles were
contained in the
nucleus. The
negatively charged
particles were
scattered outside the
nucleus around the
atom’s edge.
Bohr Model
 In
1913, the Danish
scientist Niels Bohr
proposed an
improvement. In his
model, he placed
each electron in a
specific energy level.
Bohr Model

According to Bohr’s
atomic model,
electrons move in
definite orbits around
the nucleus, much like
planets circle the sun.
These orbits, or energy
levels, are located at
certain distances from
the nucleus.
Modern Theory - Wave or the
Quantum Model
The Wave Model


Today’s atomic model is based on the
principles of wave mechanics.
According to the theory of wave mechanics,
electrons do not move about an atom in a
definite path, like the planets around the sun.
The Wave Model


In fact, it is impossible to determine the exact
location of an electron. The probable location of
an electron is based on how much energy the
electron has.
According to the modern atomic model, at atom
has a small positively charged nucleus
surrounded by a large region in which there are
enough electrons to make an atom neutral.
Electron Cloud:




A space in which electrons are likely to be
found.
Electrons whirl about the nucleus billions of
times in one second
They are not moving around in random
patterns.
Location of electrons depends upon how much
energy the electron has.
Energy and the Electron Cloud:

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Depending on their energy they are 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.
The electron cloud model is used to accurately
represent the structure of the atom
Using models for
understanding:

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The Bohr Model shows all
of the particles in the atom
In the center are circles.
Each circle represents a
single neutron or proton.
Protons should have a plus
or P written on them.
Neutrons should be blank
or have an N
In a circle around the
nucleus are the electrons.
Electrons should have a
minus sign or an e
Electrons have special rules…

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Bohr models atoms’ electrons
organized into energy levels.
Each level can hold a specific
number of electrons
The first energy level can hold up
to 2 electrons
2nd energy level – (up to) 8
electrons
Electrons are not usually added to
energy levels until the lower levels
are filled
The electrons in the outer most
shell of any element are called
valence electrons

A few examples of the Bohr Model:
Lithium has 3
electrons, 2 in
the first level
and 1 in the 2nd.
It has 1 valence
electron
Fluorine has 9
electrons, 2 in
the first level
and 7 in the 2nd.
It has 7 valence
electrons
Aluminum has
13 electrons, 2
in the first level
and 8 in the 2nd
and 3 in the 3rd
It has 3 valence
electrons
Key points:

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In the Bohr model of the atom, the nucleus contains the
majority of the mass of the atom in its protons and
neutrons.
In most cases, electrons fill the lower energy levels first,
followed by the next higher energy orbital until it is full,
and so on until all electrons have been placed.
Atoms tend to be most stable with a full outer shell (one
which, after the first, contains 8 electrons), leading to
what is commonly called the “octet rule”
Octet Rule



The octet rule refers to the tendency of atoms to prefer
to have eight electrons in the valence shell.
atoms will lose, gain, or share electrons in order to have
a full valence shell of 8 electrons. (Hydrogen is excluded
because it can hold a maximum of 2 electrons in its
valence shell. )
Atoms follow the octet rule because they always seek
the most stable electron. Low atomic weight elements
(the first twenty elements) are most likely to adhere to
the octet rule.
The Rule of 8



All atoms want to have a full valence shell - 8
electrons
 This makes them happy atoms that will not
react with other atoms.
Atoms with less than 8 electrons in their valence
shell will react chemically (bond) with other
atoms in one of 2 ways:
 Gain electrons to equal 8
 Lose their valence electrons (to equal 8)
In general, the fewer the electrons that need to
be gained or lost, the easier and faster the atom
will react
More on Energy Levels
Electrons are arranged in Energy Levels or Shells
around the nucleus of an atom.
•
first shell - a maximum of 2 electrons
•
second shell - a maximum of 8 electrons
•
third shell - a maximum of 18 electrons
Exception to the rule:

In the Bohr Model of the atom, electrons fill different
electron orbitals (also known as electron shells or energy
levels) from the inside-out.

The first orbital can only hold 2 electrons. The second
orbital can hold 8 electrons, the third can hold up to 18,
and the fourth up to 32, but the outermost orbital can
never have more than 8 electrons.

Only these outermost electrons, known as valence
electrons, can interact with other atoms in chemical
reactions.
Valence Electrons

The electrons farthest away from the
nucleus in the “outermost energy level”
are called valence electrons
 They affect the way an atom bonds
 Different atoms have a different number
of valence electrons
 Atoms can have anywhere between 1
and 8 valence electrons
Lewis (Dot) Structure

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This is another way to represent valence electrons
(only shows these)
The Lewis structure is used to represent the
covalent bonding of a molecule or ion
When an element is alone, the dots represent the
valence electrons
Crash Course video:
https://www.youtube.com/watch?v=a8LF7JEb0IA or
http://viewpure.com/a8LF7JEb0IA
Atomic Models

Limitations of the Bohr Model:
 Electrons do not orbit the
nucleus of an atom like
planets orbit the Sun
 Scale does not represent
the actual size of an atom
So, Why do we still learn the Bohr
Model?
Using this model makes it easier to
understand the general structure of
atoms.
Activity:
Homework:
First page: Identify elements on the PT
 Use the Bohr model to differentiate among the
three basic particles in the atom (proton,
neutron, and electron) and their charges, relative
masses, and locations.
 Hint: N = Atomic mass- # of P
(remember atomic mass is
P + N)
