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CHAPTER 4
ATOMIC STRUCTURE
ANCIENT GREEK MODELS OF
THE ATOM
Democritus
believed that all matter
consisted of small particles that could not
be divided.
He called these particles atoms from the
Greek word atamos meaning “uncut” or
“indivisible”
He thought there were different types of
atoms for example atoms in liquids were
round and smooth.
ARISTOTLE
 Aristotle
did not think
there was a limit to the
number of times matter
could be divided.
 He thought that all
substances were built
from four elements:
 Element-matter that is
made of one type of atom.
 Ex. Hydrogen is only
made of hydrogen atoms
DALTON’S ATOMIC THEORY
 John
Dalton, English
teacher that studied
the behavior of gases
in the air.
 Based on the way
gases exert pressure,
he correctly concluded
that a gas consists of
individual particles.
DALTON
Dalton
measured the masses of elements
that combine when compounds form.
He noticed that the ratio of masses is
always the same, they have a fixed
composition.
He proposed the theory that all matter is
made up of individual particles called
atoms, which cannot be divided.
DALTON’S THEORY – 4 parts
1. All elements are composed of atoms.
2. All atoms of the same element have the
same mass, and atoms of different
elements have different masses.
3. Compounds contain atoms of more than
one element.
4. In a particular compound, atoms of
different elements always combine in the
same way.
Was Dalton Correct?
 Dalton
used solid
spheres to represent
atoms….. Incorrect!
 “All atoms of the same
element have the same
mass” - what about
isotopes?
 Matter is composed of
indivisible particles –
atoms can be divided
(nuclear reaction)
“different
elements have different masses” -
Correct!
“elements always combine in the same
way.” - Correct!
Theory
was widely accepted just revised
once new discoveries were made.
JJ THOMSON
 Objects
such as amber and
glass can attract and repel
other objects…remember that
opposites _________.
attract
 JJ
Thomson used an electric
current to learn more about
atoms.
 He discovered first subatomic
particles – electrons-negatively
charged particles!!!! (add to
notes)
THOMSON’S EXPERIMENT
 He
hypothesized that a
stream of charged
particles that interacted
with air in the tube would
cause air to glow.
 He observed that the
beam was repelled by the
negatively charged plateAnode and attracted by
the positively charged
plate.- cathode
THOMSON’S CONCLUSION
Thomson
concluded that the particles in the
beam had a - charge because they were
attracted to the + plate.
This changed the way scientists thought
about atoms (remember they thought an
atom was a solid ball of matter)
Thomson’s experiments provided the first
evidence that atoms are made of even
smaller particles, he revised Dalton’s
model.
THOMSON’S MODEL
In
his model, negative charges were evenly
scattered throughout an atom.
“Plum pudding” model, chocolate chip ice
cream model.
QUICK REVIEW – USE CARDS
1.
•
•
•
•
Who said there were only 4 elements?
A. Aristotle
B. Democritus
C. Dalton
D. Thomson
2.
Negative charges evenly scattered
throughout.
• A. Aristotle
• B. Democritus
• C. Dalton
• D. Thomson
CARDS
 3.
Matter is made up of atoms which are solid
spheres.
•
•
•
•
A. Aristotle
B. Democritus
C. Dalton
D. Thomson
 4.
Termed small particles atoms from Greek
word atamos.
•
•
•
•
A. Aristotle
B. Democritus
C. Dalton
D. Thomson
RUTHERFORD’S ATOMIC
THEORY
 His
student, Ernest
Marsden, set up gold foil
experiment to find out what
happens to particles as
they pass through a thin
sheet of gold.
 Rutherford predicted that
most particles would travel
in a straight path from their
source.
Rutherford’s
Prediction
GOLD FOIL EXPERIMENT
 During
the gold foil
experiment, a beam
was aimed at the gold,
which would produce a
flash of light when it
struck the screen.
 By observing the flash,
he could figure out the
path of the particle. PROBLEM!!!!!!!!!!!
•More particles were deflected than
expected!
•Some even bounced straight back!
DISCOVERY OF THE NUCLEUS
Rutherford
concluded that the positive
charge of an atom-proton is not evenly
spread throughout the atom.
It is concentrated in a very small, central
area that Rutherford called the nucleus.
The nucleus is a dense, positively charged
mass located in the center of the atom.
RUTHERFORD’S MODEL
According
to Rutherford’s model, all of an
atom’s positive charge is concentrated in its
nucleus.
The particles that were reflected back came
close to the nucleus and those that went
through passed through space surrounding
nucleus.
SIZE OF AN ATOM
 If
an atom had the
same volume as a
football stadium, its
nucleus would have
the volume of a
marble!!!
 Total volume of an
atom is about a trillion
times that of the
nucleus.
(1,000,000,000,000)
QUICK REVIEW – USE CARDS
1.
One of the first people to state that matter
is made up of atoms was
• A. Aristotle
• B. Democritus
• C. Dalton
• D. Thomson
2.
Thomson’s model of an atom is best
described as a(n) ____

a. solar system
b. solid sphere

c. plum pudding
 3.
Who provided the first evidence that atoms
contain subatomic particles?
• A. Aristotle
B. Democritus
• C. Dalton
D. Thomson
 4.
Who discovered electrons?
• A. Aristotle
B. Democritus
• C. Dalton
D. Thomson

 5.
Which scientist experimented with gold foil?
• A. Aristotle
B. Democritus
• C. Rutherford
D. Thomson
6.
What was one incorrect concept in
Dalton’s model?

A. used solid spheres to represent
atoms

B. States that all atoms of the same
element have the same mass

C. Matter is composed of indivisible
particles

D. All of the above
Bohr’s Model of the Atom
Niels
Bohr was a Danish
physicist who expanded
Rutherford’s Atomic Model
Bohr focused his arrangement
around the positioning of
electrons
Energy Levels
In
Bohr’s model, electrons move with
constant speed in fixed orbits around the
nucleus, much like planets orbit a sun.
The possible energies that electrons have
in an atom are called energy levels.
If an atom gains or loses energy, the
energy of an electron can change.
Evidence for Energy Levels
Scientists
can measure the energy gained
when electrons absorb energy and move to
a high level
Example:
• Heat produced by an explosion causes some
electrons to move to higher energy levels. When
the electrons move back to lower energy levels,
they emit some energy in the form of light. (Not all
energy is lost, it has to go “somewhere” and in this
case it is released in the form of light)
Fireworks
Electron Cloud Model
Building
from Bohr’s Model scientists now
know that electrons move in a much more
unpredictable manner than Bohr proposed.
An Electron Cloud is a visual model of the
most likely locations for electrons in an
atom.
Scientists use the electron cloud model to
describe the possible locations of electrons
around the nucleus.
Electron Cloud Similar to Plane
Prop
Electron
s move
so
quickly
they are
a blur
Energy levels, Orbitals, and Electrons
SECTION 4.2
THE STRUCTURE OF AN ATOM
33
SUBATOMIC PARTICLES
PARTICLE
PROTON
ELECTRON
NEUTRON*
SYMBOL CHARGE RELATIVE LOCATION
MASS
p+
+
1
e-
1/1836
n
1
0
Nucleus
Electron
cloud –
orbiting
nucleus
Nucleus
* James Chadwick concluded that neutrons are neutral because a
charged object did not deflect the paths of the particle.
ATOMIC # AND MASS #
 Atoms
of an element
always have the same
number of protons.
 Atomic # = number of
protons
 Since electrons = protons,
atomic # also tells you the
# of electrons
 For example: Sulfur has an
atomic # of 16 so it has 16
protons and 16 electrons.
16
S
Sulfur
32.06
ATOMIC # AND MASS #
6
C
Carbon
12.011
How
many protons
does Carbon have?
6
How many
electrons does
Carbon have?
6
Mass number
Mass
number is the sum
of the protons and
neutrons in the
nucleus.
To find the # of
neutrons = Mass #
minus # of protons
For example:
Sulfur has a mass # of
32.06
Subtract # of protons
32.06-16 = 16 neutrons
16
S
Sulfur
32.06
Mass number – Round up
3
Li
Lithium
6.941
How
many neutrons
does Lithium have?
Mass
# of 7-3 protons
= 4 neutrons
ISOTOPES
 Every
atom of the same element has the same
number of protons and electrons. For example
all atoms of Carbon have 6 electrons and 6
protons.
 Every atom of an element does not have the
same number of neutrons.
 Isotopes are atoms of the same element that
have different numbers of neutrons and
therefore different mass numbers.
ISOTOPES
 Isotopes
of an element have the same atomic
number but different mass numbers because
they have different numbers of neutrons.
 For example Carbon 14 is a radioactive
isotope. Usually, carbon has a mass number of
12, but Carbon 14 has a mass number of 14
due to the 2 extra neutrons.
 6 protons + 8 neutrons = mass number of14
 How many neutrons are in Carbon 13?
 Mass of 13-6 protons = 7 neutrons
Two isotopes of sodium – sodium usually
has 11 neutrons
PRACTICE
ISOTOPE
ATOMIC #
# OF
NEUTRONS
Nitrogen-15
7
8
Iodine-131
53
78