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8th Grade Science
Alabama Course of Study
• #2 Describe the structure of atoms,
including the location of protons, neutrons,
and electrons.
– Identifying the charge of each subatomic
particle
– Identifying Democritus and Dalton as
contributors to the atomic theory
© Fall 2005, Pflugerville ISD, 8th Grade
Chapter 2- Introduction to Atoms
Section 1:
Development of the Atomic
Theory
Section 2:
The Atom
© Fall 2005, Pflugerville ISD, 8th Grade
Outcomes
• Students will:
– Describe some of the experiments that led to
the current atomic theory.
– Compare the different models of the atom.
– Explain how the atomic theory has changed
as scientists have discovered new information
about the atom.
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
What Do You Think?
Imagine that you have cut a
penny in half. Then, you take
one piece and half it again. Will
this continue forever, or will you
come to a point where no more
cutting is possible?
© Fall 2005, Pflugerville ISD, 8th Grade
• Remember, matter what everything is
made of.
• The smallest part of matter are called
atoms.
• All matter is made of atoms.
• It started with a man, a knife, and an
apple.
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Democritus was a
Greek philosopher who
theorized that all
matter was made of
invisible particles
called atoms.
Democritus of Abdera,
about 460-370 BCE
© Fall 2005, Pflugerville ISD, 8th Grade
The word atom means
uncuttable or
indivisible.
Development of the Atomic Theory
His views
contrasted those of
Aristotle, who
believed in the four
elements; earth,
water, air, fire.
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Most of our
knowledge of
Democritus comes
from negative
remarks about him
in others’ writings.
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
John Dalton 1766-1844
© Fall 2005, Pflugerville ISD, 8th Grade
Dalton, a British
chemist and
teacher, did studies
and experiments in
weather,
colorblindness, and
gases.
Development of the Atomic Theory
© Fall 2005, Pflugerville ISD, 8th Grade
He noticed that elements, a
substance that cannot be
separated or broken down into
simpler substances by chemical
means, combine in specific
proportions to form
compounds, a substance made
up of atoms of two or more
different elements joined by
chemical bonds,, and theorized
that their atoms combine at the
same proportions
Development of the Atomic Theory
Thomson’s
experiments using
a cathode-ray tube
showed that
smaller particles
make up atoms
Joseph John
“J.J.” Thomson
1856-1940
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Thomson
received the
Nobel Prize in
physics in 1906
for his discovery
of the electron.
© Fall 2005, Pflugerville ISD, 8th Grade
Plum-pudding Model
• Thomson proposed
that electrons were
located throughout an
atom like plums in a
pudding, as shown in
this model.
• Today, you might call
Thomson’s model the
chocolate chip icecream model.
© Fall 2005, Pflugerville ISD, 8th Grade
Bell Ringer
• Prediction:
• What do you think was wrong with the plumpudding model? Explain.
© Fall 2005, Pflugerville ISD, 8th Grade
VIDEO
• Jot Notes as you
watch the video
•
•
© Fall 2005, Pflugerville ISD, 8th Grade
Duell Chemistry
Gold Foil Experiment
Development of the Atomic Theory
Rutherford, a former
student of Thomson’s
from New Zealand, tested
his teacher’s theories in
his Gold Foil Experiment.
Ernest Rutherford
1871- 1937
© Fall 2005, Pflugerville ISD, 8th Grade
•He expected his alpha
particles to go straight
through the foil, and
most of them did.
Development of the Atomic Theory
•But some of the
particles were
deflected or bounced
straight back!
Rutherford’s Gold Foil
Experiment
© Fall 2005, Pflugerville ISD, 8th Grade
•This showed that a
nucleus with a
positive charge
makes up the center
of an atom.
Rutherford’s Model of the Atom
• Rutherford’s model of the
atom had electrons
surrounding the nucleus
at a distance.
• He calculated that the
diameter of the nucleus
was 100,000 times
smaller than the diameter
of the gold atom. Similar
to a stick pin on a football
field.
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Bohr, a Danish
scientist who worked
with Rutherford,
described the motion
of electrons around
the nucleus.
Niels Bohr 1885-1962
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Bohr’s Atomic Model
© Fall 2005, Pflugerville ISD, 8th Grade
Bohr said that
electrons orbit
the nucleus at
specific energy
levels, and can
move from one
level to another.
Development of the Atomic Theory
To do this, Bohr
said, the
electrons must
absorb or
release energy,
often in the form
of light.
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Erwin Schrödinger and
Werner Heisenberg’s
work with the
uncertainty principle
explained that
electrons do not travel
in orbits.
In fact, the exact path
of a moving electron
Schrödinger & Heisenberg cannot be predicted.
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
The current atomic
theory states that
there are regions
inside an atom
where electrons are
likely to be found.
Electron Cloud Model
© Fall 2005, Pflugerville ISD, 8th Grade
These regions are
called electron
clouds.
Summary
• Democritus thought that matter is composed of atoms.
• Dalton based his theory on observations of how
elements combine.
• Thomson discovered electrons in atoms.
• Rutherford discovered that atoms are mostly empty
space with a dense, positive nucleus.
• Bohr proposed that electrons are located in levels at
certain distances from the nucleus.
• The electron-cloud model represents the current atomic
theory.
© Fall 2005, Pflugerville ISD, 8th Grade
Quiz
• What error did
Thomson find in
Dalton’s atomic
theory?
• What is the name for
Thomson’s model of
the atom
• What is the current
model of the atom
called?
© Fall 2005, Pflugerville ISD, 8th Grade
Quiz
• What error did
Thomson find in
Dalton’s atomic
theory?
• What is the name for
Thomson’s model of
the atom
• What is the current
model of the atom
called?
© Fall 2005, Pflugerville ISD, 8th Grade
• Thomson discovered
that atoms are made
of smaller parts.
• The plum-pudding
model
• The electron-cloud
model
Homework
Create this table.
Scientist
Democritus
Dalton
Thomson
Rutherford
Bohr
Schrodinger and
Heisenberg
© Fall 2005, Pflugerville ISD, 8th Grade
Discovery
Bell Ringer
What Do You Think?
Which scientist do you
think made the most
important discovery in
regards to the atom?
Explain.
© Fall 2005, Pflugerville ISD, 8th Grade
Turn and Talk
Share your table with your neighbor.
Scientist
Democritus
Dalton
Thomson
Rutherford
Bohr
Schrodinger and
Heisenberg
© Fall 2005, Pflugerville ISD, 8th Grade
Discovery
• Today you are going to create a book showing
how the atomic theory has changed over time.
• Materials:
– Copy Paper (7 pieces per group)
• 1 of the pages is a title page
–
–
–
–
Colored Pencils/Markers/Crayons
Pens or Pencils
Stapler (to bind the book)
Yesterday’s Notes
© Fall 2005, Pflugerville ISD, 8th Grade
• Steps:
– 1. You will work in the groups that I assign you.
– 2. You will receive two grades for today’s assignment.
• 1 for participation
• 1 for the following criteria
– 2. You MUST include the following in your book:
• Pictures of the different atomic models
• The names of the scientists that we discussed
– What they discovered
• The dates that the discovery happened
• If time permits, you may also want to include:
– Drawings of the experiments
– Drawings of the people
– 3. Be Creative and colorful!
– 4. Staple the left side of all the pages together in the correct order.
© Fall 2005, Pflugerville ISD, 8th Grade
The Atom
What Do You Think?
What is the smallest thing
you have ever seen? How
does it compare to the size
of an atom?
© Fall 2005, Pflugerville ISD, 8th Grade
How Small is an Atom?
• Think about a penny.
• It contains about 2 X 10²²
atoms.
• (20,000,000,000,000,000,
000,000 atoms)
• That’s 20 thousand billion
billion atoms
• In other words, atoms are
really small!
© Fall 2005, Pflugerville ISD, 8th Grade
• Scientists know that aluminum is made of
average-sized atoms.
• An aluminum atom has a diameter of
about 0.00000003 cm. That’s three onehundred-millionths of a centimeter.
• Aluminum foil is about 50,000 atoms thick!
© Fall 2005, Pflugerville ISD, 8th Grade
Parts of the Atom
Nucleus
The atom is made up of even smaller
parts.
Protons
Neutrons
Electrons
Because the masses of particles in
atoms are so small, scientists made a
new unit for them. The SI unit used to
express the masses of particles in atoms
is the atomic mass unit. (amu)
© Fall 2005, Pflugerville ISD, 8th Grade
Parts of the Atom
Nucleus
The nucleus is the small,
dense, positively charged
center of the atom. It
contains most of its mass.
Protons are positively
charged particles in the
nucleus of the atom.
Neutrons are particles in the
nucleus that have no charge
© Fall 2005, Pflugerville ISD, 8th Grade
• A neutron walks into a diner and orders a
glass of orange juice at the lunch counter.
When the waiter brings the juice, the neutron
asks, “How much do I owe you?”
• The waiter replies, “For you, no charge.”
© Fall 2005, Pflugerville ISD, 8th Grade
Parts of the Atom
Nucleus
The electrons are negatively
charged particles found in
the electron clouds out side
the nucleus. The size of the
electron cloud determine the
size of the atom.
© Fall 2005, Pflugerville ISD, 8th Grade
Parts of the Atom
The current atomic
theory states that
there are regions
inside an atom
where electrons are
likely to be found.
Electron Cloud Model
© Fall 2005, Pflugerville ISD, 8th Grade
These regions are
called electron
clouds.
Parts of the Atom
The compared to protons
and neutrons, electrons are
very small in mass.
It takes more than 1,800
electrons to equal the mass
of 1 proton. The mass of an
electron is so small that it is
usually thought of as
almost zero.
Electron Cloud Model
© Fall 2005, Pflugerville ISD, 8th Grade
Charges
• The charges of protons and
electrons are opposite but
equal, so their charges
cancel out.
• Because an atom has no
overall charge, it is neutral.
• What happens if the
numbers of electrons and
protons are not equal?
© Fall 2005, Pflugerville ISD, 8th Grade
• The atom becomes a
charged particle called an
ion.
• An atom that loses and
electron becomes a
positively-charged ion.
• An atom that gains one or
more extra electrons
become a negativelycharged ion.
•
The following image shows Na losing an electron and Cl gaining an electron
•
•
Thus the Na becomes Na+
The Cl becomes Cl-
© Fall 2005, Pflugerville ISD, 8th Grade
VIDEO
• Atoms vs. Ions
© Fall 2005, Pflugerville ISD, 8th Grade
• How does an atom become a positively-charged
ion?
© Fall 2005, Pflugerville ISD, 8th Grade
• How does an atom become a positively-charged
ion?
• An atom becomes a positively-charged ion when
it loses an electron.
© Fall 2005, Pflugerville ISD, 8th Grade
Create a Visual
• Create a visual
representation of the
element Aluminum
changing from an
Aluminum atom to an
Aluminum ion.
© Fall 2005, Pflugerville ISD, 8th Grade
The Atom
What Do You Think?
What are some differences you
use to tell one of your
classmates from another?
© Fall 2005, Pflugerville ISD, 8th Grade
Brainstorm
• Brainstorm:
– How do you think you calculate the atomic
number?
– Write down your ideas.
© Fall 2005, Pflugerville ISD, 8th Grade
How do Atoms of Different
Elements Differ?
• There are more than 110 different
elements.
• The atoms of each of these elements are
different from the atoms of all other
elements.
• Imagine that you could build an atom by
putting together protons, neutrons, and
electrons.
© Fall 2005, Pflugerville ISD, 8th Grade
Starting Simply
• Let’s start with the simplest atom.
– Protons and Electrons are found in all atoms.
– The simplest atom is made of just one of
each.
– It’s so simple that it doesn’t even have a
Neutron.
– To “build” this atom, put just one proton in the
center of the atom for the nucleus. Then, put
one electron in the electron cloud.
© Fall 2005, Pflugerville ISD, 8th Grade
CONGRATULATIONS
• You just built a hydrogen atom.
© Fall 2005, Pflugerville ISD, 8th Grade
Moving on!
• Now draw an atom that has two protons.
• Both of the protons are positively charged,
so they repel one another.
• You cannot form a nucleus with them
UNLESS you add some neutrons.
• For this atom 2 neutrons will do.
• To have a neutral charge, your atom will
also need two electrons outside the
nucleus.
© Fall 2005, Pflugerville ISD, 8th Grade
Helium
© Fall 2005, Pflugerville ISD, 8th Grade
Building Bigger Atoms
• You could build a carbon atom using 6
protons, 6 neutrons, and 6 electrons.
• You could build an oxygen atom using 8
protons, 9 neutrons, and 8 electrons.
• You could even build a gold atom with 79
protons, 118 neutrons, and 79 electrons!
• As you can see, an atom does not have to have
equal numbers of protons and neutrons.
© Fall 2005, Pflugerville ISD, 8th Grade
Turn and Talk
• Why do you think the number of neutrons
is not always the same as the number of
protons or electrons?
© Fall 2005, Pflugerville ISD, 8th Grade
Atomic Number- the Number of
Protons
The Helium atom
has two protons
in its nucleus.
Helium Atom
© Fall 2005, Pflugerville ISD, 8th Grade
•This means that
it has the atomic
number 2.
Atomic Number- the Number of
Protons
How can you tell which elements
these atoms represent?
The number of protons in the
nucleus of an atom is the atomic
number of that atom. The atomic
number is the same for all atoms of
that element.
© Fall 2005, Pflugerville ISD, 8th Grade
• The story of the three bears will help us to
remember how the atom changes.
© Fall 2005, Pflugerville ISD, 8th Grade
Baby Bear
• Baby Bear is going to
symbolize the atom as
being just right.
• By this I mean that the
atom is in its original
atomic form.
• It hasn’t been changed at
all.
© Fall 2005, Pflugerville ISD, 8th Grade
Papa Bear
• Papa Bear is going to
symbolize the atom in ion
form.
• If you remember the story
Papa Bear’s stuff was too
hot.
• Ions are atoms that have
a positive or negative
charge.
• They become this way
because they either gain
or lose electrons.
© Fall 2005, Pflugerville ISD, 8th Grade
Mama Bear
• Last but not least, we
have mama bear.
• Goldilocks was not a
fan of Mama Bear’s
either because it just
wasn’t right. It was
the same as the
others except for one
little change.
• Mama bear will
represent Isotopes.
© Fall 2005, Pflugerville ISD, 8th Grade
Exit Slip
• What is the
atomic number
of Barium?
© Fall 2005, Pflugerville ISD, 8th Grade
Bell Ringer
• Isotopes have the same number of protons, but
different number of _______________.
© Fall 2005, Pflugerville ISD, 8th Grade
Isotopes
Isotopes of Hydrogen
© Fall 2005, Pflugerville ISD, 8th Grade
Notice that these isotopes of
hydrogen have the same
number of protons, but
different numbers of neutrons.
These are called isotopes.
Isotopes
These isotopes are also known as
Hydrogen-1, Hydrogen-2, and Hydrogen-3
© Fall 2005, Pflugerville ISD, 8th Grade
Isotopes
• Atoms that are isotopes of each other are
always the same element, because isotopes
always have the same number of protons.
• They have different numbers of neutrons,
however, which gives them different masses.
© Fall 2005, Pflugerville ISD, 8th Grade
Properties of Isotopes
• Each atom has a limited number of isotopes that
are found in nature.
• Some have special properties because they are
unstable.
• An unstable atom is an atom with a nucleus that
will change over time. (This is called
radioactive.)
• They spontaneously fall apart after a certain
amount of time. As they do, they give off smaller
particles, as well as energy.
© Fall 2005, Pflugerville ISD, 8th Grade
Properties of Isotopes
• However, isotopes of an element share most of the
same chemical and physical properties.
– For example, the most common oxygen isotope has 8 neutrons
in the nucleus. Other isotopes of oxygen have 9 or 10 neutrons.
• All 3 are colorless, odorless gases at room temperature
• Each has the chemical property of combining with a substance as it
burns
• Different isotopes of an element even behave the same
in chemical changes in your body.
– For example, we breathe in various isotopes of oxygen. This is
kind of like the different types of gases at the gas pump. We can
use most all of them to run the vehicle.
© Fall 2005, Pflugerville ISD, 8th Grade
Isotopes Video
• https://www.youtube.com/watch?v=GsJPxR6IfZ
I
© Fall 2005, Pflugerville ISD, 8th Grade
Telling Isotopes Apart
• You can identify each isotope of an element by its mass
number.
• The mass number is the sum of the protons and
neutrons in an atom.
• Electrons are not included because their mass is so
small that they have very little effect on the atom’s total
mass.
© Fall 2005, Pflugerville ISD, 8th Grade
© Fall 2005, Pflugerville ISD, 8th Grade
• What is the mass
number of the 3
isotopes of hydrogen?
© Fall 2005, Pflugerville ISD, 8th Grade
© Fall 2005, Pflugerville ISD, 8th Grade
Naming Isotopes
• To identify a specific isotope of an element, write
the name of the element followed by a hyphen
and the mass number of the isotope.
• Hydrogen-1
– 1 proton and no neutrons
– Mass number =1
• Hydrogen-2
– 1 proton and one neutron
– Mass number=2
© Fall 2005, Pflugerville ISD, 8th Grade
Naming Isotopes
• The carbon isotope with a mass number of 12 is
called Carbon-12.
– If you know the atomic number (listed right above the
element on the periodic table), then you can calculate
the number of neutrons in carbon-12 by subtracting
the atomic number from the mass number.
– Carbon-12
• 12-6=6
© Fall 2005, Pflugerville ISD, 8th Grade
Calculating the Mass of an Element
• Most elements contain a mixture of two or more
isotopes.
– For example, all copper is composed of copper-63
atoms and copper-65 atoms.
– The Atomic Mass of and element is the weighted
average of the masses of all the naturally occurring
isotopes of that element. (dealing with isotopes only)
– A weighted average accounts for the percentages of
each isotope that are present.
– Copper, including the copper in the Statue of Liberty is
69% copper-63 and 31% copper-65.
– Therefore, the atomic mass of copper is 63.6 amu.
© Fall 2005, Pflugerville ISD, 8th Grade
Let me walk you through what
to do.
• Copper-63 = 65% of all copper (63 X 0.65) = 43.47 amu
• Copper-65 = 31% of all copper (65 X 0.31) = 20.15 amu
• 43.47 + 20.15 = 63.62 amu
© Fall 2005, Pflugerville ISD, 8th Grade
© Fall 2005, Pflugerville ISD, 8th Grade
Your Turn
• Calculate the atomic mass of boron, which
occurs naturally as 20% boron-10 and 80%
boron-11.
© Fall 2005, Pflugerville ISD, 8th Grade
• Calculate the atomic mass of rubidium, which
occurs naturally as 72% rubidium-85 and 28%
rubidium-87.
© Fall 2005, Pflugerville ISD, 8th Grade
Exit Slip
• Calculate the atomic mass of gallium, which
occurs naturally as 60% gallium-69 and 40%
gallium-71.
• How many neutrons are there in gallium-69?
• How many neutrons are there in gallium-71?
© Fall 2005, Pflugerville ISD, 8th Grade
Bell Ringer
• What do you think keeps atoms together?
© Fall 2005, Pflugerville ISD, 8th Grade
Forces in Atoms
• You have seen that atoms are made of smaller
particles. But what are the forces (the pushes or pulls
between objects) acting between these particles?
• Four basic forces are at work everywhere, even within
the atom.
–
–
–
–
Gravitational force
Electromagnetic force
Strong force
Weak force
• They work together to give an atom its structure and properties.
© Fall 2005, Pflugerville ISD, 8th Grade
Gravitational Force
© Fall 2005, Pflugerville ISD, 8th Grade
Electromagnetic Force
• As mentioned earlier, objects
that have the same charge
repel each other, while objects
with opposite charge attract
each other.
• This is due to electromagnetic
force.
• Protons and electrons are
attracted to each other
because they have opposite
charges.
• The electromagnetic force
holds the electrons around the
nucleus.
© Fall 2005, Pflugerville ISD, 8th Grade
Strong Force
• Protons push away from one
another because of the
electromagnetic force.
• A nucleus containing two or
more protons would fly apart
if it weren’t for the strong
force.
• At the close distances
between protons and
neutrons in the nucleus, the
strong force is greater that
the electromagnetic force, so
the nucleus stays together.
© Fall 2005, Pflugerville ISD, 8th Grade
Weak Force
• This is important in
radioactive atoms.
• In certain unstable
atoms, a neutron can
change into a proton
and an electron.
• The weak force plays
a key role in this
change.
© Fall 2005, Pflugerville ISD, 8th Grade
Quiz
• What is an atom’s
mass number equal
to?
• How is the atomic
mass of an element
calculated?
• How do isotopes
differ from one
another?
© Fall 2005, Pflugerville ISD, 8th Grade
Quiz
• What is an atom’s
mass number equal
to?
• How is the atomic
mass of an element
calculated?
• How do isotopes
differ from one
another?
© Fall 2005, Pflugerville ISD, 8th Grade
• The total number of
protons and neutrons
in that atom.
• By taking a weighted
average of the mass
numbers of the
isotopes of that
element
• In the number of
neutrons that they
have