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Atomic Theories
Chapter 17 Page 506
Atomic Theories First Thoughts
• People began wondering about matter more
than 2,500 years ago.
• Some of the early philosophers thought
that matter was composed of tiny particles.
• They reasoned that you could take a piece of
matter, cut it in half, cut the half piece in half
again, and continue to cut again and again.
• Eventually, you wouldn’t be able to cut any
more. You would have only one particle left.
• They named these particles atoms, a term
that means “cannot be divided.”
A Model of the Atom- a model is a
simplified representation, a picture.
• During the eighteenth century, scientists in
laboratories began debating the existence of
atoms once more.
• Chemists were learning about matter and
how it changes.
• They found that certain substances couldn’t
be broken down into simpler substances.
• Scientists came to realize that all matter is
made up of elements.
• An element is matter made of atoms of only
one kind.
Classifications of Matter (p450)
Matter
Yes
Yes
Can it be
separated by
physical means?
Is the composition
uniform?
Homogeneous
mixture
No
Pure
substance
Mixture
Yes
Yes
No
No
No
Heterogeneous
mixture
Can it be decomposed by
ordinary chemical means?
Yes
Yes
Compound
No
No
Element
Chemical Laws
• Law of Conservation of Mass or matter states that mass
is neither created nor destroyed in ordinary chemical
reactions.
• Law of Definite Proportions- the fact that a specific
chemical compound contains the same elements in
exactly the same proportions by mass regardless of the
size of the sample or the source of the sample.
• Law of Multiple Proportions- if two or more different
compounds are composed of the same two elements,
then the ration of the masses of the second element
combined with a certain mass of the first element is
always a ration of small whole numbers.
Examples
• Law of Definite Proportions
• Carbon Dioxide
– Always 73% oxygen and 27% carbon by weight
• Law of Multiple Proportions
• Carbon Monoxide
– 6 grams of carbon, 8 grams of oxygen
• Carbon Dioxide
– 6 grams of carbon, 16 grams of oxygen
Dalton’s Concept (1766-1844)
• John Dalton, an English schoolteacher
proposed the following ideas about matter:
1. Matter is made up of atoms.
*2. Atoms cannot be divided into smaller pieces.
*3. All the atoms of an element are exactly alike.
4. Different elements are made of different
kinds of atoms.
• Dalton pictured an atom as a hard sphere that
was the same throughout.
Scientific Evidence
• In 1870, the English scientist William
Crookes did experiments with a glass tube
that had almost all the air removed from it.
• The glass tube had two pieces of metal
called electrodes sealed inside.
• The electrodes were connected to a battery
by wires.
A Strange Shadow
• One electrode,
called the anode,
has a positive
charge.
• The other, called
the cathode, has
a negative charge.
• When the battery was connected, the glass tube
suddenly lit up with a greenish-colored glow.
A Strange Shadow
• A shadow of the
object appeared at
the opposite end
of the tube—the
anode.
• The shadow
showed Crookes
that something was traveling in a straight line
from the cathode to the anode, similar to the
beam of a flashlight.
Cathode Rays
• Crookes hypothesized that the green glow
in the tube was caused by rays, or streams
of particles.
• These rays were called cathode rays because
they were produced at the cathode.
• Crookes’s tube is known as a cathode-ray
tube, or CRT. – TV or computer monitor.
J.J Thompson
• In 1897, J.J. Thomson, an English physicist,
tried to clear up the confusion.
• He placed a magnet beside the tube from
Crookes’s experiments.
• The beam is bent in the direction of the
magnet.
• Light cannot be bent by a magnet, so the
beam couldn’t be light.
The Electron
• Thomson concluded that cathode rays are
charged particles of matter.
• Based on the direction the beam bent in
the magnetic field he reasoned that the
beam was negative charges.
• He observed that these particles were
attracted to the positively charged anode,
so he reasoned that the particles must be
negatively charged.
The Electron
• These negatively charged particles are now
called electrons.
• Thomson also inferred that electrons are a
part of every kind of atom because they are
produced by every kind of cathode material.
• If atoms contain one or more negatively
charged particles, then all matter, which
is made of atoms, should be negatively
charged as well.
• But matter is not negatively charge.
Could it be that atoms also contain some
positive charge?
Thomson’s Atomic Model
• Thomson pictured a sphere of positive
charge.
• The negatively charged electrons were
spread evenly among the positive charge.
• The atom is neutral.
• Plum pudding model (chocolate chip cookie)
Chocolate Chip Cookie Model
Rutherford’s Experiments
• Ernest Rutherford and his coworkers began
an experiment to find out if Thomson’s
model of the atom was correct.
• They wanted to see what would happen when
they fired fast-moving, positively charged
bits of matter, called alpha particles, at a
thin film of metal such as gold.
Rutherford’s Gold Foil Experiment
A Model with a Nucleus
They might have drawn
diagrams like those
which uses Thomson’s
model and shows what
Rutherford expected.
Most alpha particles could
move through the foil with
little or no interference.
Some alpha particles bounced
back at them.
Shoot a tissue with a 22 and
the bullet bounces back
The Proton
• This figure shows how
Rutherford’s new model
of the atom fits the
experimental data.
• Most alpha particles
could move through
the foil with little or
no interference. But
some bounce back.
Rutherford’s Model
• The actual results did
not fit Thompson’s
model, so Rutherford
proposed a new one.
• He hypothesized that
almost all the mass of
the atom and all of its
positive charge are crammed into an incredibly
small region of space at the center of the atom
called the nucleus. Most of the atom is empty
space.
The Proton
• In 1920 scientists identified the positive
charges in the nucleus as protons.
• A proton is a positively charged particle
present in the nucleus of all atoms.
The Neutron
• An atom’s electrons have almost no mass.
• According to Rutherford’s model, the only
other particle in the atom was the proton.
• That meant that the mass of an atom should
have been approximately equal to the mass
of its protons.
The Neutron
• However, it wasn’t.
• The mass of most atoms is at least twice as
great as the mass of its protons.
• It was proposed that another particle must be
in the nucleus to account for the extra mass.
• The particle, which was later call the
neutron (NEW trahn), would have the same
mass as a proton and be electrically neutral.
The Neutron
• Proving the existence of neutrons was
difficult though, because a neutron has
no charge.
• It took another 20 years before scientists
were able to show by more modern
experiments that atoms contain neutrons.
The Neutron
• The model of the
atom was revised
again to include the
newly discovered
neutrons in the
nucleus.
• The nuclear atom has a
tiny nucleus
tightly packed with positively charged protons
and neutral neutrons. Negatively charged
(electrons) particles surround this nucleus
Further Developments
• Then, electrons would travel in orbits
around the nucleus.
• A physicist named Niels Bohr even
calculated exactly what energy levels
those orbits would represent for the
hydrogen atom.
• However, scientists soon learned that
electrons are in constant, unpredictable
motion and can’t be described easily by
an orbit.
Electrons as Waves
• Physicists began to wrestle with explaining
the unpredictable nature of electrons.
• The unconventional solution was to
understand electrons not as particles, but
as waves.
• This unconventional solution also applied
to light waves which can be thought of as
particles.
The Electron Cloud Model
• The new model of the
atom allows for the
somewhat unpredictable
wave nature of electrons
by defining a region
where electrons are most
likely to be found.
• Electrons travel in a
region surrounding the
nucleus, which is called
the electron cloud.
Size and Scale
• Drawings of the nuclear atom don’t give an
accurate representation of the extreme
smallness of the nucleus compared to the
rest of the atom.
• For example, if the nucleus were the size
of a table-tennis ball, the atom would have
a diameter of more than 2.4 km.
• A football field is 100 m so 10 football
fields is 1 km so 2.4 km = 24 football fields
Atomic Models
•
•
•
•
•
•
Ancient Greek- “cannot be divided”
John Dalton’s Theory- simple sphere
Thompson’s Model- plum pudding
Rutherford- Electron/Proton Model
Bohr- Orbital Model/ Neutron
Modern – electron Cloud Model
– Protons, Neutrons, Electrons
• Current Model
– Protons and neutrons made up of quarks
Identifying Numbers Page 512
• The atoms of different elements contain
different numbers of protons.
• The atomic number of an element is the
number of protons in the nucleus of an
atom of that element. Designated as Z
• Atoms of an element are identified by the
number of protons because this number
never changes without changing the identify
of the element.
Number of Neutrons
• Most atoms of carbon have six neutrons, some
have 7 and some have 8.
• They are all carbon atoms because they all have
six protons.
• These three kinds of carbon atoms are called
isotopes. Isotopes (I suh tohps) are atoms of the
same element that have different numbers of
neutrons.
Mass Number
• The mass number of an isotope is the
number of neutrons plus protons. Also
called the atomic mass. Designated A
Atomic Number,
Atomic Mass, and # of Neutrons
• mass number = #protons + # neutrons
• # protons = atomic number
• Number of neutrons = mass number –
atomic number
• Number of neutrons = A – Z
Find the number
• How many neutrons in Iodine-131?
• What is the atomic mass number of a
copper isotope with 34 neutrons?
• Worksheet
Strong Nuclear Force
• Because protons are positively charged, you
might expect them to repel each other just as
the north ends of two magnets tend to push each
other apart.
• It is true that they normally would do just that.
• However, when they are packed together in the
nucleus with the neutrons, an even stronger
binding force takes over.
• That force is called the strong nuclear force
and it only works at very small distances.
Radioactive Decay
• Many atomic nuclei are stable when they have
about the same number of protons and neutrons.
• As the number of protons increases, the
ratio of neutrons to protons must increase.
• A nucleus can become more stable by
changing the ration of protons to neutrons.
• In these nuclei, repulsion builds up. The nucleus
must release a particle to become stable.
• This release of nuclear particles and energy
is called radioactive decay.
Transmutation
• When the particles that are ejected from a
nucleus changes the number of protons,
the atomic number of the nucleus changes.
• When this happens, one element changes
into a different element.
• The changing of one element into another
through radioactive decay is called
transmutation.
The Nucleus
2
Radioactive Decay
• Transmutation is occurring in most of your
homes right now.
• A smoke detector makes use of radioactive
decay.
• This device contains
americium-241 (a muh
RIH shee um), which
undergoes transmutation
by ejecting energy and
an alpha particle.
The Nucleus
2
Radioactive Decay
• In the smoke detector, the fast-moving alpha
particles enable the air to conduct an
electric current.
• As long as
the electric
current is
flowing, the
smoke
detector is
silent.
The Nucleus
2
Radioactive Decay
• The alarm is triggered when the flow of
electric current is interrupted by smoke
entering the detector.
Changed Identity
• When americium expels an alpha particle,
it’s no longer americium.
Review
• Number of Protons determines the
element. #Protons is Atomic Number (Z)
• # Protons + # Neutrons is Atomic Mass
Number (A)
• #Neutrons = A-Z
• “Strong Force” keeps protons in the
nucleus from flying apart.
Review (cont)
• Neutrons provide the strong force.
• Ratio of Neutrons to Protons determines
the strength of the strong force.
• For low Atomic # elements n/p = 1 is
stable.
• For high Atomic # elements n/p >1.5 is
stable.
• Nuclei emit particles to balance n/p and
become stable.
Loss of Beta Particles
• Some elements undergo transmutations
through a different process.
• Their nuclei emit an electron from the
nucleus called a beta particle.
• A beta particle is a high-energy electron
that comes from the nucleus, not from the
electron cloud.
Neutron into a Proton
Emission of Beta Particle
+
Proton
+
+
-
Neutron
Beta Particle
Loss of Beta Particles
• Because a neutron has been changed into a
proton, the nucleus of the element has an
additional proton.
Loss of Beta Particles
• Unlike the process of alpha decay, in beta
decay the atomic number of the element that
results is greater by one.
Proton into a Neutron
Electron Capture
+
Proton
-
Electron
Capture
Neutron
Proton into a Neutron
Positron Emission
+
Proton
Neutron
+
Positron
Types of Radioactive Decay
Type
Symbol Charge Affect
Alpha
Particle
4
Electron
Capture
Beta
Particle
0
Positron
0
Gamma ray γ
2He
2+
Lowers Z and A
e-1
-1
Lowers Z,
same
-1
β
1-
Raises Z,
A same
+1
β
1+
Lowers Z, A-same
0
Energy Release
A
Particle
Energy/Penetration/Damage
Particle
Energy
Penetration
Damage
Alpha Particle
Loses energy
quickly
Sheet of paper
blocks
Damage to cells
Beta Particle
Much faster
moving
Penetrate
Damage cells
paper, stopped
by aluminum foil
Gamma Rays
Electromagnetic Stopped by
Little damage,
dense materials, pass right
lead, thick
through tissues
concrete
Rate of Nuclear Decay
• The half-life of a radioactive isotope is the
amount of time it takes for half of a sample
of the element to decay.
Calculating Half-Life Decay
• Iodine-131 has a half-life of eight days.
• If you start
with a
sample of 4 g
of iodine131, after
eight days
you would
have only 2 g of iodine-131 remaining.
Carbon Dating
• Carbon-14 is used to determine the age
of dead animals, plants, and humans.
• In a living organism, the amount of carbon-14
remains in constant balance with the levels of
the isotope in the atmosphere or ocean.
• This balance occurs because living
organisms take in and release carbon.
• As soon as the organism dies the amount of
C-14 begins to decrease.
Assumptions of
Carbon Dating
• The percentage of C-14 in a sample of
carbon (containing C-12,C-13, C-14) has
been the same for thousands of years.
• The rate of decay of C-14 has been
constant for thousands of years.
• “All things are as they have always been!”
Detecting Radioactivity
• Alpha Particles are positively charged
particles and Beta Particles are negatively
charged particles.
• We can use the properties of charged
particles to detect them.
Radiation Detectors
• Cloud Chamber – water or alcohol vapor
condenses around the ion as it moves
through the chamber leaving a trail like the
trail of a jet plane
– Alpha particle leaves a short thick trail
– Beta particle leaves a long thin trail.
• Bubble chamber is similar device
Radiation Detectors (cont)
• Electroscope- we talked about these
earlier as a way to detect electrical charge.
• Geiger counter- uses the fact that
charged particles will allow an electrical
current to flow and counts the times that a
current flows in a special tube. (Page 549)
• Fluorescent screen- remember
Rutherford’s experiment
Medical uses for radiation
• See pages 554-556
• Tracers – since radioactive elements give off
charged particles, these elements can be “trace”
as they move through the body
– Iodine 131 collects in a properly functioning
thyroid gland but not as much in a gland with
a tumor
• Destroy cancer cells- we know that radiation
particles can damage cells. They damage fast
growing cancer cells more than normal cells
Energy from the Nucleus
• In nuclear fission, a very heavy nucleus splits
into more-stable nuclei of intermediate mass.
• The mass of the products is less than the
original nucleus.
• This mass is converted into energy according
to Einstein’s equation E= mc2
• Enormous amounts of energy are released.
• Nuclear fission can occur spontaneously or
when nuclei are bombarded by particles.
Nuclear Fission
A chain reaction is a reaction in which the
material that starts the reaction is also one of the
products and can start another reaction.
The minimum amount of nuclide that provides the
number of neutrons needed to sustain a chain
reaction is called the critical mass.
Nuclear reactors use controlled-fission chain
reactions to produce energy and radioactive
nuclides.
Nuclear Power Plant Model
Nuclear Fusion
In nuclear fusion, low-mass nuclei combine to
form a heavier, more stable nucleus.
Nuclear fusion releases even more energy per
gram of fuel than nuclear fission.
If fusion reactions could be controlled, they could
be used for energy generation.
Solar Energy and Bombs
+
+
+
+
+
+
1
Energy
+
4 1H
Hydrogen
+
4
2He
Helium
2 +1 β
Positrons
Making Synthetic Elements
• Scientists now create new elements by
smashing atomic particles into a target
element.
• The absorbed particle converts the target
element into another element with a
higher atomic number.
• The new element is called a synthetic
element because it is made by humans.
• Elements with atomic numbers 93 to 112,
and 114 have been made in this way.