Transcript Study Guide

Monday
August 20
Describe subatomic
particles.
Models of the Atom
• John Dalton—pictured the atom as a hard
sphere that was the same throughout
• William Crookes—CRT
One electrode, called the anode, has a positive charge
JJ Thompson
• He concluded that the CRT produced
particles that were negatively charged.
• These particles later became know as the
ELECTRON.
• Perhaps one of the biggest surprises that
came from Thompson’s experiments was
the evidence that particles smaller than
the atom did exist. (subatomic particles)
Thompson’s Model
• Solid core with the solid part being
positively charged and the ball bearing
that were spread evenly throughout being
the negative charges.
Rutherford’s experiment
• Questioned Thompson’s model, Was it
correct.
• In Rutherford’s experiment, alpha particles bombarded
the gold foil or veered slightly from a straight path but
then some particles bounced right back. (409)
continued
• It was proposed that there must be
another particle in the NUCLEUS of the
atom.
• The particles is the neutron. It is
electrically neutral.
Niel Bohr
• Physicist even calculated the energy
levels in the nucleus.
• Scientist discovered that the electrons did
not move in an orbit as suggested by
Bohr. They were more random.
• Electrons travel in a region surrounding
the NUCLEUS, which is called the electron
cloud.
Model with a NUCLEUS
• Due to the unexpected results from
Rutherford’s model he proposes a new
model for the atom.
• From the new information, he concluded
that the atom the dense center of positive
charges known as the NUCLEUS.
• There was a problem
Important Vocabulary
• Atomic Number —the number of protons
in the nucleus
•
These three kinds of carbon atoms are
called isotopes. Isotopes are atoms of the
same element that have different numbers
of neutrons
Mass Number —The number of protons PLUS the
number of neutrons.
You can find the number of neutrons in an
isotope by subtracting the atomic number from
the mass number.
Radioactive Decay of the Nucleus
• Radioactive decay—the release of nuclear
particles and energy.
• Transmutation—the changing of one
element into another by radioactive decay.
• Alpha particle—consist of TWO protons
and TWO neutrons. Together the energy
and particles are called Nuclear Radiation.
The Nucleus
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.
The Nucleus
Changed Identity
• After the transmutation, it becomes the
element that has 93 protons, neptunium.
The Nucleus
2
Loss of Beta Particles
• Some elements undergo transmutations
through a different process.
• Their nuclei emit an electron called a
beta particle.
• A beta particle is a high-energy electron
that comes from the nucleus, not from the
electron cloud.
The Nucleus
2
Loss of Beta Particles
• During this kind of transmutation, a neutron
becomes unstable and splits into an electron
and a proton.
• The electron, or beta particle, is released
with a large amount of energy.
• The proton, however, remains in the nucleus.
The Nucleus
2
Loss of Beta Particles
• Because a neutron has been changed into a
proton, the nucleus of the element has an
additional proton.
The Nucleus
2
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.
The Nucleus
Rate of Decay
• Radioactive decay is random.
• The rate of decay of a nucleus is measured
by its half-life.
• The half-life of a radioactive isotope is the
amount of time it takes for half of a sample
of the element to decay.
The Nucleus
Loss of Beta Particles
• Some elements undergo transmutations
through a different process.
• Their nuclei emit an electron called a
beta particle.
• A beta particle is a high-energy electron
that comes from the nucleus, not from the
electron cloud.
The Nucleus
2
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.
The Nucleus
2
Calculating Half-Life Decay
• After 16 days, or two half-lives, half of the
2 g would
have
decayed
and you
would
have only
1 g left.
The Nucleus
2
Calculating Half-Life Decay
• The radioactive decay of unstable atoms
goes on at a steady pace, unaffected by
conditions such as weather, pressure,
magnetic or electric fields, and even
chemical reactions.
Carbon Dating
• Carbon-14 is used to determine the age of
dead animals, plants, and humans.
The Nucleus
Carbon Dating
• When archaeologists
find an ancient item,
they can find out how
much carbon-14 it has
and compare it with the
amount of carbon-14
the animal would have
had when it was alive.
• Knowing the half-life of carbon-14, they
can then calculate when the animal lived.
The Nucleus
2
Carbon Dating
• When geologists want to determine the age
of rocks, they cannot use carbon dating.
• Instead, geologists examine the decay of
uranium.
• Uranium-238 decays to lead-206 with a halflife of 4.5 billion years.
• By comparing the amount of uranium to
lead, the scientist can determine the age of
a rock.
The Nucleus
2
Uses of Radioactive Isotopes
• Tracer elements are used to diagnose disease
and to study environmental conditions.
• The radioactive isotope is introduced into
a living system such as a person, animal,
or plant.
• It then is followed by a device that detects
radiation while it decays.
Medical Uses
• The isotope iodine-131 has been used to
diagnose problems with the thyroid, a
gland located at the base of the neck.
• Other radioactive isotopes are used to
detect cancer, digestion problems, and
circulation difficulties.
Environmental Uses
• Radioisotopes also can be placed in
pesticides and followed to see what impact
the pesticide has as it moves through an
ecosystem.
Calculating Half-life
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Read the problem
Write down the information known
What do you want to find out?
Determine the number of half-lives
Final weight= initial weight/2 (number of halflives)
• Refer to page 419