Nuclear Chemistry
Chapter 10Modified from Mrs. Wolfe
Isotopes
• Recall that the mass number is the SUM of the protons + neutrons
• Isotopes are atoms on an element that have different numbers of neutrons.
• Why is the atomic mass a decimal? – The average of the masses of an element’s different isotopes.
Isotope Notation
Atomic Number =Protons only
Mass NumberProtons +neutrons
Isotopes
Radioactivity
Recall that atoms look like this…
• Protons (positive)• Neutrons (neutral)
– Both in the nucleus
• Electrons (negative)– Surround the
nucleus
neutron
electron
proton
nucleus
orbit of electron
Recall what an isotope is…
• When an element gains or loses a neutron it becomes an isotope of that element.
• Still the same element because the atomic number has not changed (atomic number is the number of protons)neutron
electron
proton
nucleus
orbit of electron
Radioactivity
• Radioactive materials have UNSTABLE nuclei.
• Isotopes are generally radioactive. Why do you think that is?
• Because isotopes have unstable nuclei, since they have a “WEIRD” number of neutrons.
What causes a nucleus to be unstable?
•Unstable = radioactive•When the strong nuclear force cannot overcome the electrostatic force repelling the protons away from each other in the nucleus, the nucleus becomes unstable.
What is the Strong Nuclear Force?•The attractive force that binds the protons and the neutrons together inside the nucleus.•It acts among protons, among neutrons, and between protons & neutrons
Unstable atoms tend to become stable!
• They will get rid of neutrons to stabilize themselves!
• Very similar to bailing water out of a sinking boat to keep it from sinking!
• The process of becoming stable is called RADIOACTIVE DECAY.
Radioactive DECAY
• The process of emitting particles/energy to become stable is known as Radioactive Decay.
• It’s NOT ROTTING!
3 Types of Radioactive Decay
• Alpha– Protons and Neutrons are
emitted
• Beta– Electron emitted
• Gamma– Burst of energy is emitted
Radioactive Decay is Spontaneous.
Penetrating Power of Radioactivity
• Alpha Particles – LARGE, heavy protons/neutrons can’t travel far.
– can be blocked by the dead skin layer on your hand or a thin sheet of paper
• Beta Particles– SMALL, light electrons can travel farther.
– can be blocked with a thin sheet of lead
• Gamma Particles – Energy can travel very far.
– can be blocked with thick lead or concrete.
paper
lead
concrete
How can we detect radioactive decay?
• Geiger Counters!• Go get the handout for this
LAB (it requires a computer with Shockwave on it – the MacBook Airs have this)
Think about this:
• Why do you have to wear a LEAD apron during an X-Ray?
• Because X-Rays are GAMMA particles that are and used to photograph our bones. A LEAD apron will protect the fragile parts of your body that we don’t want to expose to the X-rays.
Where does radioactivity come from?
• Isotopes from the Sun• Isotopes in Dirt & Rocks• Isotopes in the Air! • Isotopes in YOU!• Isotopes in Food/Water• Just about EVERYTHING!• This is called Background
Radiation because you’re exposed to it everyday.
How much of a Radiation Dose do you receive on a yearly basis?
• An annual dose under 350 mrem is natural and safe.
Let’s figure out your annual dose!
– Time for a WS! Estimate Your Personal Radiation Dose– IMPORTANT: You need the info. below for the WS!– The elevation you live at is 0-1000 ft.– Your home is made of wood.– You DO NOT live within 50 miles of any power plant.
When atoms emit PROTONS...
• …they are no longer the same element.
• Example: A Lead isotope is an alpha source, so it loses protons and neutrons. If it loses 3 protons, it’s atomic number changes from 82 to 79. What element did it change to?
• Gold!!
• Now, this is a CHEMICAL change, not a physical change. So it won’t LOOK LIKE gold, but it will ACT LIKE gold!
What about if neutrons/energy are lost?
• It is still the same element, but with less MASS.
• Example: An isotope of Lead emits a neutron. Its atomic NUMBER is still 82, but its atomic MASS has changed from 207 to 206. Because its atomic NUMBER doesn’t change, it’s still Lead!
• It’s Lead-206
(206 tells you the MASS of the isotope)
Rates of Becoming Stable
• How quickly/slowly do atoms stabilize?
• It’s specific to each isotope.
Isotope Stabilization Rate
Uranium-238 4,510,000,000 years
Potassium-40 1,280,000,000 years
Radium-226 1,599 years
Radon-222 3.82 days
Thorium-219 0.00000105 sec
Half-life• Half-life – the amount of TIME required for HALF of a
radioactive substance to stabilize.
• Example:• The half-life of Iodine-131 is 8 days. If you start with 50 grams
of Iodine-131, then 25 grams will remain radioactive after 8 days.
• During another half-life, half of the remaining radioactive substance will stabilize. How much Iodine remains radioactive after 16 days?
• 12.5 grams
• Time for a WS!• Half-life Calculation WS
Half-lives…&…stabilizing atoms!
• Tennis player = radioactive substance
• Little girl = Stabilized atoms
• Remember, half-life is the amount of TIME required for HALF of the substance to STABILIZE.
Let’s Take A Closer Look…
Practice Question
• What color represents the radioactive
element?
RED• What color
represents the
stable element?
BLUE
Time for a LAB!
• If you haven’t done the Skittle Lab…be sure to do it to better understand how to create and interpret a Half-Life graph!
Carbon-dating
• C-14 is a naturally-occurring isotope found in the tissue of living things.
• It can be used to determine how long a living thing has been dead!
• This is called Carbon-dating.
How does Carbon-Dating work?
• 1. The sun puts C-14 in our air.• 2. Plants take in the C-14. Animals eat plants.• 3. Humans eat the C-14 in plants/animals.• 4. The C-14 naturally decays. As long as the human keeps
eating, the C-14 supply in the body is replenished.• 5. Human dies and stops eating. The C-14 supply is not
replenished. • 6. The decay rate of C-14 is known. We can use it to figure
out how long the human has been dead.
Carbon-Dating in Forensics
• C-14 has a half-life of 5,715 years.
• You find a body that has 500 g of C-14 left. You know this body had 1000 g of C-14 when he/she died because the air holds that amount of C-14. How long has this person been dead?
• 5,715 years!
Nuclear Fission
• So far, we’ve discussed atoms EMITTING particles.
• Sometimes, a nucleus has to SPLIT to release particles/energy.
• FISSION – is the process of splitting a nucleus to release energy
Nuclear Fusion
• Fusion is the process of JOINING two nuclei.
• This happens in the sun, when two Hydrogen atoms FUSE & release light energy!
The picture below illustrates….
• ….FISSION!
A single neutron begins a Fission chain reaction and makes LOTS of energy!
Nuclear Power Plants
• 104 Nuclear Plants in the US– 20% of electricity
– 1 gram Uranium-235 = 3 tons of coal!!
How does a Nuclear Reactor Work?
• A neutron begins a FISSION chain reaction in the reactor vessel. Control rods are used to capture neutrons to control the reaction speed.
• Energy released from fission heats a chamber of water. • The water boils and becomes steam.• The steam turns a turbine and creates electricity.• The steam is CONDENSED back into liquid by cold water
pumped in from a nearby pond.• THE RADIOACTIVE MATERIALS NEVER MIX
WITH THE POND WATER.
Pressurized Water Reactor
Boiling Water Reactor
Fission Reactor
What dangers do they present?
• What do we do with the toxic waste after fission?– Right now, it’s kept on Yucca Mountain, Colorado.
• Thermal Pollution (in ponds around reactors)– Warmer water used in the condenser is returned to
ponds.
• Uranium-235 is a limited resource.
• Chance of Meltdown – (Particles/energy could accidentally be released from the
reactor vessel and out into the open…called “fallout”.)
Fusion Reactors
Tokamak Fusion Test Reactor
Princeton University
National Spherical Torus Experiment
Fusion isn’t used in power plants.
• It takes a LOT of energy to cause atoms to fuse. Energy is released when they fuse, but it’s not much more than it took to start the fusion process.
• There’s no chain reaction.
• So it won’t work for a power plant.
Nuclear Applications
• Was the Atomic Bomb nuclear fission or fusion?
• Let’s look at some historical pictures of the bomb.
Atomic Bomb Pictures
• Noriaki Teshima – (male) 12 year old junior high school student
• He was exposed to the bomb at school. He suffered major burn over his entire body, to the extent that his skin was dangling in tatters. With the help of a friend he returned home. Suffering from terrible thirst, he is said to have tried to suck the puss from his raw, nail-less fingers. He died in agony on August 7, 1945. His mother kept his fingernails and part of his skin to show his father, who had not returned from the war.
The next picture shows floor tiles that were melted from the heat.
Black Rain
• After the bombing, violent fires raged through the city, whipped up by whirling winds. Soon, heavy rain fell Northwest of downtown.
• For the first hour or two, the rain fell in huge black drops, discolored by mud and soot.
• This black rain contained highly radioactive materials that killed fish in the rivers.
• People who drank well water in the affected areas suffered from severe diarrhea for up to three months.
No One Survived.
• Anyone in the general area of the atomic bomb drop was killed, either slowly or instantly.
• The following picture is an artist’s rendition of what the civilians might have looked like immediately after the bomb drop.
• Look closely and you’ll notice their skin dripping off.
Nuclear Applications
• How is radioactivity used in our everyday life, besides in power plants and bombs?
Radioisotope Tracers
– Isotopes are absorbed by specific organs and used to diagnose diseases.
– They contain a SMALL amount of radiation. – The BENEFITS of diagnosing and curing the
disease greatly outweigh theRISKS of exposure to this small amount of radiation.
Chemotherapy
• Radiation Treatment– large doses are targeted at tumors to kill the
cancer cells.– It is difficult to avoid killing healthy cells
during chemotherapy. This is an especially serious problem when the tumor is in the brain.
Radiation treatment using-rays from cobalt-60.
Food Irradiation
– Food can be treated with radiation. This is called irradiation.
– Irradiation kills bacteria, preventing food from spoiling.
– Food can be transported farther with less spoilage.
– Irradiation causes URPs to form in the food and we don’t know whether or not these compounds are dangerous in the long run.
Watch the Video on Food Irradiation!
ON THE WIKI
The End!
• Be sure to take the Practice Quiz on Moodle!
