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1
Nuclear Changes
Physical Science
Chapter 10
2
Radioactive decay
The spontaneous breaking down of a nucleus into a slightly lighter nucleus, accompanied by emission of nuclear radiation (particles, electromagnetic radiation, or both).
3
Nuclear Radiation
Alpha particles, beta particles (positive or negative), and gamma rays.
Have different penetrating powers
4
Alpha particles
Large mass (4 amu) and charge (+2). Can’t travel far in air Low penetrating power
Cannot penetrate skin Can be stopped by a sheet of paper
Harmful if ingested or inhaled
5
Beta particles
Travel close to the speed of light Penetrate about 100 times as much as
alphas Can travel a few meters in air Can be stopped by lead or glass
6
Gamma rays
Travel at the speed of light Greatest penetrating ability Can travel indefinitely through air or
empty space Can only be stopped by thick layers of
lead or concrete.
7
Nuclear equations
The total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation.
Elements have atomic numbers 1 or greater
Neutrons have atomic numbers of 0 Electrons have atomic numbers of -1
8
Examples
HeThU 42
23490
23892
PaeTh 23491
01
23490
RhePd 10045
01
10046
9
Example
ClAr 3717
3718 _____
10
You try
KrRb 8336
8337 _____
11
You try
12
You try
TlHe 20881
42_______
13
Half-life
The amount of time it takes for half of a radioactive sample to decay
We can’t predict when an individual atom will decay, only the rate of decay for a large number of atoms.
There is a table on page 333.
14
Radioactive dating
Determining the age of a substance based on the amount of radioactive nuclides present
Carbon-14 is used for organic materials up to 50 000 years old
Others used for older materials and minerals up to 4 billion years old
15
Stability Protons repel each other through
electrostatic forces They attract each other (and also
neutrons) through nuclear forces – but only over small distances
More neutrons can increase attractive force without increasing repulsive force
Too many protons or neutrons makes the nucleus too big and unstable
Beyond bismuth (83), no stable nuclei exist
16
Nuclear fission
A very heavy nucleus splits into more-stable nuclei
Mass of products is less than mass of reactants Releases enormous amounts of energy E=mc2
17
Chain reaction
The material that starts the reaction is one of the products and can start another reaction.
Critical mass – minimum amount of nuclide that is needed to sustain a chain reaction
18
19
Nuclear fusion
Light-mass nuclei combine to form a heavier, more stable nucleus
Releases more energy per gram of fuel than fission
Takes place in stars (including the sun) Hydrogen to helium
20
Fusion requirements
High heat and pressure needed Right now, no known material can
withstand the initial temperatures (100 million K) needed for controllable manmade fusion.
Discuss
Section 2 review question #2 on page 342
21
Background radiation
Natural radiation everyone is exposed to We are adapted to survive these low
levels of radiation
22
23
rem
How radiation exposure is measured Up to 0.5 rem per year is considered
safe In the US, average yearly exposure is
0.1 rem.
24
Radiation exposure damage
DNA mutations Cancer Genetic effects
Smoke detectors
Produce current by releasing alpha particles
Smoke particles reduce the current and set off the alarm
25
26
Radioactive medicine
Used to treat cancer Used to detect cancer and other
diseases CT scans MRIs PET scans Radioactive tracers used to locate tumors
27
Radioactive agriculture
Tracers can be used to determine water movement and fertilizer effectiveness
Radiation can be used to extend shelf life by killing bacteria and insects
28
Nuclear waste containment
Waste can have a half life from a few months to thousands of years.
It must be contained to protect living organisms
Best sites for storage have low populations, little water at the surface or underground, and no earthquakes.
29
Nuclear reactors
Use controlled fission chain reactions to produce energy.
Research is being done to try to build controlled fusion reactors.