CHAPTER 7CHAPTER 7
NUCLEAR CHANGESNUCLEAR CHANGES
Chapter 7 Section 1:Chapter 7 Section 1:RadioactivityRadioactivityWhat is radioactivity?
◦Radioactivity is the process by which an unstable nucleus emits one or more particles or energy in the form of electromagnetic radiation
Nuclear RadiationNuclear RadiationRadioactive materials have
unstable nuclei
Nuclei change by releasing energy.
Elements can transform into isotopes of the same element or into entirely different elements
Nuclear DecayNuclear DecayAs elements “change” or
transform they undergo nuclear decay.◦Carbon dating uses rates of nuclear
decay
The released energy is called Nuclear Radiation◦Can cause damage to living tissue
When a radioactive atom decays, the nuclear radiation leaves the nucleus.
The radiation is now able to react with any surrounding matter
The reaction depends on the charge, mass, & energy of the nuclear radiation
Types of Nuclear RadiationTypes of Nuclear Radiation4 types of nuclear radiation:
◦Alpha Particles
◦Beta Particles
◦Gamma Rays
◦Neutron Emission
Alpha Particles Alpha Particles Positively charged & more massive than any other type of nuclear radiation
Discovered by Ernest Rutherford (also discovered the nucleus)
Consist of 2 protons & 2 neutrons
Alpha ParticlesAlpha ParticlesDo not travel far through materials
(can barely pass through paper)
The massive size of alpha particles limits their movement
Remove electrons from surrounding matter as they pass through it (_______________)
Ionization causes alpha particles to slow down
Beta Particles Beta Particles Fast-moving Electrons produced by neutron decay
Negatively charged particles
Travels farther through matter compared to alpha particles
How can electrons come from How can electrons come from a positively charged nucleus?a positively charged nucleus?
In the 1930’s, it was discovered that as a neutron (neutral charge) decays it forms a proton & electron
The electron has very little mass and gets ejected from the nucleus at a high speed as a Beta Particle
Gamma RaysGamma RaysVery High Energy Light
1898- Marie Curie isolated radioactive radium
1900- Paul Villiard discovered that radium emitted a new type of nuclear radiation…
Gamma Rays!Gamma Rays!More penetrating than Beta particles
Gamma rays are not made of matter & do not have an electric charge
Gamma rays are a form of electromagnetic energy
Have more energy than X-rays
Do not ionize surrounding matter
Can penetrate 60cm of aluminum & 7 cm of lead!
Not easily stopped by clothing or building materials◦Can be very harmful
Neutron EmissionNeutron EmissionNeutron Emission is the release of a high-energy neutron by some neutron-rich nuclei during radioactive decay.
Emitted from an unstable nucleusDo not ionize surrounding matterTravel farther through matter than or
Nuclear DecayNuclear DecayWhen an unstable nucleus emits alpha or beta particles, the # of protons or neutrons changes.
Example: Radium-226 changes to Radon-222 by emitting an alpha particle
A nucleus gives up 2 protons & A nucleus gives up 2 protons & 2 neutrons during 2 neutrons during alpha alpha decaydecayNuclear decay processes can be
written similar to a chemical reaction equation
The nucleus before the decay is like the reactant
The nucleus after the decay is like the product
A nucleus gains a proton & A nucleus gains a proton & loses a neutron during loses a neutron during beta decaybeta decayEquation is formed the same way except the symbol for a beta particle is used.
In all cases of beta decay, the mass number before & after the decay doesn’t change
When writing the equation…Write the original element on the
left side (reactants)
X is used for the unknown symbol A is used for the unknown massZ is used for the unknown atomic
#
Solve for the unknowns & rewrite the balanced equation
Radioactive decay ratesRadioactive decay rates
A substance’s half-life is the time in which half of the radioactive substance decays(A measure of how quickly a substance decays)
After each half-life passes half of the sample remains unchanged.
Practice ProblemPractice ProblemRadium has a half-life of 1599 years. How long would it take seven-eights of radium-226 sample to decay?
1- 7/8 = ______½ x ½ x ½ =______How many “half-lives”?______
Each half-life is 1599 years
It takes 3 half-lives to have 1/8 radium remaining.
3 x 1599 = 4797 years
More Practice! (pg. 228)
Carbon- 14 has a half life of 72 hours. How long would it take for 15/16 of the carbon sample to decay?
Uranium-238 decays very slowly, with a half-life of 4.47billion years. What percentage of the sample would remain after 13.4 billion years?
A sample of strontium-90 is found to have decayed to 1/ 8 of its original amount after 87.3 years. What is the half-life of strontium?
A sample of francium-212 will decay to 1/16 its original amount after 80 minutes. What is the half-life of francium-212?
What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 grams in 21.6 seconds?
The half-life of hafnium-156 is 0.025 seconds. How long will it take a 560 g sample to decay to one-fourth of its original mass?
Potassium-42 has a half-life of 12.4 hours. How much of an 848 g sample of potassium-42 will be left after 62.0 hours?
Half-life practiceHalf-life practiceGold-210 is a radioactive isotope that has a half-life of 12 hours. If a lab starts with a 13milligram sample of gold-210, how much will remain after 37 hours?
Chapter 7 Section 2Chapter 7 Section 2
Nuclear Fission & Fusion
Nuclear ForceNuclear ForceElements can have both stable &
unstable isotopes.
Example: Carbon-12 is stable Carbon-14 is unstable &
The stability of a nucleus depends on the nuclear forces that hold the nucleus together
Nuclei are held together by a Nuclei are held together by a particular forceparticular force
Strong Nuclear Force is the interaction that binds protons & neutrons together in a nucleus
Force is much stronger than the repulsion force between protons
Strong nuclear forces occur at very short distances (3 x 10-15 m)
In stable nuclei the attractive forces are stronger than repulsion forces
Too many neutrons or protons can cause a nucleus to become unstable & decay
Nuclei with too many neutrons or too few are unstable & will undergo decay
Nuclei with more than 83 protons are always unstable ◦Atomic numbers will be greater than _____
Nuclear FissionNuclear FissionThe process of producing lighter nuclei from heavier nuclei is called Fission
First observed by Otto Hahn & Fritz Strassman in 1939
Their experiment bombarded Uranium-235 with neutrons
The Product…The Product…2 lighter nuclei, neutrons, &
energy
Barium-137 & Kryton-84
The product includes 15 neutrons
Uranium can undergo many types of fission with different products
Energy is released during Energy is released during Nuclear FissionNuclear FissionDuring Fission the nucleus
breaks into smaller nuclei
The reaction releases large amounts of energy
Each dividing nucleus releases about 3.2 x10-11 J of energy
Hahn & Strassman determined the overall masses of the elements in their reaction had decreased after the reaction.
The missing mass had changed to Energy!
The Theory of RelativityThe Theory of Relativity
Presented by ___________ __________ in 1905
Describes the equivalence of mass & energy observed in nature
Equivalence means that matter can be converted into energy & energy into matter.
E = mcE = mc22
Energy = mass x Energy = mass x (speed of light)(speed of light)22
Neutrons released by fission can start a chain reaction!
When a nucleus splits into lighter nuclei, they need less neutrons so the neutrons are emitted.
The emitted neutron can then collide with surrounding nuclei and undergo fission
Nuclear Chain Reaction
A series of fission processes in which the neutrons emitted by the dividing nucleus cause the division of the other nuclei
Chain reaction is the principle behind the nuclear bomb!
Chain reactions can be controlled
The more neutrons that are produced per reaction the great the chances of creating a chain reaction
Specific materials can be used that will slow a fission chain reaction by absorbing neutrons
Nuclear Fusion (Hydrogen Bomb)Energy can be obtained when light
nuclei are combined into heavy nuclei
Fusion is the process in which light nuclei combine at extremely high temperatures forming heavier nuclei & releasing energy
Chapter 7 Section 3Chapter 7 Section 3
Dangers & Benefits of Nuclear Radiation
Dangers of Nuclear Dangers of Nuclear RadiationRadiation
Background Radiation is nuclear radiation that arises naturally from cosmic rays & from radioactive isotopes in the soil & air◦Example: Sun, soil, water & plants
Nuclear radiation can ionize atoms in living tissue.◦Example: Hemoglobin loses its ability
to carry oxygen when exposed to radiation
Skin helps keep low levels of radiation outside the body.
Nuclear radiation can cause burns in the skin as well as destroy bone marrow cells.
Radiation sickness results from exposure to high levels of nuclear radiation.
Observable effects from nuclear radiation exposure (low intensity) may not appear for days or years.◦Example: hair loss, sterility, death of bones, & cancer.
Nuclear Radiation can cause Nuclear Radiation can cause genetic mutationsgenetic mutations
Long-term effects of nuclear radiation appear when DNA molecules in the body are damaged.
Example:◦ Radioactive wastes from the Shiprock Uranium Mine contaminated water that was used for cattle & sheep.
◦ The radiation damaged the DNA in their reproductive cells causing offspring to have birth defects.
Beneficial Uses of Nuclear Beneficial Uses of Nuclear RadiationRadiation
Many uses in medicine & archeological dating
Smoke alarms
Cancer Treatment
Radioactive TracersRadioactive TracersUsed in agriculture, medicine, &
scientific research
Radioactive Tracers are added to substances so that its location can be detected later.
Used to locate tumors, measure the speed of a river, track drugs throughout a body
Nuclear PowerNuclear Power
Nuclear reactors are used to generate electricity (uses fission)
Nuclear fission has many benefits◦Produces not gaseous pollutants◦Much more energy than coal & oil reserves
Disadvantages of Nuclear Disadvantages of Nuclear FissionFissionSerious safety concerns with handling, treatment, and disposal of the Uranium
Safe operation of the nuclear reactor is also a concern◦Power plants can only be active for 40
years
Nuclear wastes must be safely stored
Half-life practiceHalf-life practiceA radioactive substance has a half-life of 10 years. What fraction of a sample of the substance would be left after 30 years?
Half-life practiceHalf-life practiceIron-109 is a radioactive isotope that has a half-life of 4 hours. What fraction of the Iron-109 will remain after 24 hours?
1. The half-life of a radioactive
isotope is 1.0 hr. a)What fraction of the original
sample of radioactive nuclides is left after 3.0 h?
b) What fraction of the original sample of radioactive nuclides is left after 1.0 d?
2. A soil sample contains 40 mg of 90Sr. An atom of 90Sr masses 1.49295 x 10-25kg. Approximately, how much 90Sr will be in the sample 150 y from now?
Strontium's half-life = 50 years.
3. In 1898, Pierre and Marie Curie isolated about 10 mg of 226Ra from 8 tons of uranium ore. The half-life of Radium is 25 years. If this sample had been placed in a museum, how much of the radium would remain in the year 2100?
A radioactive element has a
half-life of 20 days. How much of a 16mg sample would be undecayed after 80 days?
