Radioactivity*and* Nuclear*Reactions* Small*vs*Large ...
Transcript of Radioactivity*and* Nuclear*Reactions* Small*vs*Large ...
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Radioactivity and Nuclear Reactions
Ch 9.1-‐9.2, 9.4
Nucleus and the Strong Force ! Protons and neutrons are packed tightly together
! Two positives normally repel each other, so why don’t the protons in the nucleus repel?
! Strong force = one of four basic forces that causes protons and neutrons to be attracted to each other
! 100 times stronger than electric force
! Short-‐range force, so it weakens with distance
Small vs Large Nuclei ! Protons and neutrons are held together less tightly in
large nuclei. Why?
! Small nuclei have few protons, so the repulsive force on a proton due to other protons is small
! In a large nuclei, the attractive strong force is exerted only by the nearest neighbors. All the protons exert repulsive forces making the repulsive force large.
Radioactivity ! In many nuclei, the strong force keeps the nucleus
together (STABLE)
! When it can’t, the nucleus can decay and give off matter and energy in a process of radioactivity
! Larger nuclei tend to be unstable – all nuclei containing more than 83 protons are radioactive
! All elements with more than 92 protons are synthetic and decay soon after they are created (UNSTABLE)
Stable and Unstable Nuclei
! Smaller elements neutron to proton ratio is 1:1 to be stable isotopes
! Heavier elements neutron to proton ratio is 3:2 to be stable isotopes
! Nuclei of any isotopes that differ much from these ratios are unstable, whether heavy or light
Nuclear Radiation ! When an unstable nucleus decays, particles and
energy are emitted from the decaying nucleus ! Alpha Particles – (2 p and 2 n lost) massive,
comparatively speaking; loses energy quickly; can’t pass through paper; changes the element (transmutation); mass changes; can damage the body
! Beta Particles – (n turns into p and emits e) e emitted from n; transmutation changes the element; mass doesn’t change; much faster and penetrating; damage body
! Gamma Rays – electromagnetic waves that carry energy; most penetrating form; cause less damage to biological molecules
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At a glance… Radioactive Half-‐Life ! Some radioisotopes decay in less than a second,
while others take millions of years
! Half-‐life: the amount of time it takes for half the nuclei in a sample of the isotope to decay
Radioactive Half-‐Life cont Ch 21.3: Absolute-‐Age
Dating of Rocks ! Relative-‐age dating vs. Absolute-‐Age Dating
! Relative-‐age dating: compares past geologic events based on the observed order of strata in rock record
! Absolute-‐age dating: determines actual age of a rock, fossil, or other object
Radioactive Decay ! Radioisotopes are found in igneous and metamorphic
rocks, some fossils, and organic remains
! Emission of radioactive particles and the resulting change into other elements over time is called radioactive decay
! This decay stays constant regardless of the environment, pressure, temperature, or any other physical changes
! So, these atomic particles become accurate indicators of the absolute age of an object
! I love you Mrs. Sjuts! ☺
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Radioactive Dating ! Fossils and rocks can be dating using radioactive
isotopes
! Amounts of the radioisotope and its daughter nucleus are measured in a sample
! Then, the number of half-‐lives that need to pass to give the measured amounts of the isotope are calculated
! The number of half-‐lives is the amount of time that has passed since the isotope began to decay AND usually is the same as the age of the object.
Carbon Dating ! The radioactive isotope C-‐14 is often used to find the
ages of once living objects
! It is naturally found in most all living things
! An atom of C-‐14 eventually will decay into N-‐14 with a half-‐life of 5,730 years
! By measuring the amount of C-‐14 in a sample and comparing it to the amount of C-‐12, scientists can determine the approx age of plants and animals that lived within the last 50,000 years
Uranium Dating ! Some rocks contain uranium, which has two
radioactive isotopes with long half-‐lives, both decaying into isotopes of lead
! By comparing the uranium isotope and the daughter nuclei the number of half-‐lives since the rock was formed can be calculated
! U-‐235 " 0.7 billion years
! U-‐238 " 4.5 billion years
Ch 9.4 Nuclear Reactions
! Nuclear Fission – the process of splitting a nucleus into two nuclei with smaller masses
! Chain reaction – ongoing series of fission reactions
! Critical mass – the amount of fissionable material required so that each fission reaction produce approximately one more fission reaction
! Nuclear Fusion – two nuclei with low masses are combined to form one nucleus of larger mass
Nuclear Fission ! Large elements need a TON of energy in order to hold their
nucleus together.
! When the large nucleus is split into smaller nuclei, those smaller nuclei don’t require as much energy to stay together…
! So, that leftover energy is released!
! Atomic bomb – used in Hiroshima and Nagasaki
Fission -‐ Chain Reaction Nuclear Fission: Pros and Cons
Nuclear Meltdown
Cooper Nuclear Station near Brownville, NE
Fort Calhoun Nuclear Generating System between Ft. Calhoun and Blair
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Nuclear Fusion ! Need very high temperatures in order to overcome
the repulsive forces. Sun's Fusion
! Scientists cannot control fusion reactions for the purpose of power.
! We can, however, use it to make nuclear weapons. Large ones. Hydorgen Bomb -‐ Fusion
Nuclear Decay vs. Nuclear Reactions
! Decay happens spontaneously
! Reactions are controlled and self-‐sustaining and release much more energy
Nuclear Reaction: Plutonium
! Pu-‐239 Used to make nuclear weapons like the one dropped on Nagasaki in 1945