the nucleus and radioactivity radioactivity: spontaneous changes in the nucleus that emit energy as...
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The Nucleus and Radioactivity
Radioactivity: Spontaneous changes in the nucleus that emit energy as radiation (particles or rays)
Nuclei contain protons and neutrons; some combinations of these particles are unstable
Examples of Radioactive Nuclei Include:
Radioactive Decay: Emission of radiation produced by unstable nuclei changing to a more stable state
Types of Radiation Include:
Alpha rays a: positive charge
Beta rays b: negative charge
Gamma rays g: no charge
a and b rays consist of streams of particles
g rays consist of electromagnetic radiation
positron: an antiparticle of a particle (their charges are opposite, but their masses are the same)A positron has a +1 charge and is called a positive electron.positron:+ orA positron is formed when a proton is converted to a neutron.neutronproton+positron
contains 2 protons and 2 neutrons
identical to helium nucleus
travel only short distances
electrons produced in the nucleus, then emitted
travel greater distances than a particles
High-energy ray similar to an X ray
Travel great distances
Daughter Nuclei: New nuclei that result from unstable nuclei undergoing radioactive decay
Example: Uranium-238 gives up an a particle, resulting in a daughter nucleus of a different element, Thorium (Th)
Summary of Radiation Types
Alpha DecayWhen a radioactive nucleus emits an alpha particle, a new nucleus results.
The mass number of the new nucleus is 4 less than that of the initial nucleus.
The atomic number is decreased by 2.
Nuclear Reactions: Alpha EmissionAlpha emission is the decay of a nucleus by emittingan a particle.
In a balanced nuclear equation, the sum of the mass numbers and the sum of the atomic numbers for the nuclei of the reactant and the products must be equal.
251Cf 247Cm + 4He 98 96 2 24195Am42He+23793Np
Write an equation for the alpha decay of Rn-222.
222Rn new nucleus + 4He 86 2Mass number: 222 4 = 218Atomic number:86 2 = 84Symbol of element 84 = Po
222Rn 218Po + 4He 86 84 2
Beta DecayThe unstable nucleus converts a neutron into a proton (emitting an electron from the nucleus)
The mass number of the new nucleus remains the same
The atomic number of the new nucleus increases by 1
1n 0e + 1H 0 -1 1
Beta emission is the decay of a nucleus by emitting a particle; 1 neutron is lost and 1 proton is gained. Nuclear Reactions: Beta Emission
Example: Potassium - 42 is a beta emitter. 42K new nucleus + 0e 19 -1
Mass number : (same) = 42 Atomic number: 19 + 1 A= 20 Symbol of element 20 = Ca
42K 42Ca + 0e 19 20 -1
Learning CheckWrite the nuclear equation for the beta decay of Co-60.
SolutionWrite the nuclear equation for the beta decay of Co-60.
60Co 60Ni + 0e 2728 1
*Nuclear Reactions: Positron Emission Positron emission is the decay of a nucleus by emitting a positron, +; 1 proton is lost and 1 neutron is gained.
Gamma RadiationGamma radiation is energy emitted from an unstable nucleus indicated by m.
In a nuclear equation for gamma emission, the mass number and the atomic number are the same.
99mTc 99Tc + 43 43
Summary of Radiation
Some radioactive isotopes are more stable than others, and therefore decay more slowly
Half-Life: Time required for half of the unstable nuclei in a sample to decay
Example: A Potassium-38 sample weighs 100 grams. 8 minutes later, the sample is weighed again and found to weigh 50 g. The half-life of potassium-38 is 8 minutes
Note: The half-life of a radioactive isotope is a property of a given isotope and is independent of the amount of sample, temperature, and pressure.
Half-Lives Vary Dramatically Between Elements
Half-Life CalculationsAfter one half-life, 40 mg of a radioisotope will decay to 20 mg. After two half-lives, 10 mg of radioisotope remain.
40 mg x 1 x 1 = 10 mg 2 2 1 half-life 2 half-lives
Initial40 mg20 mg
If the half-life of iodine-131 is 8.0 days, how much of a 100. mg sample remains after 32 days?Determine how many half-lives occur in thegiven amount of time.32 days1 half-life8.0 daysx=4.0 half-lives
For each half-life, multiply the initial mass by one-half to obtain the final mass:100. mginitial massx12x12x12x12The mass is halved four times.= 6.25 mg final mass
Learning Check The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 26 hours?
Half life = 13 hrs Number of half lives = 2Amount remaining = 64 mg x 1 x 1 = 16 mg 2 2 13 hrs 13 hrs64 mg 32 mg 16 mg
Radiation and Health
Free Radicals: Very reactive compounds that can cause mutations, cancer; usually caused by long-term exposure to low-level radiation
Radiation Sickness: Illness and symptoms caused by short-term exposure to intense radiation
Medical: diagnosing and disease (cancer, thyroid, brain scans)
Uses of Radioisotopes
Common Imaging Techniques
PET Scans (Positron Emission Tomography): gamma rays create a 3D image of organs, used to analyze blood flow, metabolic activity and brain function
CT (Computed Tomography): X-rays are used to create series of images of the brain, identifying brain damage and hemorrhaging
MRI (Magnetic Resonance Imaging): H protons in magnetic field are used to create color images of soft tissue
Health/Agriculture: food irradiation
Radioactive dating: determine age of fossils
Nuclear Power Plants: Alternative energy source
Units of Radiation
Curie (Ci): number of disintegrations per second per gram of radium; 3.7 x 1010 disintegrations per second
Rad (Radiation Absorbed Dose): amount of material able to deliver 2.4x10-3 cal of energy to 1 kg of tissue
Rem (Radiation Equivalent in humans): amount of biological damage caused by different types of radiation
In 1934 Radioactivity was Artificially Induced for the first time!!
High-energy particles (such as neutrons) can create unstable nuclei that then undergo radioactive decay (Cyclotrons and Linear Accelerators)
Nuclear Fission: Process in which large nuclei split into smaller nuclei when bombarded with neutrons, releasing large amounts of energy
Example: When a neutron bombards U-235, an unstable nucleus of U-236 forms smaller nuclei such as Kr-91 and Ba-142.
Chain Reaction: Nuclear reaction in which the products of a reaction cause that reaction to occur repeatedly
Nuclear Fusion: Process in which small nuclei combine (fuse) to form larger nuclei
Example: Hydrogen nuclei combine to form Helium nuclei