Chemistry Unit 11

Radioactivity

Process in which an unstable atomic nucleus emits charged particles and energy


Radioactive Isotope (radioisotope)

An atom containing an unstable nucleus


Nuclear Decay

When the composition of a radioisotope changes. Radioisotopes spontaneously change into other isotopes over time. During nuclear decay, radioisotopes change into the atoms of a different element (ex: uranium-238 can change into thorium-234)


Nuclear Radiation

Charged particles and energy that are emitted from the nuclei of radioisotopes. There are 3 types:

1. Alpha particles

2. Beta particles

3. Gamma rays – energy that is released


Alpha Decay

Decay that releases alpha particles. An alpha particle is a positively charged particle made up of 2 protons and 2 neutrons (same as Helium) – can only travel a few cm in air and can be stopped by a sheet of paper. Common symbol: He or ?

Nuclear Equation: [image]

The superscript represents the mass number (# of protons + # of neutrons) while the subscript represents the atomic # (the # of protons that determine the element). Uranium(U) is the parent isotope while Thorium(Th) is the daughter isotope and Helium(He) is the alpha particle.

*mass number decreases by 4 and atomic # decreases by 2 in alpha decay

Product Isotopes has 2 fewer protons and 2 fewer neutrons than the reactant isotope.

Equation is balanced:

-atomic mass on left=atomic masses on right (238)

-atomic number on left=atomic numbers on right (92)


Beta Decay

Decay that releases beta particles. A beta particles is an electron emitted by an unstable nucleus (smaller and faster – can penetrate through paper but stopped by metal)

Common symbol: e or ?

Eutron decomposes into a proton and an electron – proton gets trapped in nucleus, electron is emitted

Nuclear Equation: [image]

*atomic # increases by 1 in beta decay

Product isotope has one proton more and one neutron fewer than the reactant isotope.

Equation is balanced:

-atomic mass on left=atomic masses on right (234)

-atomic number on left=atomic numbers on right (90)


Gamma Decay

penetrating ray of energy emitted by an ustable nucleus (from ? and ? decay). No mass and no charge. Energy waves that travel through space at the speed of light.

Common symbol: [image]…Often accompanies ? or ?

Nuclear Reaction:

[image]

*? and e are equivalent as beta decay

*[image]is the energy released – not a particle and therefore doesn’t affect anyting

Much more penetrating than ? or ? rays. Can take several cm of lead or several meters of concrete to stop it


Penetration

Alpha: stopped by a sheet of paper

Beta: stopped by a layer of clothing or by a few millimeters of a substance such as aluminum

Gamma: stoped by several feet of concrete or a few inches of lead


Half-life

time required for life of a readioactive sample to decay


Nuclear Forces

attractive force that binds protons and neutrons together in the nucleus. A nucleus is unstable when the forces cannot overcome the repulsion – causes nuclear decay (when nucleus gets bigger – forces work best on short distances)


Fission

splitting of an atomic nucleus into 2 smaller parts. Tremendous amounts of energy result from very small amounts of mass.

Energy released by fission of 1 kg of uranium-235 = energy produced by burning more than 17,000 kg of coal


Mass into Energy

In fission, some mass is “lost” during the reaction and converted into energy. Albert Einstein describes how mass and energy are related: E = mc2

(c= the speed of light)

Total amount of mass and energy remains constant


Chain Reaction (spreading rumors simulated)

The teacher has a secret. She tells 3 people – each of those 3 people tell 3 more people – each of those 9 people tell 3 more people – each of those 27 people tell 3 more people – each of those 81 people tell 3 more people. Within a day, 243 people know this “rumor” that the teacher has spread

Neutrons released by split element nuclei such as uranium release even more neutrons which release uranium and so on.

[image]


Energy From Fission

Nuclear power plants generage ~30% of electricity in U.S. Controlled fission of uranium-235 occurs in a “fission reactor”

Advantages: small amounts of “fuel” – uranium rods (remember:energy released by fission of 1 kg of uranium-235 = energy produced by burning more than 17,000 kg of coal)

Do not amit air pollutants (sulfur and nitrogen oxides)

 

Disadvantages: Workers wear protective clothing. Waste can remain active for hundreds or thousands of years (has to be isolated and stored while it decays). Operators can lose control. Meltdown: core of reactor melts and radioactive material is released

 

*A nuclear reaction in an atomic bomb is uncontrolled while a nuclear reaction in a nuclear reactor is supposed to be controlled


Fusion

A reaction in whcih small nuclei combine to form a heavier nucleus.

Hydrogen nuclei combine to form a larger helium nucleus. Hydrogen isotopes deuterium (H-2) and tritium (H-3) are used in fusion.

Fraction of mass is converted into energy.

Sun and stars are powered by the fusion of hydrogen into helium. 300 million tons of hydrogen undergo fusion every second and ~4 million tons is converted into energy.

Required extremely high temperatures and you must contain “plasma”


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