nuclear chemistry chapter 22. unstable isotopes called radioisotopes become stable by making...
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Nuclear ChemistryChapter 22
Unstable isotopes called radioisotopes become stable by making changes in nuclei. Changes are accompanied by the emission of large amounts of radiation energy.
Unlike chemical reactions, nuclear reactions are NOT affected by changes in temperature, pressure, or the presence of a catalyst! They are also unaffected by the compound the element is in.
Nuclear reactions can NOT be slowed down, sped up, or turned off.
Atomic nuclei are made of protons and neutrons, which are collectively called nucleons.
In nuclear chemistry, an atom is referred to as a nuclide and is identified by the number of protons and neutrons in its nucleus.
Ra22888 is the same as Radium-228.
Example: 42He
2 protons: (2 x 1.007276amu) = 2.014552 amu2 neutrons: (2x1.008665 amu) = 2.017330 amu2 electrons: (2 x 0.0005486 amu) = 0.001097 amu
_______________ Total combined mass: 4.032979 amu
The atomic mass of helium has been measured to be 4.00260 amu.
The measured mass of 4.00260 amu is 0.03038 amu less than the calculated mass, 4.03298 amu.
The difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons is called the mass defect.
The mass of the p + n + e does not add up to the total mass of an atom.
The difference is called the mass defect and occur because some of the mass is literally converted to energy to hold the nucleus/atom together.
What causes the loss in mass? Einstein’s equation E = mc2 describes
the relationship. According to Einstein’s equation E=mc2,
mass can be converted to energy, and energy to mass.
The mass defect is caused by the conversion of mass to energy upon formation of the nucleus.
The mass units of the mass defect can be converted to energy units by using Einstein’s equation.
This energy is the nuclear binding energy, the energy released when a nucleus is formed from nucleons.
Not all nucleons are stable. When you plot the # of protons vs. the # of
neutrons, you get a “band of stability”. This is related to electrostatic force (repulsion)
between protons! It’s somewhat balanced by the nuclear force
holding the nucleus together. But beyond atomic # 83, you can’t have a
stable nucleus no matter how many neutrons are in there—repulsions are too strong.
In general, the higher the neutron-to-proton ratio in the nucleus, the more unstable the nucleus!
Depending on the type of nucleus, the type of radioactive decay changes.
Stable nuclei tend to have even numbers of nucleon, which implies that they are somehow paired!
The most stable nuclei have 2, 8, 20, 28, 50, 82, or 126 p, n, or total nucleons.
This implies that nucleons –like electrons--also exist in certain energy levels, each of which holds a specific number of nucleons!
The nuclear shell model proposes this, and calls these numbers that complete the energy levels the magic numbers.
All nuclei with atomic number > 83 are radioactive!
Elements above 92 are called the transuranium elements—none occur in nature, and all are radioactive (created in nuclear accelerators)
Radioactive Decay In 1896, Henri Becquerel was studying
uranium compounds. He wrapped a photographic plate in a
lightproof covering and put a uranium compound on it.
The plate was exposed, even without light!
He proposed unknown rays must be causing this, and called them “x-rays.”
Marie Curie and her husband, Pierre, also worked in his lab.
They found that uranium and thorium were radioactive. They won the 1903 Nobel Prize in Physics for identifying radioactive materials.
Later, Marie identified and purified polonium and radium.
She won the 1911 Nobel Prize in Chemistry.
Marie was the first woman to win a Nobel Prize, and the only to win two.
She is the only person to win 2 in two different science fields (Linus Pauling won for Chemistry and for Peace).
She was the first female professor at the University of Paris. She founded the Curie Institutes in Paris.
Pierre died in a traffic accident; Marie died in 1934 from the effects of radiation. T
Her daughter also won a Nobel Prize in science.
Radioactive decay: the process by which an unstable nucleus loses energy by emitting radiation.
Eventually, unstable radioisotopes of one element are converted to stable, non- radioactive isotopes of a different element!
This finally destroys Dalton’s atomic theory: Isotopes of an element can be
different and act differently An atom can be broken apart Elements can change from one kind to
another
Types of Radiation Alpha radiation: He nuclei are emitted. These particles, called α particles, have two
protons and 2 neutrons, and a double + charge. They are also written as He42
o example: HeThU 42
23490
23892
o α particles are heavy and do not penetrate very far. They can be stopped by a piece of paper. But once inside the body, they can cause severe tissue damage.
o This occurs with very heavy nuclei that need to reduce the protons and neutrons to become more stable.
Beta radiation: neutrons break into a proton and an electron. (Technically, this should violate the law of conservation of energy! Wolfgang Pauli proposed a particle called a neutrino that is also emitted. The presence of neutrinos has been confirmed!)
o eHn 01
11
01 the proton stays in the nucleus, and the electron, also called a β particle, is ejected.
o Example: eNC 01
147
146 nitrogen is a stable isotope
o β particles have half the charge and are much lighter than α particles. They are therefore much more penetrating! β particles can be stopped by wood or foil.
o This occurs with elements that have too many neutrons.
Positron emission: a proton can be converted into a neutron by emitting a positron, a particle that has the same mass as an electron but a positive charge. o 0
110
11 np the positron is ejected from the nucleus
o Example: 01
3818
3819 ArK
o This occurs when elements have too many protons to be stable
Electron capture: in this case, an inner orbital electron is captured by the nucleus of its own atom! The electron combines with a proton, and a neutron is formed.
o npe 1
011
11
o PdeAg 10646
01
10647
o This can also occur when there are too many protons
Gamma rays are often emitted along with α or β radiation, when the decay leaves the nucleus in an excited state.
Gamma rays have no mass and no charge (they are just energy!) so they can pass through anything but several meters of concrete or centimeters of lead.
o Examples: HeRaTh 42
22688
23090 or
ePaTh 01
23491
23490
Gamma radiation: high-energy electromagnetic waves that are emitted from a nucleus as it changes from an excited state to a ground state. The fact that gamma rays are emitted supports the nuclear shell model, in which particles are in ground-state energy levels but can become excited the way electrons do.
Half-Life
Half life is the time required for one-half of the atoms of a radioisotope to decay.
half-life is a characteristic of every radioisotope; it can vary from a fraction of a second to several million years!
o Po21484 163.7 µs
o C146 5715 years
o U23892 4.46 x 109 years
Nuclear Radiation Rem: roentgen equivalent, man. One rem is the amount
of ionizing radiation that does as much damage to human tissue as is done by 1 roentgen of high voltage x-rays.o Everyone is exposed to environmental background
radiation! o Average exposure in the US is about 0.1 rem/year. o Maximum allowable is 0.5 rem/year. o But high altitudes and airline travel increase
exposure, as does naturally occurring radon-222. o Every x-ray you get also increases exposure.
Can detect radiation using: Film badges—use the exposure of film Geiger-Muller counters—count electric
pulses carried by gas ionized by radiation Scintillation counters—convert scintillating
light (from substances that absorb radiation and emit visible light) to electric signals
Applications of nuclear radiation are significant Radioactive dating: C-14 is the most commonly
used, but is good only to 50,000 years. Can use nuclides with longer half-lives to date rocks more than 4 billion years old.
Medicine: Co-60 is used to destroy cancer cells. Other radioactive tracers can be used to detect cancers or other irregularities in the body.
Agriculture: tracers in fertilizer can be used to determine the effectiveness of the fertilizer. Can trace it through the plant to see how it is taken up. Can also use radiation to kill bacteria and insects in meat and other foods (controversial!)
Nuclear Fusion and Fission Fission: the splitting of a nucleus into
smaller fragments. This occurs when nuclei of certain
isotopes are bombarded by neutrons.
UU 23692
23592
neutron
Energy!
+ neutron
+ 2 neutrons
go on to start a chain reaction
Ba14256
Kr9136
fissionable
very unstable
Atomic bombs are uncontrolled nuclear chain reactions!
Nuclear reactors are controlled nuclear chain reactions. They are used to produce useful energy. Energy appears as heat, which is removed by a coolant fluid (water). The water heats other water to steam, which drives a turbine which generates electricity.
One pellet of uranium is equal to the energy in 150 gallons of oil!!
Nuclear reactors are controlled by:o Neutron moderation: this slows the neutrons so that they
can be captured by the reactor fuel (U-235) to continue the chain reaction. Many neutrons move so fast that they pass right through the nucleus without being absorbed. Water and carbon slow down the neutrons so they can affect the next atom of U-235.
o Neutron absorption: this decreases the number of slow neutrons. We don’t want the chain reaction to go out of control, so we trap many neutrons before they can hit another U-235. Use control rods of cadmium, which absorbs neutrons.
Nuclear fission reactors produce radioactive waste, from leftover fuel to contaminated control rods to radioactive building materials.
Also there is a danger of having the chain reaction go too fast, overheating the core, and causing a meltdown. ** there can never be a nuclear explosion because the fuel elements are to spaced out and can’t physically connect to provide the critical mass needed for a bomb.
Fusion: the combining of two nuclei to produce a nucleus of heavier mass. In the sun, this reaction happens:
One kg of fusion fuel would produce the same amount of energy as 10,000,000 kg of fossil fuel! Also, the fuel, hydrogen, is infinitely available. The products are not radioactive, and there is no waste. It’s the holy grail of science!
4 01
42
11 2 HeH particles + energy!
Problems are the extremely high temperatures—over 40 million °C—in which fusion reactions take place.
So we can’t get temperatures that high to start the reaction, and then no materials we have can exist at those temperatures.
We also don’t know how to control fusion. We might be able to contain a fusion reaction
in magnetic fields. Research is being done.