Download Chapter 25 Nuclear Chemistry

Chapter 25 Nuclear Chemistry
Prior Knowledge
Recall, the atom of any element is composed of a dense center that is surrounded by electrons.
In Chapter 4 we learned that the nucleus can be subdivided into particles called
Recall that the number of protons in a particular nucleus is called the
And the sum of the protons and neutrons is called the
Atoms that have identical atomic numbers, but different mass numbers are called
The uniqueness of an isotope can be expressed the following way
term: nuclide-
When speaking of isotopes/nuclides we always say the name of the atom (determined by the # of protons)
For example, consider the isotopes of carbon, and the isotopes of hydrogen
carbon-12
hydrogen-1
carbon-13
hydrogen-2
carbon-14
hydrogen-3
25.1 Nuclear Radiation- In this section, you will learn the different types of radiation and their effects.
1. Radioactivity
Marie Curie (1867-1934) and Pierre Curie (1859-1906) were able to show that rays emitted by uranium atoms caused
fogging in photographic plates (film exposure). Marie Curie named the process by which materials give off such rays
radioactivity.
Radiation - The penetrating rays and particles emitted by a radioactive source are called radiation.
Nuclear reactions differ from chemical reactions in a number of important ways: (1) In chemical reactions, atoms tend
to attain stable electron configurations by gaining, losing, or sharing electrons, (2) In nuclear reactions, the nuclei of
unstable isotopes, called radioisotopes, gain stability by releasing radiation, (3) Nuclear reactions are unaffected by
catalysts and changes in temperature and pressure - the speed of a nuclear reaction is always the same! The rx rate
cannot be increased or slowed down.
A radioactive atom has an unstable nucleus due to lack of neutrons or too many neutrons relative to the amount of
protons.
An unstable nucleus releases energy by emitting radiation during the process of radioactive decay. Radioactive decay
turns the unstable nucleus into a stable nucleus of a different element.
2. Types of Radiation (table O)
The three main types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation.
a. Alpha radiation () - consists of helium nuclei that have been emitted from a radioactive source.
These emitted particles, called alpha particles, contain two protons and two neutrons (just like the Helium nucleus)
and have a double positive charge.
The mass numbers (superscripts) are balanced as well as the atomic numbers (subscripts). Mass number(A) always
decreases by 4, while the atomic number( Z), which determines the identity of the atom, decreases by 2.
Alpha particles have a large mass, do not travel very far, and are not very penetrating (paper or skin easily stops them).
Radioisotopes that emit alpha particles (table N) are dangerous when ingested because they penetrate soft tissue that is
in close proximity.
Ex. Write the radioactive decay for Plutonium -239 and Uranium-235 below (both nuclei emit alpha particles - table N)
b. Beta Radiation() - An electron resulting from the breaking apart of a neutron in an atom is called a beta particle.
The neutron is composed of a proton with an electron stuck to it. When the electron is released, a proton and beta
particle (electron) are released.
Ex. Carbon-14 emits a beta particle as it undergoes radioactive decay (table N) to form Nitrogen-14
The atomic mass stays the same, but the atomic number has increased by 1
Ex. Write the nuclear equation for radioactive decay of Iodine-137 below.
Beta particles have less charge than alpha particles and much less mass. The small mass gives a beta particle more
penetrating power. They pass through paper and skin, but are stopped by metal foils and thin pieces of wood.
c. Gamma Radiation() - A high-energy photon emitted by a radioisotope is called a gamma ray. The high-energy
photons are electromagnetic radiation with no mass or charge (its energy).
Gamma radiation is often emitted along with  and  particles during radioactive decay.
Gamma rays are extremely penetrating and can be very dangerous. They can only be stopped by several meters of
concrete or several centimeters of lead.
Section Review
1. How does an unstable nucleus release energy?
2. What are the three main types of nuclear radiation?
3. What part of an atom undergoes change during radioactive decay?
4. How is the atomic number (Z) of a nucleus changed by alpha decay? By beta decay? By gamma decay?
5. How is the atomic mass number(A) changed of a nucleus changed by alpha decay? By beta decay? By gamma decay?
6. Which of the three types of radiation discussed is most penetrating?
Section 25.2 Nuclear Transformations - In this section, you will learn about the decay rates of radioactive substances.
1. Nuclear Stability and Decay
All nuclei contain protons and neutrons (exception - hydrogen-1 has no neutrons). Since protons are positively charged,
it would be expected that they would repel and separate, but this does not occur. A force holds them together.
The nuclear force is an attractive force that acts between all nuclear particles that are extremely close together, such as
protons and neutrons in a nucleus. At such short distances, the nuclear force dominates over electromagnetic
repulsions and holds the nucleus together.
More than 1,500 different nuclei are known. Of those, only 264 are stable and do not decay or change with time.
Nuclei stability and the type of radiation emitted depends on the nuclei's neutron to proton ratio.
Nuclei with too many neutrons compared to protons emit alpha or beta particles.
Nuclei with many more protons than neutrons gain stability by converting a proton into a neutron. This can occur by
electron capture or a positron emission (table O)
Positron emissions (+) have the same mass an electron (-), but have a positive charge and are called positrons
Ex. of positron emission Iron-53
All nuclei with an atomic number over 83 are radioactive, most emit alpha particles to become stable.
2. Transmutation Reactions - The conversion of an atom of one element to an atom of another element is called
transmutation.
Natural transmutation - Transmutations that occurs naturally by radioactive decay.
Sample Problems - Using table N and O, write out the natural transmutation reactions for the following nuclides
Ex. francium-220
Ex. plutonium-239
Ex. strontium-90
Ex. neon- 19
Ex. nitrogen-16
Ex. technetium-99
Ex. calcium-37
Ex. radon-222
Artificial Transmutations - produce radioactive elements by bombarding the nuclei of stable atoms with high energy
particles such as protons(
) and neutrons(
), and alpha particles (
) in machines called particle
accelerators or "atom smashers."
Ex. Nitrogen-14 is smashed with alpha particles to make oxygen-17, what other particle is emitted?
Ex. Aluminum-27 is smashed with alpha particles to make phosphorous-30, what other particle is emitted?
2. Half-Life - Radioactive isotopes emit nuclear matter, and therefore disintegrate as time passes. The amount of time it
takes for an isotope to disintegrate has been calculated and it is measured in half lives.
A half-life (t1/2) is the time required for one-half of the nuclei of a radioisotope sample to decay to products.
After each half-life, half of the existing radioactive atoms have decayed into atoms of a new element.
Half-lives can be as short as seconds or as long as billions of years (Table N). Half -lives of radioisotopes found in nature
can be used to determine the age of fossils. Short lived radioisotopes are used in nuclear medicine and pose little risk.
Half Life Problems. - Table N is usually a must
Ex. Based on Reference Table N, what fraction of a sample of gold-198 remains radioactive after 2.69 days?
Ex. If the half life of a substance is 2.5 years, what fraction of the original substance will remain after 10 years?
Ex. Exactly how much time must elapse before 16 grams of potassium-42 decays, leaving 2 grams of the original isotope?
Ex. How many days are required for 200. grams of radon-222 to decay to 50.0 grams?
Ex. An 82.5 gram sample of potassium-42 will have what mass after 62 hours?
Ex. What fraction of an original sample of nitrogen-16 will remain after 21.6 seconds?
Ex. Approximately what fraction of an original Co-60 sample remains after 21 years?
3. Carbon Dating - Scientists often find the age of an object that was part of a living system by measuring the amount of
carbon-14 in it. Carbon-14 has a half life of 5730 years, and exists in constant proportion in all living things. The amount
of carbon-14 in living things is 0.05 % of all the carbon.
Ex. Carbon-14 emits beta radiation and decays with a half-life (t1/2) of 5730 years. Assume you start with a mass of 2.00
X 10-12 grams of Carbon-14.
a. How long is three half lives?
b. How many grams of the isotope remain at the end of three half-lives?
Section Review Questions
1. What determines the type of decay a radioisotope will undergo?
2. How much of a radioisotope remains after one half-life? After two half-lives?
3. What are two ways a transmutations can occur?
4. A radioisotope has a half-life of 4 days. How much of a 20-gram sample of this radioisotope remains after 8 days?
25.3 Fission and Fusion of Atomic Nuclei
1. Nuclear Fission - When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the
splitting of a nucleus into smaller nuclei.
Uranium-235 and Plutonium-239 are the only fissionable isotopes. Ex.
Nuclear Power involves the conversion of mass to energy. Each time the atom spits, mass is converted into energy.
Chain reaction- after each nucleus splits, 3 neutrons are released that will collide with 3 more nuclei and so on. The
splitting grows exponentially. Atomic bombs are uncontrolled nuclear chain reactions.
Nuclear Power Plants - Control the chain rx with control rods (carbon and cadmium) that control the released neutrons.
The heat from the fission is used to boil H2O and the steam turns the parts of a turbine creating electricity.
Nuclear power is good because the fuel lasts longer than fossil fuels with no emissions of CO2(greenhouse gas) or
SO2(acid rain gas). 1 kg of Uranium-235 releases more energy than 20,000 tons of dynamite (that's 40 million pounds!)
Nuclear power is bad because the nuclei left after fission are radioactive and must be contained and stored. In addition,
2 major nuclear accidents have occurred which leads to radioactive nuclei circumnavigating the globe. It is estimated
that the 1986 Chernobyl melt down will have caused 20 million cases of human cancer by the year 2050.
Note: Radioactive gases decay fast and are stored until levels are save and are ultimately released. Ex. radon-222,
krypton-85, and nitrogen-16
2. Nuclear Fusion - occurs when nuclei combine to produce a nucleus of greater mass.
In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei and two positrons.
Solar Fusion
Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large
nuclei split, but the process only occurs at extremely high temperatures.
3. Detecting Radiation - Radiation cannot be seen, heard, or smelled, it can be detected because it ionizes inert gases
such as Argon.
A Geiger counter uses a gas-filled metal tube to detect radiation. An argon gas filled chamber is hooked up to high
voltage, but electricity cannot travel through the gas unless the gas contains ions. When radiation is present, the gas is
ionized and the electricity flows. The more gas ionization, the higher the amount of electricity flows and the Geiger
counter will have a higher radiation reading.
Geiger counters can detect alpha, beta, and gamma radiation.