Unit 8 Nuclear Chemistry

Lesson 8.07

Nuclear Fission and Fusion

Lesson 8.02

Radioactivity and Half-Life

Applications of nuclear chemistry:

The strong nuclear force is a powerful one, so nuclear reactions release a great deal of energy, usually in the form of radiation. We can use this energy for several purposes: ♦Nuclear medicine uses the energy from nuclear reactions for diagnosis and treatment. →Visualizing parts of the human body for diagnosis (X rays, CT scans) →Treating illnesses (radiation therapy for cancers) ♦Nuclear power uses the heat released from nuclear reactions to make electricity. ♦Nuclear bombs release heat and radiation from nuclear reactions.

Lesson 8.06

Transmutation of Elements

Radioactively decaying atoms release matter or energy:

Upon breaking down, unstable nuclei release energy of three major types: →Alpha particle: (a helium nucleus ((2 protons and 2 neutrons)), often released from a large atom ((2^4He^2+ or 2^4He)) →Beta particle: (a high-speed electron from the nucleus ((not the orbitals)) →Gamma ray: Electromagnetic waves

Calculating half-life example:

♣^14C has a t1/2 of 5,730 years. ♣If you start with 2.00 x 10^-6 g of ^14C, how much will you have after 17,190 years? ◘Given →t1/2 = 5,730 →t = 17,190 →N0 = 2.00 x 10^-6 g ◘Calculate the number of half-lives: → #half-lives = t/t1/2 →17,190 / 5,730 → = 3 ◘Calculate fraction remaining from the number of half-lives: → fraction remaining = 1/2^n → 1/2^3 → = 1/8 ◘Multiply the fraction remaining by the initial amount. N0 is the initial amount.Nt is the final amount after time t.: →Nt = (fraction remaining) x N0 → = 1/8 x 2.00 x 10^-6 g → = 2.50 x 10^-7 g

Lise Meitner & Otto Hahn

♦1938, they were studying what happens when neutrons bombard uranium: 92^235U + 0^1n → 56^141Ba + 36^92Kr + 30^1n + energy ♦At first, they were puzzled by the production of such light atoms as Ba and Br, but later they realized that the neutron had split the uranium atom. →They had discovered fission.

When radioisotopes decay, they can change into new elements:

♦A neutron within the iodine nucleus decayed into a proton and an electron. →This decay increased the atomic number by 1, thereby producing an isotope of the element xenon.

Radiation of three types:

♦Alpha emission →a helium nucleus (2 protons, 2 neutrons), often released from a large atom →Alpha particles can be stopped by paper or skin ♦Beta emission →a high-speed electron or positron (antielectron) from the nucleus (not from the orbitals) →Beta particles can be stopped by aluminum foil or Plexiglas ♦Gamma emission →electromagnetic waves emitted from the nucleus →Gamma emission can be stopped by concrete or lead

Decay:

♦An unstable nucleus will spontaneously change—scientists use the word decay—to become a more stable nucleus. ♦It does so by emitting a particle or particles and/or energy, which are collectively called radiation.

Besides the previous major types of radiation, some nuclei can:

♦Emit a positron, a positively charged particle similar to an electron (+1^0e). →A proton decays to produce a neutron and a positron. ♦Capture an electron. →The nucleus captures an electron from the surrounding cloud. →The captured electron combines with a proton to form a neutron. →The neutron may subsequently be emitted.

Enrico Fermi and the first nuclear fission reactor in 1942:

♦Enrico Fermi developed the first fission reactor, or pile, by achieving a critical mass of uranium-235. ♦Critical mass is the amount of ^235U (or other fissile material) necessary to sustain a continuous fission chain reaction. ♦If the mass is below the critical mass, then the outward pressure from the heat, energy, and motion of the fragments will push the target atoms too far apart to absorb the emitted neutrons and continue the fission reaction. ♦The critical mass provides enough attractive force to position the target atoms close enough together to initiate the chain reaction and to hold the fissile material together against this outward pressure. ♦Thus, it is enough mass to sustain the chain reaction.

Ernest Rutherford and James Chadwick:

♦Ernest Rutherford discovered that almost the entire mass of the atom is in its nucleus, yet the volume of the nucleus is extremely small compared with the rest of the atom. →Rutherford later discovered the first nuclear particle, the proton. The proton has a charge of +1. ♦Later James Chadwick discovered the other nuclear particle, called the neutron. →He found that the neutron had about the same mass as a proton, but was not charged.

Fission provides energy for electricity.......ha ha

♦Fermi's nuclear reactions could be controlled in a contained nuclear reactor and could produce heat to make electricity. In a nuclear power plant, the following happens: →Fission occurs in the reactor core, and boron rods control the reaction by absorbing excess neutrons. →The heat from fission transfers to pressurized water circulated through the core (the primary coolant loop). →The hot pressurized water transfers heat to water in another loop, called the secondary loop, creating steam. →Steam drives turbines, the steam gets condensed to water, which is then recirculated. →The reactor's concrete structure prevents radiation leaks from the reactor to the environment.

Nuclear reactions change the composition of the atom's nucleus:

♦For example: 92^238U is an isotope of uranium that has an atomic number of 92 (as do all uranium atoms) and a mass number of 238. →It is an isotope of uranium with 92 protons and 146 neutrons. →In a nuclear reaction, the composition of the nucleus actually changes. →For example, the isotope uranium-238 (92^238U) will change into the isotope thorium-234 (90^234Th) by emitting 2 protons and 2 neutrons (a helium atom) from its own nucleus. →Thus, uranium becomes a totally different element. →→This happens in a nuclear reaction, but not in a chemical reaction.

Two ways to initiate nuclear reactors:

♦Gun-type →explosives fire a piece of 92^235U into subcritical 92^235U to create supercritical mass. ♦Implosion →explosions create shock waves that are directed inside a sphere to compress the subcritical 94^244Pu core into supercritical mass.

The different types of radiation...

♦Have different amounts of energy and are capable of penetrating materials in different ways ♦For example, alpha particles can be stopped by paper or skin; beta particles, by aluminum foil or Plexiglas; and gamma emissions, by concrete or lead. ♦Some types of decay also yield positrons- positively charged electrons (+1^0e)- or neutrons

Calculating the half life:

♦If you are given the half-life for the decay of a particular radioisotope, then you can calculate how much of that radioisotope remains after several half-lives: fraction remaining (FR) = 1/2^n →n = the number of half-lives

Unstable nuclei decay to become stable:

♦If you plot the number of neutrons (N) versus the number of protons (Z) for the various elements, you see that the N/Z ratio = 1 for elements up to the atomic number of 20. ♦Above Z = 20, there are more neutrons than protons. →Within this region is a line of stable nuclei, a band of stability. →Elements above the band of stability have too many neutrons, decreasing the strong force and destabilizing the nucleus. →Elements below this band of stability have too many protons, increasing the repulsive force and destabilizing the nucleus. ♣All nuclei with an atomic number above 83 (Z > 83) are unstable, some more so than others.

Fission:

♦In fission, the unstable target nucleus absorbs a neutron and immediately splits into two lighter element fragments (products) and one or more neutrons. ♦The mass difference between the products and reactants is converted to energy (E=mc^2). ♦The emitted neutrons can split other atoms in a chain reaction.

What is nuclear chemistry?

♦In nuclear chemistry, you will study the composition and changes of the nucleus of an atom. ♦Nuclear chemistry began in the late 19th and early 20th centuries, as scientists discovered radioactivity and the subatomic particles that make up the nucleus. →These scientists include Ernest Rutherford, Henri Becquerel, Pierre and Marie Curie, and James Chadwick. →In studying radioactive atoms, they found that the nucleus itself could undergo changes.

Fusion:

♦In nuclear fusion, lighter elements combine to make both heavy elements and energy.

Nuclear bombs and fission:

♦In the early days of World War II, physicist Robert Oppenheimer and General Leslie Groves led the U.S. effort to develop an atomic bomb in what was called the Manhattan Project. ♦The problems involved enriching 92^235U, storing subcritical 92^235U mass in a bomb casing, and finding a way to initiate a chain reaction of 92^235U fission.

E=mc^2

♦In this equation, mass has equivalent energy and energy has equivalent mass. ♦In any reaction where energy is change, there is a slight change in mass. ♦Conversely, if there is a change in mass, then there is a change in energy.

The nucleus of an atom:

♦Is made up of protons and neutrons

Radioactive decay changes the nucleus:

♦Large unstable nuclei (atomic number, Z > 83) tend to decay by alpha emission. →The atomic number decreases by 2 and atomic mass decreases by 4: 92^238U → 90^234 Th +2^4 He^2+ ♦In high N/Z nuclei, neutrons decay into a proton with the release of a beta particle →The atomic number increases by 1, but mass remains the same: 6^14C → -1^0 e +7^14 N ♦In nuclei with many protons (low N/Z), protons can decay into neutrons and positrons (also called antielectrons) →The atomic number decreases by 1, but mass remains the same: 8^15 O → 7^15 N + +1^0e ♦Alternatively, low N/Z nuclei can capture an electron from the electron cloud (electron capture): 37^81Rb + -1^0e → 36^81 Kr

Nucleus:

♦Made up of protons (+1 charge) and neutrons (0 charge) →Both are made of quarks →Quarks attract one another to hold protons and neutrons together via the strong nuclear force →Quarks from one proton also attract quarks from another nucleon with the residual strong force ♦The strong nuclear force—or, simply, the strong force—acts over short distances to hold the nucleus together. ♦In contrast, an electromagnetic force tends to repel the protons and tends to make the nucleus fly apart.♦ ♣Neutrons help stabilize the nucleus. →In stable atoms, these opposing forces are balanced. →However, when there are too many protons relative to the number of neutrons, these forces are out of balance, and the nucleus is unstable.

Some particle accelerators are straight:

♦Not all particle accelerators are circular; some, including the Stanford Linear Accelerator Center (SLAC), are straight-line accelerators. ♦SLAC uses a long (3 km) vacuum tunnel to accelerate particles. ♦The rapid particles then travel around a storage ring, where they hit different types of targets, depending upon the experiment. ♦The electromagnets of SLAC use copper tubing and wave guides to accelerate the electrons. ♦Detectors at the end look for radiation, particles, particle velocities, particle tracks, and even subatomic particles. ♦Subatomic particles like quarks and leptons leave trails in bubble chambers after collisions. ♦In particle-accelerator collisions, transmutations with short half-lives are often formed and decay further into other particles.

Quick question: What are electrons made of??

♦Now that you know what makes up protons and neutrons, are you wondering what makes up electrons? ♦Electrons aren't made of anything else.♦ ♦Like up quarks and down quarks, electrons are a type of fundamental particle called a lepton.♦ ♦Leptons are not involved in holding the nucleus together. ♦While quarks exhibit a strong nuclear attraction, leptons exhibit a weak nuclear attraction.

Transmutation can occur through artificial means:

♦Physicist Ernest Rutherford bombarded nitrogen-14 with alpha particles emitted from radium: ♦7^14N + 2^4He → 8^17O + 1^1H ♦The result was the first realization of an alchemist's dream, changing one element into another, even though no precious metal was involved. ♦Since Rutherford's time, nuclear scientists have artificially produced an entire series of elements (transuranium elements, Z > 92) and new radioisotopes by colliding atoms, particles, and ions together in devices called particle accelerators.

QUARKS OoO

♦Protons and neutrons are together called nucleons, and they have been found to be composed of even smaller particles. ♦The discovery was made by way of a particle collider or atom-smasher. ♦A particle collider is a particle accelerator (I am totally not thinking about the Flash XD ) with two opposite beams of particles. ♦The particles are accelerated to extremely high kinetic energy. ♦Those particles hit other particles, and scientists observe the resulting reaction. →It's like dropping a television from a skyscraper and examining the pieces after it hits the ground. ♦Believe it or not, such experiments can reveal the smallest particles that make up matter.

Protons, Neutrons, and Quarks:

♦Protons are made of 2 up quarks and 1 down quark → [(+2/3) + (+2/3) + (-1/3) = +1 charge] ♦Neutrons are made of 1 up quark and 2 down quarks → [(+2/3) + (-1/3) + (-1/3) = 0 charge]

More on quarks:

♦Quarks have strong nuclear attractions and charges, which hold the nucleus together. ♦Although there are six types of quarks, you will only learn about two types here: ♣Up quarks are particles with a + 2/3 charge ♣Down quarks are particles with a - 1/3 charge

Isotopes (in case you forgot, cause honestly I did):

♦Remember, isotopes of an element are atoms that have the same number of protons but a different number of neutrons.

Fusion and electricity:

♦Scientists at many institutions are attempting to develop a sustained fusion reactor. ♦To do this, they must duplicate conditions inside the sun. ♦They heat deuterium and tritium to a high-temperature plasma state to ionize the atoms. ♦Strong electromagnets produce powerful magnetic fields that squeeze the isotopes together for fusion. ♦While the first fusion reactions have been achieved, a sustained fusion reactor is still being developed. ♦A fusion nuclear power plant would work much the same as a fission one. ♦Heat from the plasma would be transferred to a moderator substance, which would then transfer heat through two water-coolant loops to a steam turbine and a generator. ♦Fusion power plants would have the advantage of producing energy for electricity without making radioactive waste (fission products).

Particle accelerators:

♦Scientists use particle accelerators to study the structures of atomic nuclei. ♦A particle accelerator contains a source of electrically charged particles. ♦The particles may be electrons, alpha particles, or ions. ♦The particles travel in a vacuum. ♦As they pass through a series of electromagnets, they are accelerated to near the speed of light (186,000 miles/s or 3.00 × 108 m/s). ♦Even small particles like electrons have great energies at this velocity. ♦The high speeds are necessary for charged particles to make it past the electron clouds to the nucleus. ♦The energized particles smash into target atoms. ♦Upon collision, the target atoms may form new elements (transmutations) and release other particles and radiation. ♦In some accelerators, the protons and neutrons of the target atoms break down into quarks and other subatomic particles.

Some particle accelerators have circular paths:

♦The first accelerator was made by Ernest Lawrence in 1929 and was called a cyclotron. ♦The cyclotron had two D-shaped electromagnets. ♦Electrons were injected into the cyclotron, and the alternately charged "dees" created an oscillating electric field that accelerated the electrons. ♦The electrons traveled outward in a circular path as they accelerated. ♦At the end, they hit a small metal foil target. ♦A detector sensed the types of radiation and particles. ♦Compared to today's cyclotrons, Lawrence's was rather weak and could not reach velocities near the speed of light. ♦However, today's accelerators use rings of electromagnets that are kilometers in diameter to generate the necessary speeds. ♦The cyclotron of the Fermi National Accelerator Laboratory, in Illinois, is 10 square miles, or 25.6 square kilometers.

The charges of protons:

♦The like charges of protons repel each other, causing the atomic nucleus to come apart. →The nucleus of an atom contains positively charged protons. →Because like charges repel, the protons should move away from each other and the nucleus should fly apart. ♦But why doesn't this happen? →First, neutrons create a buffer that offsets the repelling interaction of protons. →Second, as you will see, another force inside the nucleus is working to hold the protons together.

So, why, again, doesn't a nucleus fly apart because of the protons opposition to each other?

♦The neutrons keep protons apart because if protons get too close, they will push each other away. →However, protons still need a force to keep them together... ♦QUARKS →the attraction between quarks holds protons together →This force of attraction is called the strong nuclear force. →The strong nuclear force is so strong that it goes beyond the proton and pulls on quarks inside other protons, keeping the atomic nucleus together. →This force that attracts quarks in adjacent protons is called a residual strong force.

How long does it take for a sample of an isotope to decay?

♦The time it takes for a specific amount of radioactive material to decay is measured in a unit called half-life (the plural is half-lives). ♦The half-life (t1/2) if a radioisotope is the amount of time required for one-half of the radioactive nuclei of the parent substance to decay into the daughter substance(s). ♣So if you start with 100% parent substance, you will have the following over time: →After one t1/2 - 50% parent remaining →After two t1/2 - 25% parent remaining →After three t1/2 - 12.5% parent remaining →After four t1/2 - 6.25% parent remaining (The half-life varies with different radioisotopes.)

Medieval alchemists:

♦Tried to change lead into gold: failed to do so ♦Their work led to many chemical discoveries and techniques, such as: →Distillation →Metalworking →Refining to produce dyes, paints, extracts liquors and other products

How long does it take for an isotope to decay?

♦When a radioisotope (a single atom) decays, the original isotope, called the parent isotope, changes into another isotope called the daughter isotope. ♦In a single atom, this change takes place almost instantaneously. ♦However, scientists are often interested in how long it takes for a sample of the isotope to decay.

Einstein:

♦While Albert Einstein was thinking about what it would be like to travel at the speed of light and why the speed of light was a sort of cosmic speed limit, he formulated his theory of special relativity. ♦In his theory, he realized that mass has energy even when it is not moving: →He derived an equation that related mass and energy: E=mc^2 →Where E is energy →m is mass in kilograms →c is the speed of light (3 x 10^8 m/s)

Particle accelerator:

♦a complex electromagnetic apparatus used to study the structure of the nucleus

Quarks:

♦fundamental subatomic particle that composes protons and neutrons; theoretically, six types of quarks exist and their existence has been demonstrated experimentally

Transmutation:

♦in nuclear chemistry, the change from one element to another

Radioactivity:

♦the decay of an unstable atomic nucleus (too many protons, not enough neutrons) followed by the release of radiation ♦The radiation can take the form of a particle or energy

Half-life:

♦the time needed for half of a sample of radioactive material to decay