NUCLEAR CHEMISTRY

The discovery of radiation

In 1896 Henri Becquerel made an important discovery. He accidentally had placed a piece of uranium ore on top of an unexposed photographic plate. Later, when the plate was developed, the image of the rock was found on the plate. Based on further experiments, he concluded that the plate had been exposed by rays given off by the uranium.

Madame Curie discovers Radium and Polonium

Following Becquerel’s discovery, Marie Sklodowska Curie and her husband, Pierre Curie, attempted to isolate the “radioactive” material from the uranium ore.In doing so they discovered two new elements, Radium and Polonium, both of which were more radioactive than the original ore.

Ernest RutherfordRutherford investigated this new property of matter and discovered that, in the process of emitting radiation, atoms of one element became atoms of another element.Today, we describe the process of an atom of one element becoming an atom of a different element as transmutation.Rutherford was the first to identify and name two different types of radiation given off when an atom of one element underwent transmutation and became an atom of another element. The two types of radiation he found were:

• The alpha particle (a)• The beta particle (b)

A third type of radiation that was discovered later is called:• Gamma radiation (g)

The nucleus is composed of nucleons

– protons

– neutrons

A nucleus is characterized by two numbers

– atomic mass number(A; total number of nucleons)

– atomic number (Z; number of protons)

The Nucleus

A nuclide that has 26 protons and 33 neutrons is used to study blood chemistry. Write its nuclide symbol in the form of

Because this nuclide has 26 protons and 33 neutrons, so atomic number, Z, is 26 and nucleon number A is 59 (26 protons +33 neutrons).

Isotopes

An isotope is an Atoms with the same number of protons, but differing numbers of neutrons. Isotopes are different forms of a single element.

Hydrogen Isotopes:1H protium most abundant isotope, nucleus consists of a single proton2H deuterium one neutron and one proton often given the symbol “D”. forms the hydrogen component of heavy water (D2O).

3H tritium one proton and 2 neutrons radioactive isotope: half-life of 12.3 year not found in nature.

Properties that depend on mass will be different for different isotopes of the same element:

Eg. boiling points of heavy water and conventional water are slightly different allowing them to be separated by fractional distillation.

Why does the atom break up?

Remember that the nucleus of the atom is held together by the strong nuclear force. This force is normally strong enough to hold the protons and neutrons together.

However, sometimes the force of repulsion due to the protons having the same charge overcomes the strong nuclear force and the atom breaks apart.

RADIOACTIVITY

Most naturally occurring isotopes of elements up to atomic number 19 have stable nuclei. Elements with higher atomic number (20-83) consist of a mixture isotopes, some of which may have unstable nuclei. When the nucleus of an isotope is unstable, it is radioactive, which means that it will spontaneously emit energy to become more stable. This energy, called radiation, may take form of particles such as alpha (α) particles or beta(β) particles or pure energy such as gamma (γ) rays. Elements with atomic numbers of 84 and higher consist only of radioactive isotopes. So many protons and neutrons are crowded together in their nuclei that the strong repulsions between the protons makes those nuclei unstable.

TYPES OF RADIATION

Alpha Decay:

Alpha particles are helium nuclei, containing two protons and two neutrons. They are deflected slightly in an electric of magnetic field. Their penetrating power is very low, being stoppable by a thin sheet of aluminum or paper.

Beta particles are electrons capable of travelling at speeds approaching the speed of light. Their low mass allows them to be deflected greatly in an electric or magnetic field, in the opposite direction as the deflection of alpha particles. Their high speed gives them greater penetrating power than alpha particles. Some beta particles can penetrate several centimetres of aluminum. Beta particle emissions change the composition of the nucleus.

n p + e-

Beta Decay:

A radioactive nucleus that undergoes positron emission has a proton in its nucleus convert into a neutron and an positron, then it ejects the positron. A positron is an anti-electron: it has the same mass, but has a +1 change instead of a -1 charge. The remaining nucleus has one less proton and one more neutron: the atomic number decreases by one and the mass number stays the same. n p + e+

Positron Emission:

Electron CaptureElectron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron, thereby changing a nuclear proton to a neutron and simultaneously causing the emission of an electron neutrino. Various photon emissions follow, as the energy of the atom to falls to the ground state of the new nuclide. e-+p →n

Gamma rays are high energy electromagnetic radiation with short wavelengths. Gamma rays, unlike alpha and beta particles, do not change the composition of the nuclide. They have the highest penetrating power, being able to penetrate at least 30 centimetres of lead.

Gamma Emission:

Penetrating Power of Radiation

• Alpha radiation is least penetrating and can penetrate the outer layer of skin. Alpha radiation is stopped by a sheet of paper.

• Beta radiation can penetrate through a few cm of skin and tissue. Beta radiation is stopped by a sheet of aluminum foil.

• Gamma radiation will pass right through a body. Gamma radiation requires several cm of lead to stop.

Penetrating Power of Radiation

Mode of radioactive decay:

Nuclear Equations:

Nuclear Changes:

Write nuclear equations for (a) alpha emission by polonium-210, used in radiation therapy, (b) beta emission by gold-198, used to assess kidney activity, (c) positron emission by nitrogen-13, used in making brain, heart, and liver images, and (d) electron capture by gallium-67, used to do whole body scans for tumors.

Solution:

A

B

C

D

Because the different radioactive nuclides have different stabilities, the rates at which they decay differ as well. These rates are described in terms of a nuclide’s half-life,The half-life is defined as the time that it takes for one half of a sample of a radioactive element to decay into another element.

Rates of Radioactive Decay

2

For example, radioactive carbon-14, which decays to form nitrogen-14 by emitting a beta particle, has a half-life of 5730 years. After 5730 years, one-half of a sample remains, and one-half has become nitrogen-14. After 11,460 years (two half-lives), half of that remainder will have decayed to form nitrogen-14, bringing the sample down to one-fourth of its original amount. After 17,190 years (three half-lives), half of what remained after 11,460 years will have decayed to form nitrogen-14, so one-eighth of the original sample will remain. This continues, with one-half of the sample decaying each half-life.

Here's a look at the effect of different doses of radiation on the human body after acute, whole-body exposure. RAD - radiation absorbed dose - is the amount of radiation that bombards a body.

Over 2,000 RAD: Death is a certainty.

At doses above 5,000 RAD, the central nervous system (brain and muscles) can no longer control the body functions, including breathing and blood circulation. Everything happens very quickly. Death occurs within days or hours. Nothing can be done, and medical care is for comfort only.

1,000 to 2,000 RAD:

Probability of death increases to 100%within one to two weeks. The initial symptoms appear immediately. Within a few days later the body breaks down very quickly since the gastrointestinal system is destroyed. Once the GI system ceases to function, nothing can be done, and medical care is for comfort only.

150 to 1,100 RAD:

Severe blood changes will be noted and symptoms appear immediately. Approximately two weeks later, some exposed casualties may die. At 300-500 RAD, up to one half of the people exposed will die within 30 days without intensive medical attention. Death is due to the destruction of the blood forming organs. Without white blood cells, infection is likely. At the lower end of the dose range, isolation, antibiotics, and transfusions may provide the bone marrow with time to generate new blood cells and full recovery is possible. At the upper end of the dose range, a bone marrow transplant may be required to produce new blood cells.

50 to 150 RAD:

Slight blood changes including temporary drop in production of new blood cells will be noted and likely symptoms of nausea, fatigue and vomiting for one or two days.

5 to 50 RAD:

Slight blood changes may be detected by medical evaluation.

Less than 5 RAD: No immediate observable effects.

Ionizing radiation has two very different uses in medicine — for diagnosis and therapy. Both are intended to benefit patients and, as with any use of radiation, the benefit must outweigh the risk.

Medical Uses

Radiation Therapy: Nuclear radiation can be used to kill cancerous cells because cancerous cells are more susceptible to radiation than healthy ones. Radiation damages rapidly dividing cancer cells because they are sensitive to and easily damaged by radiation. Radiation is aimed at the cancerous tissue.

Patients undergoing radiation therapy often experience nausea and vomiting, which are early signs of radiation sickness.

Radioisotope therapy is a procedure in which a liquid form of radiation is administered internally through infusion or injection. RIT's ultimate purpose is to treat cancerous cells with minimal damage to the normal surrounding tissue.

Common Radioisotopes in Treating Cancer

Diagnostic Uses of Radiation:

Positron Emission Tomography (PET)Computer Tomotgraphy (CT)

CT scan● Computed tomography (CT) scan is an

imaging method that uses x-rays to create pictures of cross-sections of the body.

Positron Emission Tomography (PET): A patient inhales or is injected with positron-emitting isotopes such as carbon-11 or oxygen-15. When positrons encounter electrons, they emit two gamma rays, which exit the body in opposite directions. PET scans can be used to image dynamic processes.

A PET scan (positron emission tomography) looks at the biologic activity of a tumor, not the shape and size of a tumor. A CT scan give anatomic information like exact location, size, and shape of a tumor with its relationship to other structures. Nowodays, this twoo methods are a combined to one PET/CT scanner in so that it is obtain both the biologic activity overlayed with the anatomic structure. This gives us more information. Most institutions have moved to combined PET/CT to get the best information.

In a typical nuclear fission process, a neutron collides with a large atom, such as uranium-235, and forms a much less stable nuclide that spontaneously decomposes into two medium sized atoms and 2 or 3 neutrons.

neutron +large nuclide →unstable nuclide

unstable nuclide →2 medium sized nuclides +2 or 3 neutrons

Nuclear Fission

The nuclides produced in the reaction pictured above are only two of many possible fission products of uranium-235. More than 200 different nuclides form, representing 35 different elements. Two possible reactions are:

Nuclear reactions such as these are used to power electrical generating plants.

The reason the fission of uranium-235 can generate a lot of energy in a short period of time is that under the right circumstances, it can initiate a chain reaction, a process in which one of the products of a reaction initiates another identical reaction. In the fission of uranium-235, one or more of the neutrons formed in the reaction can collide with another uranium-235 atom and cause it to fission too

A chain reaction is a series of reactions where the products contribute to the reactants of another reaction without outside influence.

A nuclear chain reaction is a fission reaction where the neutrons generated by the fission process go on and initiate fission in other atoms.

Uranium-235 Chain Reaction

The nuclear reactor in a nuclear power plant is really just a big furnace whose job is to generate heat and thus convert liquid water to steam in order to turn a steam turbine generator that produces electricity. The electricity-generating portion of a nuclear power plant is typically no different than the electricity-generating portion of a plant that generates heat from burning fossil fuels.

Nuclear power plant

Generating Electricity from Nuclear Power