Chapter 21: Nuclear Chemistry

The study of nuclear reactions with an emphasis on their uses in chemistry

and their effects on biological systems.

Reactivity• In nuclear reactions, the nuclei of unstable isotopes,

called radioisotopes, gain stability by undergoing changes accompanied by the emission of large amounts of energy.

• The process by which materials give off such energy, in the form of waves (rays), is called radioactivity.

• Waves and particles emitted are called radiation.• Types of Radioactive Decay

– Beta: an electron ejected from or captured by the nucleus.– Positron: positively charged particle of same mass as an

electron ejected from the nucleus– Neutron: particle given off during fission process– Alpha: Helium (He) nucleus with no electrons– Gamma: High energy wave (no mass/no charge).

• Sample Exercises 21.1 and 21.2 pg 896 and 897.

Patterns of Nuclear Stability• The stable nuclei on a

neutron-versus-proton plot are located in a region called the band of stability. Unstable nuclei undergo spontaneous radioactive decay. The type of decay that occurs depends on the neutron-to-proton ratio of the unstable nucleus.

Nuclear Transmutations• Naturally occurring

– Have already talked about several• Radioactive Decay

– Another is the production of N-14 from naturally occurring C-14 (half life of 5715 years)

• Earliest artificial transmutation was performed in 1919 by Ernest Rutherford by bombarding nitrogen gas with alpha particles.

• Elements with Atomic Number above 92, the transuranium elements, all undergo transmutations. None of them occurs in nature. These elements have been synthesized in nuclear reactors and accelerators.

Rate of Radioactive Decay• Every radioisotope has a characteristic rate of decay

measured by its half-life• A half-life is the time required for one-half of the nuclei of a

radioisotope sample to decay to products.– After one half-life, half of the original radioactive atoms have decayed into

atoms of a new element.– How many are left after two half-lives?

• Half-life equation: Activity final = Activity initial (1/2) time passed/half-life

• Half-lives may be as short as a fraction of a second or as long as billions of years.

• Many artificially produced radioisotopes have very short half-lives, a feature that is a great advantage in nuclear medicine.

– The rapidly decaying isotopes do not pose long-term biological radiation hazards to patients.

• When radioisotopes decay, they may decay to another element that is also unstable

• These elements that are unstable have half-lives of their own.– Elements that are unstable and undergo further decay until a stable nucleus configuration

is reached are called radioactive intermediates.

Detection of Radioactivity• Ionizing radiation: radiation with enough energy to knock

electrons off some atoms of the bombarded substance to produce ions. Ionizing radiation penetrates a thin window at end of detector. Gas becomes ionized, free electrons are produced. Each time this occurs, current flows. Current flows drive a counter or cause and audible “click”– One such device, a Geiger-counter, uses a gas-filled metal tube to detect

radiation.• Geiger-counters are used primarily to detect beta/positron and gamma radiation.

• Neutron Detectors: The fast neutrons from fission are slowed down (thermalized) by the material that surrounds the detector. The thermal neutrons then interact with a material in the detector tube that has a high cross section for absorption. After the neutron is captured, free electrons are given off. The gas becomes a conductor and causes current flow through the detector. The current drives a counter or causes an audible “click”.

• Alpha Radiation: A scintillation counter uses a specially coated phosphor surface to detect radiation. Ionizing radiation striking the phosphor surface causes flashes of light. The number of flashes are detected electronically, converted into electronic pulses, then measured and recorded. Similar to what takes place inside some television tubes that are coated with phosphor on the inside.

• Film Badges and Dosimeters: Film badges consist of several layers of photographic film which darken when exposed to radiation

– Workers wear the badges the entire time at work, the badges are “developed” at regular intervals to monitor the worker’s exposure to ionizing radiation

– Dosimeters are hand held devices that are designed for short duration use only. They consist of a filament that has been charged and is calibrated such that at full charge the dosimeter indicates zero exposure. As the worker is exposed to ionizing radiation the charge in the dosimeter is reduced and the filament moves across a scale indicating how much ionizing radiation has been received by the worker.

Nuclear Power: Fission and Fusion• When the nuclei of certain isotopes are bombarded

with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments.– Isotopes – What is the difference between U – 238 and U –

235?

– An example of U – 235 fission

• Fusion occurs when nuclei combine to produce a nucleus of greater mass.– In solar fusion, hydrogen nuclei (protons) fuse to make helium

nuclei.

– An example, shows that the reaction also requires two beta particles. What is a beta particle?

Nuclear Reactor

Two Steps Involved in Nuclear Reactors• Step 1 – Neutron Moderation

– Neutrons produced from fission move so fast they will pass right through a nucleus without being absorbed.

– Water and carbon (graphite) are good moderators because they slow the neutrons (close to elastic collisions) so the chain reaction can be sustained.

• Step 2 – Neutron Absorption– To prevent the reaction from going too fast some of the slowed

neutrons must be trapped before they hit fissionable atoms.– Carried out by control rods made of materials such as Cadmium.– Some unintended absorbers are created by the fission process and

impact reactor operation (Xenon).• Despite other dangers, a nuclear reactor cannot produce a

nuclear explosion. The fuel elements are widely separated and cannot physically connect to produce the critical mass required. Once a nuclear reactor is started, however, it remains highly radioactive for many generations (nuclear waste discussion today, half- life discussion on Friday).