Chapter 20

Nuclear Chemistry

In chemical reaction, only the outer electrons of the atoms are disturbed and the

nuclei of the atoms are not affected.

Two types of nuclear reactions:

1. Radioactive decay - the process in which a nucleus spontaneously disintegrates,

giving off radiation.

2. Nuclear Bombardment reaction - a nuclear reaction in which a nucleus is

bombarded, or struck, by another nucleus or by a nuclear particle.

Radioactivity

Writing an equation for the nuclear reaction corresponding to the decay of

uranium-238 just as you would an equation for a chemical reaction.

Alpha rays consist of a helium -4 nuclei as seen in the uranium example.

Beta rays consist of high speed electrons.

Gamma rays are similar to x rays except they are higher in energy.

Nucleon is a collective name for two particles: a proton and a neutron

The nuclear force is a strong force of attraction between nucleons that act only at a

very short distances ( about 10-15

m).

The shell model of the nucleus is a model in which protons and neutrons exist in

levels, or shells, analogous to the shell structure that exists for electrons in an atom.

Certain numbers of protons or neutrons appear to be more stable than others, the

numbers of interest are called magic numbers - the number of nuclear particles in

a completed shell of protons and neutrons

For protons the magic numbers are: 2, 8, 20, 28, 50 and 82.

For neutrons they are: 2, 8, 20, 28, 50, 82 and 126.

Types of Radioactive Emission

1. Alpha emission

γphoton Gamma

βor ePositron

βor eElectron

nNeutron

Por HProton

0

0

0

1

0

1

0

1-

0

1-

1

0

11

11

2. Beta emission

3. Positron emission

4. Electron capture

5. Gamma emission

6. Spontaneous Fission

All nuclei with an atomic number greater than Z= 83 are radioactive. Many decay

by alpha emission. Alpha particles are, or �

� He nuclei are especially stable.

Naturally radioactive elements, such as uranium-238, give a radioactive decay

series until it reaches a stable nucleus, which is an isotope of lead.

Transmutation is the change of one element to another by bombarding the

nucleus of the element with nuclear particles or nuclei.

Transuranium elements are elements with atomic numbers greater than that of

uranium. (Z = 92), the naturally occurring element of greatest Z.

Transuranium elements have a number of commercial uses.

Example: Plutonium-238 and Americium-241

Biological Effects and Radiation Dosage

To monitor the effect of nuclear radiations on biological tissue, it is necessary to

have a measure of radiation dosage.

The rad - radiation absorbed dose, is the dosage of radiation that deposits 1 × 10-2

J

of energy per kilogram of tissue.

The biological effect of radiation depends on not only the energy deposited, but the

type of radiation.

Neutrons are more destructive than gamma rays of the same dosage measured in

rads.

Rem - unit of radiation dosage used to regulate various kinds of radiation in terms

of biological destruction.

Rem = rad × RBE ( relative biological effectiveness)

Rate of radioactive decay is the number of nuclei disintegrating per unit time

Rate = k Nt

Where Nt = number of radioactive nuclei at time t, and k = the radioactive decay

constant.

Example: A 1.0 mg sample of technetium-99 has an activity of 1.7 × 10-5

Ci,

decaying by beta emission. What is the decay constant for ��

�� Tc?

The half life of a radioactive nucleus is the time it takes for one half of the nuclei

in a sample to decay. The half life is independent of the amount of the sample.

��

�

= �.��

�

Example: The decay constant, k, for the beta decay of ��

��Tc was found to be 1.0

× 10-13

/s. What is the half life of this isotope in years?

Radioactive Dating - because the rate of radioactive decay of a nuclide is constant,

this can serve as a clock for dating very old rocks and human implements. Once

you know the decay constant for a radioactive isotope, you can calculate the

fraction of the radioactive nuclei that remains after a given period of time by the

following equation

ln �

�

= -kt

Carbon -14 Dating Method Example

A piece of charcoal from a tree killed by the volcanic eruption that formed the crater in Crater

Lake, Oregon, gave 7.0 disintegrations of carbon-14 nuclei per minute per gram of total carbon.

Present day carbon gives 15.3 disintegrations per minute per gram carbon. Determine the date of

the volcanic eruption. Recall that the half life of carbon-14 is 5730 y.

Nuclear Power Plants and Nuclear Fission and Fusion

Nuclear reactions, like chemical reactions, involve changes in energy. The changes

in nuclear reactions are enormous by comparison with those in chemical reactions.

Nuclear fission and fusion are reactions in which nuclei attain sizes closer to an

intermediate range (approx mass number s around 50). Nuclear fission of uranium-

235 is employed in nuclear power plants to generate electricity.

Critical mass is the smallest mass of fissionable material in which a chain reaction

can be sustained.

A Nuclear fission reactor is a device that permits a controlled chain reaction of

nuclear fissions. A power plant with a nuclear reactor is used to produce heat,

which is then used to produce steam to drive an electric generator.

Fuel rods are the cylinders that contain fissionable material.

Control rods are cylinders composed of substances that absorb neutrons, such as

boron and cadmium, and can therefore slow the chain reaction.

Fission of uranium nuclei produce

approximately 30 different elements

of intermediate mass.

When the uranium-235 nuclei splits,

approx 2 or 3 neutrons are released.

If the neutrons from each nuclear

fission are absorbed by other

uranium-235 nuclei, these nuclei split

and release even more neutrons. In

this way, a chain reaction can occur.

Light water nuclear reactor consists of fuel rods with interspersed control rods.