CHAPTER 19

CHAPTER 19

NUCLEAR CHEMISTRY

Within the nucleus of the atom is a tremendous amount of energy. The release of this energy can range from the barely noticeable to the catastrophic.

Discovery of Radioactivity

This discovery of radioactivity belongs to Antone Henri Becquerel who showed that particles emitted from uranium salts could pass through paper to expose photographic film in the absence of light.

Marie Curie coined the term ‘Radioactivity’ for the spontaneous emission of particles or rays from the nucleus of atoms. In 1898 Marie Curie and her husband Pierre Curie discovered two radioactive elements; polonium and radium.

In 1899 Ernest Rutherford discovered that two different types of rays emitted from the nucleus, alpha and beta rays. Alpha rays consisted of streams of helium nuclei where beta rays consisted of streams of electrons. The third type of nuclear emission, gamma rays, was discovered by Paul Villard in 1900.

Isotopic Notation

Isotopic notation describes the nuclear composition of a particular isotope of an element. For example:

^{Mass number ---> 1₆⁴C_{<------atomic number}

In the nucleus of carbon-14 there are 6 protons and 8 neutrons. Neutrons and protons are called nucleons. Carbon-14 is a particular nuclide of carbon.

Natural Radioactivity

Radioactive nuclides disintegrate spontaneously over time. The length of time required for 1/2 of the atoms to disintegrate is called the half-life. For instance 1,000,000 carbon-14 atoms will disintegrate to about 500,000 carbon-14 atoms in 5668 years.

Radioactive Decay Processes

When a nucleus decays all the conservation laws must be observed-conservation of mass, charge and energy. The sum of charges and particles will be the same on each side of the equation.

Consider the decay of the uranium-235 nuclei:

^{2 3}₉⁵₂U ---> ^{2 3}₉¹₀Th + ⁴₂He
where ⁴₂He is an alpha (a) particle.

Complete the following equation for the a disintegration of thorium-231.
^{2 3₉¹₀Th ---> + a

Consider the following b emission example:

^{2 3₉⁴₀Th ---> ^{2 3}₉⁴₁Pa + _-1^oe

where _-1^oe is a beta (b) particle or simply an electron.

Complete the following equation for the b decay of ^{2 3}₉⁴₁Pa
^{2 3₉⁴₁Pa ---> + b
There are two ways for gamma rays to be produced in nuclear reactions: electron capture and positron emission.

Example: Electron capture usually occurs with heavy nuclei having too many protons to be stable.
^{2 0}₈³₂Pb + _-1^oe ---> ^{2 0}₈³1Tl + 2g

Where g is a gamma photon of high energy light given off when an electron cancels the charge on a proton.

Complete the following equation for the electron capture of mercury-197
^{1 9₈⁷₀Hg + _-1^oe --->

Example: Positron emission usually occurs with light elements having too many protons to be stable.
²₁⁰₁Na ---> ²₁⁰₀Ne + ₊⁰₁e
Where ₊⁰₁e is a positron or positive electron.

Show the positron emission of Magnesium-23

Measurement of Radioactivity: Radiation units

Curie (Ci) 1Ci = 3.7x10¹⁰ disintegrations /sec which is how much
1 gram of radium gives off.
Roentgen (R) 1R = 2.1x10⁹ ions of air molecules/cm³ of air
Rad Radiation Absorbed Dose which is a measure of the
amount of energy absorbed/kg of matter
1 rad = .01 J/kg
Rem Roentgen Equivalent to Man. 1(0) rem= 1rad x factor where the factor is 1 for
beta and gamma radiation and alpha a factor of 10 determine by the biological
harm caused by the particular type of radiation.

Nuclear Fission

The fat man and little boy atomic bombs dropped of Hiroshima and Nagasaki were the only two nuclear weapons used in any war. In less than 10 seconds 4.5 square miles of Hiroshima was destroyed and 70,000 people died and as many were injured. 35,000 died in the explosion over Nagasaki. The enormous amount of energy released is the result of a conversion of a tiny fraction of mass into energy. When uranium -235 is struck by a neutron it splits into two fragments such as krypton and barium and releases 2 neutrons which in turn strike two uranium atoms which release 2 neutrons each for a total of 4 neutrons. Those 4 neutrons produce 8 -> 16 -> 32 -> 64 -> 128 -> 256 -> 512 -> 1024 -> 2048 -> 4096 -> 8192 etc.

So long as the neutrons can escape as fast as they are produced everything is fine, and you have a subcritical mass of uranium. But if the rate of production of neutrons becomes larger than the rate at which the neutrons can escape, a nuclear explosion can result.

Nuclear Power

Electricity is nearly always produced by boiling water to turn a turbine (exceptions are hydroelectric and solar voltaic cells). Coal, oil or nuclear fission can be used to boil the water. A nuclear reactor is simply the source of heat to boil water to turn turbines which turn generators to produce electricity.

Nuclear Fusion

Fusion is the putting together of nuclei to produce heavier nuclei. Nuclear fusion takes place naturally in stars and in our sun. It is quite interesting to consider that every element on earth was once formed in the center of a star billions of years ago and thrown out into space when that star exploded as a nova or super nova.

Fusion occurs most readily between light elements such as hydrogen or helium.

³₁H + ²₁H ---> ⁴₂He + ¹₀n + energy

³₁H + ¹₁H ---> ⁴₂He + energy

This source of energy holds great promise as a non polluting source of energy from sea water... the source of tritium and deuterium.

Mass-energy Relationship in Nuclear Reaction

When nuclear reactions occur a small amount of the mass is converted into energy. Consider the reaction between tritium and hydrogen

³₁H + ¹₁H ---> ⁴₂He + energy
3.01495amu 1.00782amu 4.00260amu
The total mass of the left hand side is 4.02277amu. The loss of mass is a mere
.02017 amu. However as we shall see this is not a small amount.

Problem:
Calculate the energy released when 1 mole of tritium fuses with 1 mole of protium. Hint: Use MKS units.

Mass Defect
The nuclei ⁴₂He consists of 2 protons and 2 neutrons. The total mass of 2 protons and 2 neutrons is slightly greater than the mass of one helium nuclei!! Where did the excess mass go? The excess mass was released as energy when the helium nuclei was formed and it is the energy that is responsible for holding the positive charges of the nuclei together. This energy is called the binding energy.

Problem :
Calculate the binding energy for Helium. Look up the mass of 2 protons and 2 neutrons and the mass of the helium-4 nuclei. Take the difference and convert to mks units. Use E=mc².

Biological Effects of Radiation

X-ray or gamma radiation is damaging to the genetic material within the nucleus of living cells. The damage is most severe to cells which are rapidly dividing such as cancer cells. By far the most damaging radiation is alpha radiation. This is due to the fact that alpha particles are very massive and can inflict greater damage to individual cells; however, alpha particles are not very penetrating because of their size and maximum damage can only occur if the alpha source is ingested or inhaled. Gamma or x-ray radiation does much less damage than alpha; however, it is extremely penetrating and can cause damage deep within the body from outside the body. Long term exposure to low level radiation shortens the average life span of man kind.

Application of Nuclear Chemistry
Tracers for following a chemical process.

Dating organic and inorganic materials.

Diagnosis of thyroid with ¹³¹I.

Pet scan by positron emission of C-11, O-15 and N-13.

Radiation therapy and Chemotherapy}}}}}