Radioactive isotopes of lead, uranium, and thorium
Isotopes of the radioactive elements like uranium, thorium or lead are the same element with different atomic weights. These elements are identical chemical properties because these isotopes are identical in electronic structure.
Thus isotopes of a particular element have the same number of protons but different numbers of neutrons inside the nucleus of an atom.
Emission of radioactive particles
When an alpha particle emitted within the nucleus the mother element loses two units of atomic number and four units of mass number.
Let a radioactive element,
mass number= M
Atomic number = Z
Then the new element,
mass number = (M – 4)
atomic number = (Z – 2)
But when beta particle emits from the nucleus, the atomic number of the element one unit greater than the mother element.
Thus when an element ejected a beta particle mass number of the element has the same but atomic number = (Z + 1).
88Ra226 → 88-2Rn226-4 + 2He4(α)
90Th234 → 91Pa234 + 1e0(β)
Radioactive decay series equation
Uranium is the first discovered radioactive elements. Uranium in the 6th group in the periodic table with atomic number 92 and mass number 238. Thus it undergoes successive decay till the daughter elements become the most stable isotopes of lead.
The mother element along with all the daughter elements down to the most stable isotope of lead is called a radioactive decay series.
Radioactive isotopes of uranium-238
Uranium-238 decay ultimately to a stable isotope of lead. The entire route involves eight alpha and six beta radiation.
All the above decay products are given by (4n +2) where n = mass number of radioisotopes.
Uranium-235 or (4n+3) series starts with uranium – 235 and ends with lead – 207. Thus seven alpha and four beta radiate in the entire route of emission. So the overall process is
Thorium 232 decay chain
Thorium-232 undergoes successive decay to form the most stable radioactive isotopes of lead with mass number 208.
So the entire route involves six alpha and four beta emission. This is the 4n radioactive decay series.
Group displacement law
In 1913 Soddy proposed the law for the position of radioisotopes in the periodic table. But this time the knowledge of the structure of an atom was still incomplete.
So the decay products of different series predicted this law.
When an alpha particle emitted in a radioactive decay step, the product displaced two places to the left in the periodic table.
But the radiation of beta particles results in a displacement of the product to one place to the right.
Soddy observed that more than one product belonging to the same group on the periodic table.
He further established that the product belonging to the same group had identical chemical properties but their radioactivity was different.
Thus Soddy used the term isotope for such elements occupying the same place on the periodic table.
But a study of the displacement law further reveals when an element emits one alpha and two beta particles. Thus the product isotope occupies the same group in the periodic table with the parent element.
Uses of radioactive isotopes in various fields
We used radioactive isotopes in many filed like medicine, biology, agriculture, trace analysis. Thus the uses of radioisotopes broadly classified under the heads.
Uses of radioactive isotopes in medicine
We use radioactive iodine-131 for thyroid gland disorder. Thus when we drink a solution of sodium iodide containing sodium-131.
And radioactive iodine moves to the thyroid gland. So the radiation or beta emission destroys the malignant cells without affecting the rest of the body.
Radioactive isotopes for cancer treatment
Cobalt-60 is a good gamma rays emitter. Thus cobalt-60 is used to inhibit the growth of malignant tissue for the treatment of cancer.
For the abnormality of the circulation of blood, we use a small amount of a sodium chloride solution labeled with sodium-24. Thus sodium chloride solution injected into a vein of the patient.