### What is meant by radioactive isotopes?

Radioactive isotopes of an element are atoms of the same element with different atomic weights. It posts identical chemical properties because of their identical electronic structure. Isotopes of a particular element have the same number of protons but varying the number of neutrons inside the nucleus of an atom.

1. Alpha particle emits from within the nucleus the mother element loss two units of atomic number and four units of mass number.
If a radioactive element with mass number M and atomic number Z ejected an alpha particle. The newborn element has mass number = (M – 4) and atomic number = (Z – 2).
2. Beta particle emits from the nucleus, the daughter element nucleus has an atomic number one unit greater than that of the mother element nucleus.

If a radioactive element with mass number M and atomic number Z ejected a beta particle the newborn element has a mass number the same and atomic number = (Z + 1).

$_{88}^{226}\textrm{Ra}\rightarrow&space;_{88-2}^{226-4}\textrm{Rn}+_{2}^{4}\textrm{He}\left&space;(&space;\alpha&space;\right&space;)$

$_{90}^{234}\textrm{Th}\rightarrow&space;_{91}^{234}\textrm{Pa}+_{1}^{0}\textrm{e}\left&space;(&space;\beta&space;\right&space;)$

#### What is the most stable radioactive isotope?

Uranium is the first discovered radioactive elements. It is in the 6th group or actinides in the periodic table with atomic number 92 and mass number 238. Uranium undergoes successive disintegration 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.

#### Why are U-235 and U-238 different isotopes?

Uranium – 238 disintegrate ultimately to an isotope of lead. The entire route involves eight alpha and six beta radiation to form the isotopes of lead with atomic number 82 and mass number 206.

$_{92}^{238}\textrm{U}\rightarrow&space;_{82}^{206}\textrm{Pb}+8\alpha&space;+6\beta$

All the above disintegration products are given by (4n +2) where n = mass number of radioactive isotopes.

Uranium – 235 or (4n+3) series starts with uranium – 235 and ends with the most stable isotope of lead – 207. Seven alpha and four beta radiation in the entire route of the overall process,

$_{92}^{235}\textrm{U}\rightarrow&space;_{82}^{207}\textrm{Pb}+7\alpha&space;+4\beta$

#### Thorium 232 decay chain

Thorium – 232 undergoes successive disintegration till the daughter elements become the most stable radioactive isotopes of lead with mass number 208.

The entire route involves six alpha and four beta emission called the thorium- 232 disintegration series or 4n series.

$_{90}^{234}\textrm{Th}\rightarrow&space;_{82}^{208}\textrm{Pb}+6\alpha&space;+4\beta$

#### Group displacement law in radioactivity

In 1913 Soddy proposed the law for the position of radioisotopes in the periodic table when the knowledge of atomic structure was still incomplete.

But the disintegration products of different series preceded the announcement of this law.

When an alpha particle is emitted in a radioactive disintegration step, the product is 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 elements belonging to the same group had identical chemical properties though their radioactivity’s were different.

Soddy coined the term isotope for such elements occupying the same place on the periodic table.

$_{90}^{234}\textrm{Th}\overset{\alpha&space;}{\rightarrow}&space;_{88}^{228}\textrm{Ra}&space;\overset{\beta&space;}{\rightarrow}&space;_{89}^{228}\textrm{Ac}&space;\overset{\beta&space;}{\rightarrow}_{90}^{228}\textrm{Th}$

$_{92}^{238}\textrm{Th}\overset{\alpha&space;}{\rightarrow}&space;_{90}^{234}\textrm{Th}&space;\overset{\beta&space;}{\rightarrow}&space;_{91}^{234}\textrm{Pa}&space;\overset{\beta&space;}{\rightarrow}_{92}^{234}\textrm{U}$

A study of the displacement law further reveals that when a parent radio element emits one alpha and two beta particles successively the product isotope occupies the same group in the periodic table with the parent element.

Variety filed like medicine, biology, agriculture, trace analysis, and many other fields we used radioactive isotopes. The uses of isotopes broadly classified under the heads.

#### Uses of isotopes in medicine

For the treatment of thyroid gland disorder, we can use radioactive iodine-131. After drinking a solution of sodium iodide containing sodium-131.

The radioactive iodine moves preferentially to the thyroid gland. The radiation or beta emission destroys the malignant cells without affecting the rest of the body.

#### Isotopes in cancer treatment

Cobalt-60 uses as a good gamma-ray emitter. Thus cobalt-60 used to inhibit the growth of malignant tissue in the treatment of cancer.

Abnormality of the circulation of blood, a small amount of a sodium chloride solution labeled with sodium-24, a beta emitter used. Thus sodium chloride solution injected into a vein of the patient.

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