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An atom is made up of three fundamental subatomic particles protons, neutrons and electronsOut of these three particles, protons and neutrons located at the center of the atom as a hard and dense part known as nucleus. The rest of the part of atom contains negatively charged particles called as electron which balance out the charge of the protons and make the atom electrically neutral.

The total mass of an atom accumulate at the center of atom in the form of nucleus as the mass of electrons is negligible. Hence, the sum of total number of protons and neutrons is called as mass number

There must be some nuclear force which maintains the existence of nucleus, because there is a repulsion force between positively charged proton which are collected in a small region of nucleus. If the number of proton is less in an atom, other forces can hold the protons together and atom becomes stables. But as the ratio of protons to neutrons is increases, protons cannot be held firmly together and hence form an unstable nucleus.

Atoms become unstable due to large neutron to proton ratio. Such unstable nucleus emitted some radiations and convert in to some other stable nucleus and known as radioactive elements. These radiations are termed as radioactive rays. Generally these radiations consist some particles like alpha and beta particle in some time charge less gamma rays emitted.

• For convenience, Rutherford called the three types of radiation alpha ($\alpha$), beta ($\beta$) and gamma ($\gamma$) rays. The alpha-rays were deflected in a direction opposite to that of beta-rays.
• This showed that the $\alpha$-rays carried a positive charge, $\beta$-rays carried a negative charge and those which passed un deviated were neutral or uncharged were $\gamma$-rays.
• Similarly, if the radiations given out by a radioactive substance are subjected to an electric field perpendicular to their path, they separate into three constituents.
• Those which turn towards the negative plate are the positively charged alpha particles.
• Those, which turn towards the positive plate, are the negatively charged beta particles.
• Those, which pass un deviated, are the uncharged gamma radiations.
• Further investigation has shown that an alpha ray is a stream of helium nuclei, a beta ray is a stream of electrons and a gamma ray is an electromagnetic radiation whose frequency is higher than that of X-rays.

Generally, elements with atomic number more than 82 show radioactivity and disintegrated to small nuclei with the emission of alpha, beta, proton, neutron particles or gamma rays. This nuclei with decomposed is called as parent nuclei and the product nuclei is termed as daughter nuclei.

The atomic number and mass depends upon the type of radioactive rays emitted during nuclear reaction. The decay of radioactive parent nuclei to stable nuclei is known as radioactive decay or nuclear decay.

The type of decay depends on the type of radioactive particles emitted in decay. For example,
1. Alpha decay
2. Beta decay
3. Gamma decay

On 22 December, 1895; Wilhelm Conrad Roentgen discovered Roentgen rays, later termed as X-rays. In 1896, A French physicist, Henri Becquerel who was aware about the discovery of X-rays, found that a photographic plate wrapped in a thick black paper which had been placed by chance in the same drawer which contained uranium salts, had become affected or fogged.

He concludes that uranium slats emitted some radiation like X-rays which can causes ionization in air. He called these rays as radioactive rays and this property as radioactivity.

The husband and wife team of Pierre and Marie Curie were aware about the Henri Becquerel experiment and study with their own uranium-containing ores. They proposed the term "radioactivity" which was the spontaneous emissions of radiation from certain elements. These elements called as radioactive elements

They studied some radioactive components like uranium and discovered two new radioactive elements which they named polonium and radium. The next important step on the road of discovery of radioactivity was taken by Ernest Rutherford in 1904. He established the concept that the rays emitted by the radioactive substance are of three types designated as alpha, beta and gamma rays. He purposed the concept of nucleus in an atom by using Gold foil experiment and interpreted that an atom was made up of mostly empty space with a small dense portion at center which deflected the positively charged alpha particles.

The unit of radioactivity called as curie (ci). One curie defined as "the amount of a radioactive substance which has a decay rate of 3.7 x 1010 disintegration's per second." This large number is based on the observation that one gram of radium disintegration at the rate of 3.7 x 1010 disintegration per second.

The non SI unit of radioactivity is defined as curie which is equals to amount of a radioactive substance which has a decay rate of 3.7 x 1010 disintegration per second given after the study of radium isotope 226Ra by the Curies.

1 Ci = 3.7 × 1010 decays / second

The SI unit of radioactivity is called as becquerel (Bq), which is equals to one decay per second.

1 becquerel = 1 radioactive decay / second = 2.703 x 10-11 Curie
1 Curie = 3.7 × 1010 Becquerel = 37 Giga Becquerel
1 Bq ≅ 2.703 × 10−11 Ci

For small measurements; millicurie (mci) and microcurie(μci) is used.

1mci = 3.7 × 107 disintegration/sec
1 μci = 3.7 × 104 disintegration/sec or 2.22 × 106 disintegrations per minute

The relation between curie and becquerel (Bq) with decay constant is as follows.

N (atoms) * λ (1/s) = 1 Ci = 3.7 × 1010 (Bq)

Where,
λ = Decay constant in (1/s)
N = Amount of radioactive substance

The unstable radioactive elements show nuclear decay which is a spontaneous cleavage of an atomic nucleus with the formation of some other nucleus and release a certain amount of energy. Since, radioisotopes have unstable nuclei which do not have enough binding energy to hold the nucleus together and constantly try to get stabilize.

This process of transformation of one unstable nucleus to stable nuclei by the emission of energy in the form of radiation is called as transmutation. The nuclear decay and transmutation process will continue until a new formed element also called as daughter nuclei has a stable nucleus and not radioactive in nature.

The radioactive decay or transmutation can occur naturally or by artificial means. On this basis, transmutation classified as two types.

The atoms of radioactive elements on the emission of alpha particles and beta particles would change into atoms of another element. This change is spontaneous and occurs due to instability of heavy nuclei. Such type of radioactive decay termed as natural radioactive decay and phenomenon called as natural radioactivity.

Rutherford was first to observed the decay of radon by the loss of alpha and beta particles by the preceding elements. Natural radioactive elements decay naturally without any external effect until the convert in stable nuclei.

For example; Uranium-238 disintegrated in to Thorium-234 by the emission of alpha particle which further changes in Protactinium-234 by the loss of a beta particle and anti-neutrino.

92U238 $\rightarrow$ 90Th234 + 2He4
90Th234
$\rightarrow$ 91Pa234 + -1e0+ Ï…

The emission of an alpha particle results in the formation of an element which lie two place to the left in periodic table and the emission of beta particle results in the formation of an element which lies one place to the right. This is called as group displacement law. Natural radioactivity series.

The series of continued disintegration of radioactive elements is known as radioactive series. There are total four successive radioactive serieson the basis of starting element name.
• The Uranium series
• The Thorium series
• The Actinium series
• The Neptunium series

The uranium series starts from uranium-23 and finally converts to lead-206 by the emission of eight alpha particles and six beta particles.

92U238 $\rightarrow$ 82Pb206 + 8 2He4 + 6 -1e0

• All first three series; Uranium, Thorium and actinium series end with an isotope of lead that is Pb-206, Pb-208, Pb-207.
• The mass number of all elements in thorium series are multiples of 4, hence can be represented as 4n series.
• In the same way uranium series and actinium series can be represented as 4n+2 and 4n+3 series respectively.
• The neptunium series end with an isotope of bismuth-209.
• The neptunium series represented as 4n+1 series.

Sometimes radioactivity created artificially in elements that are called as artificial radioactivity. Artificial radioactivity can be created in two ways.
1. Artificial transmutation
2. Induced transmutation
Artificial transmutation

The conversion of one element into another by artificial means is known as artificial transmutation. It first observed in 1919 by Rutherford during the bombardment of alpha particles on the nucleus of nitrogen to form oxygen isotope and proton.

7N14 +2He4 8O17 +1H1

Later in 1932 the discovery of neutrons by James Chadwick was also an application of artificial transmutation in which he used the bombardment of alpha particles on the nucleus of beryllium to form neutron and carbon nucleus.

4Be9 + 2He4 6C12 + 0n1

Artificial transmutation can be done by using several particles like; alpha particles, neutrons , deuteron and protons. For example,
1. 4Be0 + 2He4 6C12 + 0n1
2. 94Pu239 + 2He4 94Cm242 + 0n1
3. 9F19 + 2He4 10Ne22 + 1H1
4. 7N14 + 2He4 8O17 + 1H1
5. 26Fe59 + 2He4 29Cu63 + –1e0
6. 15P31 + 1H1 16S31 + 0n1
7. 6C12 + 1H1 N13 + γ
8. 4Be9 + 1H1 4Be8 + 1H2
9. 8O16 + 1H1 7N13 + 2He4
10. 13Al27 + 0n1 12Mg27 + 1H1
11. 8O16 + 0n1 6C13 + 2He4
12. 92U238 + 0n1 92U238 + λ
13. 8O18 + 0n1 9F19 + –1e0
14. 3Li6 + 1H2 3Li7 + 1H1
15. 32As75 + 1H2 32As76 + 1H1
16. 4Be9 + γ 4Be8 + 0n1

The radioactive elements used in many different fields. Like; in atomic energy, agriculture, in different industries and in rock dating process.
1. Discovery of new elements particles - The study of radioactivity has led to the discovery of new fundamental particles like neutrons, positrons, deuterons, alpha particles etc. These particles are highly useful in causing artificial transmutation of elements and adding to our knowledge of atomic structure.
2. Discovery of isotopes and isobars - The isotopes and isobars were first discovered in radioactive series and were afterwards looked for amongst non-radioactive elements.
3. Discovery of new elements: The discovery of trans uranic elements of atomic numbers higher than 92 not known to exist in nature. A large number of different radioactive isotopes of known elements have been discovered by artificial means.
4. Release of atomic energy: Nuclear reactions like nuclear fusion and nuclear fission release enormous amount of energy which can be used for different purpose.
5. Radioactive tracer: Many radioactive isotopes are used in tracing various processes in surgery, medicine, biology, agriculture, industry and chemistry. In tracer technique a radioactive isotope or its compound is introduced at one point of a system and its movement is then trace by measuring radioactivity in different parts of the system. Such isotopes are known as radioactive tracers. For example; Phosphate containing radioactive isotope of phosphorus is used for patients suffering from bone fracture to check of the phosphorus is being absorbed by the bone or not. Similarly the injection of radioactive iodine Uptake is a test of thyroid function.
6. Rock dating and carbon dating: Radioactive isotope of carbon(C-14) is used to estimate the age of earth and for the estimation of age of fossils. The half life for C-14 is around 5568 years.
6C14 7N14 +beta particles

7N14 +0n1
6C14 +1H1

Because of these two reactions, the quantity of C-14 and carbon dioxide (12CO2) present in the atmosphere has been constant over long periods of years. This C-14 has been consumed by plants as well as by other living organism and remains constant for a long time, hence can be detected easily.

1. The radiation exposure at 200 rems or higher creates clumps and responsible for quick hair loss.
2. Brain cell damages only at a very high exposure of radioactive rays around 5,000 rems or greater.
3. Some parts of body are more susceptible for exposure to radioactive radiation sources. For example; thyroid gland is susceptible for radioactive iodine, hence radioactive iodine can destroy thyroid gland.
4. The exposure of radioactive radiations around 100 rems affected the blood system by decreasing the count of the blood's lymphocyte cell which makes the victim more susceptible to infection known as mild radiation sickness.
5. Intense exposure to radioactive substances can damage to small blood vessels and cause heart failure and death directly.
6. Radioactive rays can damage the intestinal tract and cause bloody vomiting, nausea and diarrhea.
• Hiroshima and Nagasaki nuclear accident occur in 1945 during World War II.
• Three Mile Island accident happen in nuclear power plant accidents on 28th march 1979.
• Chernobyl nuclear accident occurred at the Chernobyl Nuclear Power Plant in Ukraine on 26 April 1986.