Classify the nuclides based on the number of protons and neutrons of the nuclei
Nuclides can be classified in four ways based on the number of protons and
neutrons of the nuclei. These are:
a. Isotopes: These are the nuclei that belong to the same element and have the
same number of protons. They occupy the same position in the periodic table and
hence have the same atomic number. The isotopes of given element contains same
number of electrons and hence have the same chemical properties, thus it is
difficult to separate them from one another using chemical methods. However the
Isotopes of given element differ in mass dependent physical properties like
rate of diffusion and thus can be separated from one another using physical
properties.
b. Isobars: These are the nuclei of the neighbouring group having the same mass
number but different atomic number. The isobaric nuclei belong to the different
group and hence occupy different positions in the periodic table. They also
differ in their chemical properties.
c. Isotones: These are the nuclei that contain same number of neutrons but they
differ in the number of protons and also in the mass number.
d. Isomers: These are the nuclei that contain same number of protons and
neutrons and hence they have the same mass number but they differ in their
energies. In order to reach the ground state these nuclei emit the excess
energy as gamma ray photon. The unstable nucleus is said to be in metastable
state which on emission of gamma rays becomes stable.
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Define radioactivity? Explain α, β and γ rays.
a. The emission of radiation by uranium and its compound is an atomic
phenomenon. It is independent of the chemical and physical state of the
element. Such phenomenon is known as radioactivity and such elements are said
to be radioactive.
b. α rays: These rays consist of particles which are positively charged.
If an a particle is emitted by the radioactive parent element then formation of
the daughter element takes place which have atomic number less by 2 units and
mass number less by 4 units.
c. β rays: These rays consist of electrons. When the conversion of a
neutron into proton takes place then an electron is ejected out, along with
electron another particle is also ejected out which is known as anti-neutron.
The mass of this anti-neutron in negligible. When a β ray is emitted by
the parent element then the atomic number of the daughter is more by one unit
than the parent element. However the mass number of both remains the same.
d. γ rays: These rays are similar to electromagnetic radiation and possess
very short wavelength. The daughter nucleus formed generally exists in the
excited state. While returning to the ground state they generally emit its
excess energy as γ-ray photon. Here the atomic number and the mass number of
the daughter nucleus remains the same as of the parent nucleus.
Give the general characteristics of radioactive decay.
It is seen that a parent nucleus on emission loses its identity and gets
converted into a daughter nucleus. This phenomenon is known as radioactive
decay. The following characteristics are generally applied to natural
radioactivity.
a. Loss of mass: In the radioactive decay the mass of the parent nucleus is
usually greater than that of the daughter nucleus and hence in radioactive
decay loss of mass occurs. According to Einstein’s equation the lost mass
appears as an energy which is shared between the emitted particle and the
daughter nucleus.
b. Range and specific ionisation of the emitted particles: The radiations
emitted by the radioactive nuclei are highly energetic and due to this the
radiations can penetrate through the matter. The depth of these penetrations
into the matter is proportional to the density of the matter. The distance
covered by these radiations in the matter is called their range. The number of
ion pairs per unit distance, the emitted particle covers in a medium is known
as specific ionisation.
c. Nature of path: α – particles are more massive than the β –
particles. The a- particles generally travel in a straight line and on
collision with gas molecules they are not scattered. The β- particles follow a
zigzag path and on collision with gas molecules they are scattered.
d. Physical and chemical changes: The energy associated with α and β
particles are sufficient to break the bonds in the molecules of the medium
which lead to the formation of free radicals. These free radicals initiate
various chemical reactions. The passage of α and β particles also
produce some physical changes.
What are tracers? Explain how Thyroidisis and Brain Tumour Location can be
detected with the help of tracer I131.
The element which is labelled is called as a Tracer element. In the series of
chemical reactions the path of the element or a compound containing the element
can be traced using the tracer element.
a. Thyroidisis: The iodine plays an important role in our well-being as it
controls the growth and our metabolism. Most of the iodine that we intake from
our food is stored in the thyroid gland. The thyroid gland in some people may
become over-active or under-active. Both these conditions are considered to be
critical. The condition of thyroid in patients are understood with the help of
tracer as follows: about 10 µCi of I131 in the form of NaI is given
to the patient through orange juice and the counting of the γ activity
emitted is started immediately. The counting is taken for several hours and
then the ratio of counts D/P (dummy/patients) are plotted as a function of
time. The curve obtained is compared with the curve of the normal person and
thus hypothyroidisis or hyperthyroidisis is detected.
b. Brain Tumour Location: Dyes like fluorescein, rose Bengal are absorbed by
cancerous cells, thus in order to locate the brain tumour location the dye
labelled with I131 is given to the patient. Then the entire space
around the skull is scanned by the help of special counters and the place where
ever I131 is accumulated is found and thus it is possible to locate
the brain tumour to some extent.
What is C14 dating?
C14 dating technique is usually used to find the age of dead
organism. In this technique generally the concentration ratio of C14
to C12 is used in order to determine ages upto 20,000 years. This is
widely used in archaeology. C14 has a half-life period of 5700
years. When an organism dies the C14 content in them begins to decay
with its characteristic to the half-life period. The concentration of C14
/ C12 decreases and this decrease from the equilibrium is used as a
measure of the age of the sample.
C14 emits α β ray of low energy. In living samples ,
special counters with low background are used as the activity due to C14
is low in them.
Explain the use of radioisotopes in agriculture.
Radioisotopes have been used to determine the requirement of different elements
in the plant. Some of these applications are as follows:
a. Use of fertilizers: By the help of tracer Ca45 it has been found
that the uptake of calcium by the plants in acidic soil is same for CaO and
CaCO3 but is less for CaSO4. P32 is used to
determine the rate of the uptake of phosphorous in the plants. Due to this
technique it is found that it is good to add phosphorous fertilizer to the
plants during its sowing period. During this period the uptake is more than 60%
but if the fertilizer is added at the later stage then the uptake is only 35%.
b. Irradiation of seeds: With the help of radioisotopes people have speed up the
development of the plants. They have also increased the yield and quality of
the crops. Exposing of seeds to the γ radiations are beneficial for the
growth of the plant.
c. Control of insects: Radioisotopes have also been used to gain information
about the migration and breeding habits of predatory insects. This is done by
the help of Phosphorous or Cobalt. Generally the insects are labelled with P32
or Co60. In order to label them with Co60 the insects are
dipped in cobalt chloride solution where the cobalt is labelled as Co60.
Each insect absorbs a radiation dose of 300 rads and this lasts for 6-8 months.
Then the counting device is used to follow the migration and location of
labelled insects. Now with the help of insecticide the predators are
destroyed.
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