Nuclear Chemist

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.

We know that

Qp = ΔH and qv = ΔE

At constant pressure

ΔH = ΔE + PΔV ----------------------------i

Where ΔV is the change in volume, thus above equation can be written as

ΔH = ΔE + P (V2 – V1)

=ΔE + (PV2 – PV1) ---------------------------ii

Where V1 is the initial volume and V2 is the final volume of the system

For ideal gases:

PV = nRT

So we have

PV1 = n1RT

PV2 = n2RT

Here n1 is the number of moles of the gaseous reactants and n2 is the number of moles of the gaseous products.

Substituting these in equation ii, we get

ΔH = ΔE + (n2RT – n1RT)

=ΔE + (n2 – n1) RT

ΔH = ΔE + Δng RT

Where Δng = n2 – n1 is the difference between the number of moles of the gaseous products and those of the gaseous reactants.

Substituting the value of ΔE and ΔH the above equation becomes

qp = qv + Δng RT

The above equation gives us the relationship between heat of reaction at constant pressure and that at constant volume.

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.

Nucleoside 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.

The third law of thermodynamics states that:

“The entropy of a perfect crystal of each element and a compound is zero at absolute zero.”

Limitations: If any disorder like impurity or imperfection is found in a substance then the entropy of such crystal is non-zero at 0 K. For example: The entropy of pure carbon dioxide and nitric oxide is zero at 0K. This shows that there exists disorder in the arrangement of such molecules.

This law is applicable only to pure compounds. Thus we can say that, this law is not applicable to glass which is a supercooled liquid. It is also not applicable to amorphous substance and supercooled solutions.

Importance:

a. With the help of this law Thermodynamic properties can be calculated and chemical affinity can be measured.

b. This law helps in explaining the behaviour of solids at very low temperature.

This thermodynamic quantity states that the decrease in value during a process is equal to the useful work done by the system. It is denoted by G and the mathematical equation is:

G = H – TS

Where,

H = heat content

T = absolute temperature

S = entropy of the system

For isothermal process we have

G1 = H1 – TS1 for the initial state

G2 = H2 – TS2 for final stage

Therefore,

G2 – G1 = (H2 – H1) – T(S2 – S1)

Now,

ΔG = G2 – G1 is the change in Gibbs free energy

ΔH = H2 – H1 is the change in enthalpy of the system

ΔS = S2 – S1 Is the change in entropy of the system

Thus the above equation becomes:

ΔG = ΔH – TΔS is known as Gibbs-Helmoholtz equation.

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.

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.