PHYSICAL CHEMISTRY OF MILK

Module 11. Molecular spectroscopy

Lesson 28
NUCLEAR CHEMISTRY, ISOTOPES

28.1 Introduction

In any compound the electrons around the atom’s nucleus are involved in various chemical reactions. Discovery of radiation emitted by uranium has expanded this field to nuclear changes. Marie Sklodowska Curie has initiated the study of radioactivity and contributed much to the study of nuclear changes. In the present lesson we will be learning what the isotopes are their nature and about the radio isotopes.

28.2 Radiation and Nuclear Reactions

According to the theory proposed by Frederic Soddy "radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element." In nuclear reactions, the particles in an atom’s nucleus will change and thus changing the atom itself due to nuclear reactions. In chemical reactions the molecules are formed whereas due to the nuclear reactions there will be transmutation of one element into a different isotope or a different element altogether. All elements heavier than bismuth (Bi) exhibit natural radioactivity and thus can “decay” into an altogether lighter elements. Normally the number of protons in an atom will determine a particular element. As the protons are changing in the nucleus due to radiation the atom of that element will also change.

28.3 Types of Radiations

There are three common types of radiation and nuclear changes.

28.3.1 Alpha radiation (α )

Emission of an alpha particle from an atom’s nucleus is called the alpha radiation. An alpha particle contains 2 protons and two neutrons and is similar to He Nucleus (24 He). When an atom emits α particle the atom’s atomic mass will decrease by four units (because two protons and two neutrons are lost) and the atomic number (z) will decrease by two units. The element is said to "transmute" into another element that is two z units smaller. e. g. Uranium decays into Thorium by emitting alpha particle.

(Note: in nuclear chemistry, element symbols are traditionally preceded by their atomic weight (upper left) and atomic number (lower left).

28.3.2 Beta radiation (β)

The transmutation of a neutron into a proton and an electron followed by the emission of the electron from the atom’s nucleus When an atom emits β particle the atom’s mass will not change since there is no change in the total of nuclear particles. There will be an increase in the atomic number by one because of the transmutation of a neutron to an additional proton. e.g. Carbon -14 an isotope of Carbon into the element Nitrogen.

28.3.3 Gamma radiation (γ)

It is the emission of electromagnetic energy (Similar to light energy) from an atom’s nucleus. There will be no emission of particles during gamma radiation and gamma radiation does not itself cause the transmutation of atom. It is often observed that γ-radiation during the transmutation of atoms to, α or β radioactive decay. e.g. X-rays emitted during the beta decay of Cobalt -60.

28.4 Isotopes

Isotopes may be defined as the atoms having the same nuclear charge but are having different masses. Such atoms have practically same structure of their electrons shells and belong to the same chemical element of the periodic system. Consequent to the development of mass spectroscopy, it became possible to discover isotopes among the naturally occurring compounds of non radio active elements. Further with the development of nuclear physics it has become possible to obtain different isotopes for different elements artificially.

28.4.1 Nature of isotopes

The composition of Isotopes of naturally occurring elements is markedly constant.

Fig. 28.1 Radioactive decay of bismuth-210 (T 1/2=5 DAYS)

(Source: www.vissionlearning.com)

28.5 Half-Life

Radioactive decay proceeds according to a principal called the half-life. The half-life (T½) is the amount of time necessary for one-half of the radioactive material to decay. For example, the radioactive element bismuth (₂₁₀Bi) can undergo alpha decay to form the element thallium (₂₀₆Tl) with a reaction half-life equal to five days. If we begin an experiment starting with 100 g of bismuth in a sealed lead container, after five days we will have 50 g of Bismuth and 50 g of Thallium in the jar. After another five days (ten from the starting point), one-half of the remaining bismuth will decay and we will be left with 25 g of Bismuth and 75 g of Thallium in the jar. As illustrated in fig. 28.1, the reaction proceeds in halfs, with half of whatever is left of the radioactive element decaying every half-life period.

The fraction of parent material that remains after radioactive decay can be calculated using the equation:

Fraction remaining = 1/2n

(where n = number of half-lives elapsed)

The amount of a radioactive material that remains after a given number of half-lives is therefore:

Amount remaining = Original amount * Fraction remaining

The decay reaction and T½ of a substance are specific to the isotope of the element undergoing radioactive decay. For example, Bi₂₁₀ can undergo decay to Tl₂₀₆ with a T½ of five days. Bi₂₁₅, by comparison, undergoes decay to Po₂₁₅ with a T½ of 7.6 minutes, and Bi₂₀₈ undergoes yet another mode of radioactive decay (called electron capture) with a T½ of 368,000 years!

28.6 Stimulated Nuclear Reactions

While many elements undergo radioactive decay naturally, nuclear reactions can also be stimulated artificially. Although these reactions also occur naturally, we are most familiar with them as stimulated reactions. There are two such types of nuclear reactions:

28.6.1 Nuclear fission

Reactions in which an atom's nucleus splits into smaller parts, releasing a large amount of energy in the process. Most commonly this is done by "firing" a neutron at the nucleus of an atom.

The energy of the neutron "bullet" causes the target element to split into two (or more) elements that are lighter than the parent atom.

During the fission of U₂₃₅, three neutrons are released in addition to the two daughter atoms. If these released neutrons collide with nearby U₂₃₅ nuclei, they can stimulate the fission of these atoms and start a self-sustaining nuclear chain reaction. This chain reaction is the basis of nuclear power. As uranium atoms continue to split, a significant amount of energy is released from the reaction. The heat released during this reaction is harvested and used to generate electrical energy.

Two Types of Nuclear Chain Reactions.

Concept simulation – Reactions controlled and uncontrolled nuclear chain reactions.

28.6.2 Nuclear fusion

Reactions in which two or more elements "fuse" together to form one larger element, during the process energy is being released. A good example is the fusion of two "heavy" isotopes of hydrogen (deuterium: ₂H and tritium: ₃H) into the element helium.fig. 28.2 shows such reaction.

Fig. 28.2 Nuclear fusion of two hydrogen isotopes
(Source: Vision Learning.com)

Fusion reactions release tremendous amounts of energy and are commonly referred to as thermonuclear reactions. Although many people think of the sun as a large fireball, the sun (and all stars) is actually enormous fusion reactors. Stars are primarily gigantic balls of hydrogen gas under tremendous pressure due to gravitational forces. Hydrogen molecules are fused into helium and heavier elements inside of stars, releasing energy that we receive as light and heat.