Isotopes and Radioactive DecayPresented to: Madam Bushra KhalidPresented by: Sairah Akber Sidra Butt FatimaWaleed

Isotopes

What are isotopes ?Any of two or more forms of a chemical

element, having the same number of protons in the nucleus, but having different numbers of neutrons in the nucleus.

History•Henri Becquerel (1852-1908) discovered the

existence of multiple masses for the same element when he realized a product of uranium's radioactive decay, ionium, was unable to be retrieved again by chemical means from the element thorium.

• Because chemical uniqueness is a defining characteristic of an element, it had to be concluded that ionium was not a new element, just a different variation of thorium.

History• Frederick Soddy (1877-1956) was an English

scientist who worked along with Ernest Rutherford on his research with radioactivity.

• They hypothesized in 1913 that elements existed with different atomic masses that were chemically inseparable.

• He concluded that these were all the same element and should reside in the same place on the periodic table.

• He coined these different atoms in the same element isotopes.

Occurrence in nature

•All isotopes are not equally abundant in nature.

• For example, naturally occurring isotopes of Hydrogen (Hydrogen-2 is the only common isotope which has its own name, and is generally called Deuterium).

Radioactive and Stable isotopesRadioactive isotopes Stable isotopes

• Radioactive isotope or radioisotope, natural or artificially created isotopes of a chemical element having an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached.

• The stable end product is a nonradioactive isotope of another element, i.e., radium-226 decays finally to lead-206.

• Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half lives too long to be measured.

• Only 90 nuclides from the first 40 elements are energetically stable .

• there are 255 known stable nuclides of the 80 elements which have one or more stable isotopes.

Radioactive and stable isotopes

•Diagram for radioactive isotopes:

•Diagram for stable isotopes: stable isotopes of carbon

Uses of Radioactive isotopes• In therapy, they are used to kill or inhibit specific

malfunctioning cells. • Radioactive phosphorus is used to treat abnormal cell

proliferation, e.g., polycythemia and leukemia.• Radioactive iodine can be used in the diagnosis of thyroid

function and in the treatment of hyperthyroidism. • In research, radioactive isotopes as tracer agents make it

possible to follow the action and reaction of organic and inorganic substances within the body.

• They also help to ascertain the effects of radiation on the human organism.

• In industry, radioactive isotopes are used for a number of purposes, including measuring the thickness of metal or plastic sheets, testing for corrosion or wear, and monitoring various processes.

Uses of Stable isotopes• With growing demand for petroleum products

methods such as isotopic analysis is becoming more common.

• Stable Isotopes can enhance prospecting as well as production in a petroleum company.

• Stable isotopes have helped uncover migratory routes, trophic levels, and the geographic origin of migratory animals.

• They can be used on land as well as in the ocean and have revolutionized how researchers study animal movement.

Chemical and molecular properties

• Chemical behavior of an atom is largely determined by its electronic structure, so different isotopes exhibit nearly identical chemical behavior.

• The main exception to this is the kinetic isotope effect: due to their larger masses, heavier isotopes tend to react somewhat more slowly than lighter isotopes of the same element.

• This is most pronounced for protium (1H) and deuterium (2H).

• The mass effect between deuterium and the relatively light protium also affects the behavior of their respective chemical bonds.

Chemical and molecular properties

• However, for heavier elements, which have more neutrons than lighter elements, the ratio of the nuclear mass to the collective electronic mass is far greater, and the relative mass difference between isotopes is much less.

• For these two reasons, the mass-difference effects on chemistry are usually negligible.

• In similar manner, two molecules that differ only in the isotopic nature of their atoms (isotopologues) will have identical electronic structure and therefore almost indistinguishable physical and chemical properties.

Nuclear properties and Stability

• Atomic nuclei consist of protons and neutrons bound together by the residual strong force.

• Proton repel each other and neutron stabilize the atom in two ways:

• Their copresent pushes protons slightly apart, reducing the electrostatic repulsion between the protons, and they exert the attractive nuclear force on each other and on protons.

• For this reason, one or more neutrons are necessary for two or more protons to be bound into a nucleus.

• As the number of protons increases, so does the ratio of neutrons to protons necessary to ensure a stable nucleus .

Application of isotopes• Isotope analysis is the determination of isotopic

signature, the relative abundances of isotopes of a given element in a particular sample. For biogenic substances in particular, significant variations of isotopes of C, N and O can occur.

• The identification of certain meteorites as having originated on Mars is based in part upon the isotopic signature of trace gases contained in them.

• Isotopic substitution can be used to determine the mechanism of a chemical reaction via the kinetic isotope effect.

Applications of isotopes• Isotopic labeling, the use of unusual isotopes

as tracers or markers in chemical reactions. Normally, atoms of a given element are indistinguishable from each other.

• However, by using isotopes of different masses, even different nonradioactive stable isotopes can be distinguished by mass spectrometry or infrared spectroscopy. For example, in 'stable isotope labeling with amino acids in cell culture (SILAC)' stable isotopes are used to quantify proteins.

Radioactivity• Radioactivity

▫ emission of high-energy radiation from the nucleus of an atom

• Nuclide▫ nucleus of an isotope

• Transmutation▫ process of changing one element into another via nuclear decay

• The nuclei of some atoms are unstable. The nucleus of an unstable atom will decay to become more stable by emitting radiation in the form of a particle or electromagnetic radiation.

Radioactive Decay

Radioactivity

• Random process : Random process means there is no

way to tell which nucleus will decay, and cannot predict when it is going to decay.

• Spontaneous process : A spontaneous process means the

process is not triggered by any external factors such as temperature of pressure.

History

• Radioactivity was discovered in 1896 by the French scientist Henri Becquerel, while working on phosphorescent materials.

• Rutherford and his Student where first to realize that many decay processes resulted in transmutation

• Radioactive Displacement law of Fajjans And Soddy were Formulated to Describe alpha and Beta Decay.

Radioactive Particles• By the end of the 1800s, it was known that certain

isotopes emit penetrating rays. Three types of radiation were known:

•1) Alpha particles (a)

2) Beta particles (b)

3) Gamma-rays (g)

Alpha decay(a)

•In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it's a group of two protons and two neutrons. A helium nucleus is very stable.

Beta Decay(b)

•A beta particle is often an electron, but can also be a positron, a positively-charged particle that is the anti-matter equivalent of the electron. If an electron is involved, the number of neutrons in the nucleus decreases by one and the number of protons increases by one.

Gamma Decay ( )g

•The third class of radioactive decay is gamma decay, in which the nucleus changes from a higher-level energy state to a lower level. Similar to the energy levels for electrons in the atom, the nucleus has energy levels.

Properties of Radioactive Particles

Penetrating power

• The penetrating effect of alpha, beta and gamma radiation depends on their ionizing power.

• Radiation which has a stronger ionizing power will have a lower penetrating effect.

• The radiation emission loses some of its energy each time an ion pair is produced.

• Alpha particles lose energy very quick as they move through a medium. After a short distance in the medium, the alpha particles would have lost almost all energy. So alpha particles have the lowest penetrating power.

Interaction with electrical field

• Alpha and beta particles are deflected in an electric field because they are charged. The deflections are in opposite direction because they carry opposite charges. The deflection of beta is larger than alpha because mass of beta < mass of alpha

• Gamma rays are not deflected because they do not carry any charge.

Ionizing effect

• Radioactive emission has an ionizing effect

• The 3 types of radiation are highly energetic and use their energy to remove electrons from the air molecules when they pass through air.

• The ionization of an atom produces positive ion and negative ion (electron)

• Due to their different charges and masses, they have different ionizing abilities

Interaction with magnetic field

•Alpha particles and beta particles are also deflected when they pass through a magnetic field while gamma rays are unaffected.

•The direction of the deflection of alpha particles in the magnetic field can be found using Fleming’s left-hand rule.

Radioactive Decay

• Radioactive decay is the process by which unstable atomic nuclei emit subatomic particles or radiation.

• When a radioactive nucleus decays, its nucleus breaks up, emits an alpha particle or beta particle and energy, and forms a new atom of a different element.

• A parent nuclide X changes into a daughter nuclide Y.

During radioactive decay, principles of conservation apply. Some of these we've looked at already, but the last is a new one:

•conservation of energy •conservation of momentum (linear and

angular) •conservation of charge •conservation of nucleon number Conservation of nucleon number means that

the total number of nucleons (neutrons + protons) must be the same before and after a decay.

What Causes Radioactive Decay?

• As we know that a nucleus consists of protons and neutrons, they are bound together by strong interaction. The attractive force of strong interaction and repulsive force of electrostatic force between protons is responsible for the nature of the nucleus in terms of its stability.

• Atoms which have low atomic number have approximately same neutron and protons. As the value of atomic number increases, the number of neutrons inside the stable nucleus increases than the number of protons. As a result, a point is obtained where there is no stable nucleus.

Rate of Decay Beyond knowing the types of particles which are emitted

when an isotope decays, we also are interested in how frequentlyone of the atoms emits this radiation.

A very important point here is that we cannot predict when aparticular entity will decay.

We do know though, that if we had a large sample of a radioactive substance, some number will decay after a given amount of time.

Some radioactive substances have a very high “rate of decay”,while others have a very low decay rate.

To differentiate different radioactive substances, we look toquantify this idea of “decay rate”

Decay Rates For a given element, the decay or disintegration rate is

proportional to the number of atoms and the activity measured in terms of atoms per unit time. If "A" represents the disintegration rate and "N" is number of radioactive atoms, then the direct relationship between them can be shown as below:  

                      A proportional to N                   Or mathematically speaking                                    A= λ N                                                   

Equation 1Where λ is constant of proportionality or decay constant. 

Radioactive nuclei decay by first-order kinetics. The rate of radioactive decay is therefore the product of a rate constant (k) times the number of atoms of the isotope in the sample (N).

Rate = kN

Danger of Radioactive Decay• Alpha particles may be completely stopped by a sheet of paper,

beta particles by aluminum shielding. Gamma rays can only be reduced by much more substantial mass, such as a very thick layer of lead.

• The dangers of radioactivity and radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when electrical engineer and physicist Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to X-rays. His injuries later healed.

• The genetic effects of radiation, including the effect of cancer risk, were recognized much later. In 1927, Hermann Joseph Muller published research showing genetic effects, and in 1946 was awarded the Nobel prize for his findings.

Effect of Radioactive Decay

Unit of Radioactivity Decay

•The rate at which a radioactive isotope decays is called the activity of the isotope. The most common unit of activity is the curie (Ci), which was originally defined as the number of disintegrations per second in 1 gram of 226Ra. The curie is now defined as the amount of radioactive isotope necessary to achieve an activity of 3.700 x 1010 disintegrations per second.

Half-Life

•The half-life of a radioactive element is the time that it takes for one half of the atoms of that substance to disintegrate into another nuclear form. These can range from mere fractions of a second, to many billions of years. In addition, the half-life of a particular radionuclide is unique to that radionuclide, meaning that knowledge of the half-life leads to the identity of the radionuclide.