Isotopes and Radioactive DecayPresented to: Madam Bushra KhalidPresented by: Sairah AkberSidra ButtFatimaWaleed
IsotopesWhat are isotopes ?Any of two or more forms of a chemical element,having the same number of protons in thenucleus, but having different numbers ofneutrons in the nucleus.
History• Henri Becquerel (1852-1908) discovered theexistence of multiple masses for the sameelement when he realized a product of uraniumsradioactive decay, ionium, was unable to beretrieved again by chemical means from theelement thorium.• Because chemical uniqueness is a definingcharacteristic of an element, it had to beconcluded that ionium was not a newelement, just a different variation of thorium.
History• Frederick Soddy (1877-1956) was an Englishscientist who worked along with Ernest Rutherfordon his research with radioactivity.• They hypothesized in 1913 that elements existedwith different atomic masses that were chemicallyinseparable.• He concluded that these were all the same elementand should reside in the same place on the periodictable.• He coined these different atoms in the same elementisotopes.
Occurrence in nature• All isotopes are not equally abundant in nature.• For example, naturally occurring isotopes ofHydrogen (Hydrogen-2 is the only commonisotope which has its own name, and is generallycalled Deuterium).
Radioactive and Stable isotopesRadioactive isotopes Stable isotopes• Radioactive isotope orradioisotope, natural orartificially created isotopes of achemical element having anunstable nucleus thatdecays, emittingalpha, beta, or gamma raysuntil stability is reached.• The stable end product is anonradioactive isotope ofanother element, i.e., radium-226 decays finally to lead-206.• Stable isotopes are chemicalisotopes that may or may notbe radioactive, but ifradioactive, have half lives toolong to be measured.• Only 90 nuclides from the first40 elements are energeticallystable .• there are 255 known stablenuclides of the 80 elementswhich have one or more stableisotopes.
Radioactive and stable isotopes• Diagram for radioactive isotopes:• Diagram for stable isotopes: stable isotopes ofcarbon
Uses of Radioactive isotopes• In therapy, they are used to kill or inhibit specificmalfunctioning cells.• Radioactive phosphorus is used to treat abnormal cellproliferation, e.g., polycythemia and leukemia.• Radioactive iodine can be used in the diagnosis of thyroidfunction and in the treatment of hyperthyroidism.• In research, radioactive isotopes as tracer agents make itpossible to follow the action and reaction of organic andinorganic substances within the body.• They also help to ascertain the effects of radiation on thehuman organism.• In industry, radioactive isotopes are used for a number ofpurposes, including measuring the thickness of metal orplastic sheets, testing for corrosion or wear, and monitoringvarious processes.
Uses of Stable isotopes• With growing demand for petroleum productsmethods such as isotopic analysis is becoming morecommon.• Stable Isotopes can enhance prospecting as well asproduction in a petroleum company.• Stable isotopes have helped uncover migratoryroutes, trophic levels, and the geographic origin ofmigratory animals.• They can be used on land as well as in the ocean andhave revolutionized how researchers study animalmovement.
Chemical and molecular properties• Chemical behavior of an atom is largely determinedby its electronic structure, so different isotopesexhibit nearly identical chemical behavior.• The main exception to this is the kinetic isotopeeffect: due to their larger masses, heavier isotopestend to react somewhat more slowly than lighterisotopes of the same element.• This is most pronounced for protium (1H) anddeuterium (2H).• The mass effect between deuterium and therelatively light protium also affects the behavior oftheir respective chemical bonds.
Chemical and molecular properties• However, for heavier elements, which have moreneutrons than lighter elements, the ratio of thenuclear mass to the collective electronic mass is fargreater, and the relative mass difference betweenisotopes is much less.• For these two reasons, the mass-difference effectson chemistry are usually negligible.• In similar manner, two molecules that differ only inthe isotopic nature of their atoms (isotopologues)will have identical electronic structure and thereforealmost indistinguishable physical and chemicalproperties.
Nuclear properties and Stability• Atomic nuclei consist of protons and neutrons boundtogether by the residual strong force.• Proton repel each other and neutron stabilize the atomin two ways:• Their copresent pushes protons slightly apart, reducingthe electrostatic repulsion between the protons, and theyexert the attractive nuclear force on each other and onprotons.• For this reason, one or more neutrons are necessary fortwo or more protons to be bound into a nucleus.• As the number of protons increases, so does the ratio ofneutrons to protons necessary to ensure a stable nucleus.
Application of isotopes• Isotope analysis is the determination of isotopicsignature, the relative abundances of isotopes of agiven element in a particular sample. For biogenicsubstances in particular, significant variations ofisotopes of C, N and O can occur.• The identification of certain meteorites as havingoriginated on Mars is based in part upon theisotopic signature of trace gases contained in them.• Isotopic substitution can be used to determine themechanism of a chemical reaction via the kineticisotope effect.
Applications of isotopes• Isotopic labeling, the use of unusual isotopes astracers or markers in chemical reactions. Normally,atoms of a given element are indistinguishable fromeach other.• However, by using isotopes of different masses, evendifferent nonradioactive stable isotopes can bedistinguished by mass spectrometry or infraredspectroscopy. For example, in stable isotopelabeling 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 anunstable atom will decay to become more stable by emittingradiation in the form of a particle or electromagnetic radiation.
Radioactivity• Random process :Random process means there is no way totell which nucleus will decay, and cannot predictwhen it is going to decay.• Spontaneous process :A spontaneous process means the processis not triggered by any external factors such astemperature of pressure.
History• Radioactivity was discovered in 1896 by the French scientist HenriBecquerel, while working on phosphorescent materials.• Rutherford and his Student where first to realize that many decay processesresulted in transmutation• Radioactive Displacement law of Fajjans And Soddy were Formulated toDescribe alpha and Beta Decay.
Radioactive Particles• By the end of the 1800s, it was known that certainisotopes emit penetrating rays. Three types ofradiationwere known:•1) Alpha particles (a)2) Beta particles (b)3) Gamma-rays (g)
Alpha decay(a)• In alpha decay, the nucleus emits an alphaparticle; an alpha particle is essentially a heliumnucleus, so its a group of two protons and twoneutrons. A helium nucleus is very stable.
Beta Decay(b)• A beta particle is often an electron, but can alsobe a positron, a positively-charged particle thatis the anti-matter equivalent of the electron. Ifan electron is involved, the number of neutronsin the nucleus decreases by one and the numberof protons increases by one.
Gamma Decay (g)• The third class of radioactive decay is gammadecay, in which the nucleus changes from ahigher-level energy state to a lower level. Similarto the energy levels for electrons in the atom, thenucleus has energy levels.
Penetrating power• The penetrating effect of alpha, beta and gammaradiation depends on their ionizing power.• Radiation which has a stronger ionizing power will havea lower penetrating effect.• The radiation emission loses some of its energy eachtime an ion pair is produced.• Alpha particles lose energy very quick as they movethrough a medium. After a short distance in themedium, the alpha particles would have lost almost allenergy. So alpha particles have the lowest penetratingpower.
Interaction with electrical field• Alpha and beta particles are deflected in an electric fieldbecause they are charged. The deflections are in oppositedirection because they carry opposite charges. Thedeflection of beta is larger than alpha because mass ofbeta < mass of alpha• Gamma rays are not deflected because they do not carryany charge.
Ionizing effect• Radioactive emission has an ionizing effect• The 3 types of radiation are highly energetic anduse their energy to remove electrons from the airmolecules when they pass through air.• The ionization of an atom produces positive ionand negative ion (electron)• Due to their different charges and masses, theyhave different ionizing abilities
Interaction with magnetic field•Alpha particles and betaparticles are also deflected whenthey pass through a magneticfield while gamma rays areunaffected.•The direction of the deflectionof alpha particles in themagnetic field can be foundusing Fleming’s left-hand rule.
Radioactive Decay• Radioactive decay is the process by whichunstable atomic nuclei emit subatomic particlesor radiation.• When a radioactive nucleus decays, its nucleusbreaks up, emits an alpha particle or betaparticle and energy, and forms a new atom of adifferent element.• A parent nuclide X changes into a daughternuclide Y.
During radioactive decay, principles of conservationapply. Some of these weve looked at already, butthe last is a new one:• conservation of energy• conservation of momentum (linear andangular)• conservation of charge• conservation of nucleon numberConservation of nucleon number means that the totalnumber of nucleons (neutrons + protons) must bethe same before and after a decay.
What Causes Radioactive Decay?• As we know that a nucleus consists of protons andneutrons, they are bound together by stronginteraction. The attractive force of strong interactionand repulsive force of electrostatic force betweenprotons is responsible for the nature of the nucleusin terms of its stability.• Atoms which have low atomic number haveapproximately same neutron and protons. As thevalue of atomic number increases, the number ofneutrons inside the stable nucleus increases thanthe number of protons. As a result, a point isobtained where there is no stable nucleus.
Rate of Decay Beyond knowing the types of particles which are emittedwhen 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 radioactivesubstance, 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 RatesFor a given element, the decay or disintegration rate isproportional to the number of atoms and theactivity measured in terms of atoms per unit time. If "A"represents the disintegration rate and "N" is number ofradioactive atoms, then the direct relationship betweenthem can be shown as below:A proportional to NOr mathematically speakingA= 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 theproduct of a rate constant (k) times the numberof atoms of the isotope in the sample (N).Rate = kN
Danger of Radioactive Decay• Alpha particles may be completely stopped by a sheet ofpaper, beta particles by aluminum shielding. Gamma rays canonly be reduced by much more substantial mass, such as avery thick layer of lead.• The dangers of radioactivity and radiation were notimmediately recognized. Acute effects of radiation were firstobserved in the use of X-rays when electrical engineer andphysicist Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning theburns that developed, though he attributed them to ozonerather than to X-rays. His injuries later healed.• The genetic effects of radiation, including the effect of cancerrisk, were recognized much later. In 1927, Hermann JosephMuller published research showing genetic effects, and in1946 was awarded the Nobel prize for his findings.
Unit of Radioactivity Decay• The rate at which a radioactive isotope decaysis called the activity of the isotope. The mostcommon unit of activity is the curie (Ci), whichwas originally defined as the number ofdisintegrations per second in 1 gram of 226Ra.The curie is now defined as the amount ofradioactive isotope necessary to achieve anactivity of 3.700 x 1010 disintegrations persecond.
Half-Life• The half-life of a radioactive element is the timethat it takes for one half of the atoms of thatsubstance to disintegrate into another nuclearform. These can range from mere fractions of asecond, to many billions of years. In addition,the half-life of a particular radionuclide isunique to that radionuclide, meaning thatknowledge of the half-life leads to the identity ofthe radionuclide.