Dr Gihan Gawish. Atomic structure A = Z + N (N) (Z)

Dr Gihan GawishDr Gihan Gawish


Atomic structureAtomic structure

A = Z + N

(N)

(Z)


IsotopeIsotope• Isotopes are any of the different types of atoms

(Nuclides) of the same chemical element, each having a different atomic mass (mass number)

• Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons.

• Therefore, isotopes have different mass numbers, which give the total number of

nucleons, the number of protons plus neutrons.


IsotopeIsotope• About 339 nuclides occur naturally on

Earth, of which 250 (about 74%) are stable.

• Counting the radioactive nuclides not found in nature that have been created artificially, more than 3100 nuclides are

currently known


IsotopeIsotope• Elements are composed of one or more

naturally occurring isotopes, which are normally stable.

• Some elements have unstable (radioactive) isotopes


IsotopeIsotope

C1 26

E lem en ts in ch em is try a re rep resen ted b y a sym b o l w ith tw o n u m b ers * A to m ic n u m b e r = n u m b e r o f p ro to n s o r e le c tro n s * A to m ic m a ss = n u m b e r o f n u c le o n s (p ro to n s + n e u tro n s) . C h an g e th e a to m ic n u m b er = a d iffe ren t e lem en t.Iso to p es a re a to m s w ith id en tica l a to m ic n u m b ers b u t d iffe ren t m ass n u m b ers

H3

1H2

1H1

1 C1 2

6 C1 4

6 P3 1

1 5P3 2

1 5C1 3

6 I1 3 1

5 3 I1 2 7

5 3


IsotopeIsotope• Some isotopes are stable, others are unstable

or radioactive • Radiation is emitted when an unstable

nucleus spontaneously changes, or disintegrates into more stable one.

• Every element in the periodic table has at least one radioactive isotope.

• Radioactivity is a form of nuclear reaction )nucleus( not chemical reaction )electrons(


Nuclear and chemical reactionsNuclear and chemical reactions

• A nuclear reaction involves changes in an atom’s nucleus, usually producing a different

element. Chemical reaction never changes the nucleus, it only rearranges the outer shell

electrons.

– Different isotopes of an element have essentially the chemical reactivity )same

electrons(, but often have completely different behavior in nuclear reactions.


Nuclear and chemical reactionsNuclear and chemical reactions

• The rate of nuclear reaction is not affected by the change in temperature, pressure or addition of a catalyst, or the chemical form

(compound or element).

• The energy change accompanying a nuclear reaction can be several million

times greater than that of a chemical reaction.


Radioactive decayRadioactive decay• Radioactive decay is the process in

which an unstable atomic nucleus loses energy by emitting ionizing particles and

radiation.

• This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type,

called the daughter nuclide.


RadioactivityRadioactivityNuclear decay or Radioactivity is the

spontaneous emission of radiation from a nucleus.– One element can change into another element via

radioactive decay or transmutation

– Discovered by Henry Becquerel in 1896. • He concluded that uranium gave off some

radiation.


RadioactivityRadioactivity• The radiation or radioactivity was later shown to be separable

by electric (and magnetic) fields into three different types:

1. Alpha (); a helium nucleus, He2+, emitted as alpha particle.

2. Beta (); an electron emitted from the nucleus

3. Gamma (); radioactivity consisting of high-energy light waves.


Alpha emissionAlpha emissionWhen an atom emits an alpha particle, the nucleus loses two

protons and two neutrons. Example

Emission of an alpha particle from uranium-238 produces an atom of thorium-234

• The alpha particle is emitted by elements:– of mass number greater than 140, or – of atomic number greater than 83

• These elements are seldom used in Biochemistry

U Th + He9 2 2

2 3 4 4

9 0

2 3 8A lp h a P a rtic le


Beta EmissionBeta Emission• Most of the radioisotopes commonly used in Biochemistry

are Beta emitters. The beta particle is one of two types:

• Electron emission (or ):– decomposition of neutron electron + proton

• The nucleus ejects electron as a beta particle and retains the proton.

– An example is the radioactive decay of carbon-14

• Positron emission (): – conversion of proton neutron + +.

• A positron has the same mass as an electron but a positive charge.

– An example is the decay of Zn-65

01147

146 NC

016529

6530 CuZn


Gamma emissionGamma emission• A few radioisotopes of biochemical significance are

gamma emitters• Gamma emission )( causes no change in the mass

or atomic number -emission is often a secondary process occurring after

initial decay by or emission. Surplus energy is sometimes emitted.

rays are high energy waves, corresponding to radiation with a wavelength of about 10-12 m.

• The most dangerous kind of radiation for humans.– Cobalt-60 is used in cancer therapy as a source radiation

that kills cancerous tissue.• Example of -emitters:

13154

13154

13153 XeXeI


Ionizing RadiationIonizing Radiation• Energy of particles and rays is fixed because they

are of specific composition or wavelength.

• Energy of particles varies with the atom they originate from.

– E.g. 32P releases high-energy particles, while tritium 3H release low-energy particles during the decay.

and emissions are all ionizing radiation, because they have the potential, upon encountering an atom, to

knock out its electrons, thereby creating ions. This is why these radiations are harmful.


Ionizing RadiationIonizing Radiation• Ionizing Radiation: A general name for

high-energy radiation of all kinds, such as particles, particles, rays, x-rays, and

cosmic rays.

– X-rays and -rays are electromagnetic radiation.

– Cosmic rays: A mixture of high-energy particles – protons and various atomic nuclei – that come from

space.


Radioactivity Half-LifeRadioactivity Half-Life

• Nuclear decay is a first order process

• Rates of nuclear decay are measured in units of half life (t1/2), defined as the time required for one half of the radioactive sample to decay.

693.0

2/10

ttN

NLnN

dt

dN

IsotopeParticle typeHalf life3H-12.3 yr

14C-5570 yr32P-14 days

22Na- & 15 hr125I60 days131I-8 days

238U>billions yrs


Units of RadiationUnits of Radiation• The SI unit of radioactive decay (the

phenomenon of natural and artificial radioactivity) is the becquerel (Bq).

• One Bq is defined as one transformation (or decay) per second.


Units of RadiationUnits of Radiation• In the meteric system, radioactivity unit is

Becquerel (Bp); 1 Bp= 1 disintegration per second )dps(.

•The basic unit of radioactivity is Curie (Ci), and its subdivisions: mCi, Ci

•The two units can be interconverted:– 1 Ci= 3.7 x 1010 Bp or dps.


Units of RadiationUnits of Radiation• Instruments ( or counters) report

radiation as cpm or (count per minute).

cpm = dpm * (counting efficiency of machine)

dpm= disintegration per minute


Detecting RadiationDetecting Radiation

Three methods are commonly employed in Biochemistry to

detect radiation:

1. Geiger-Muller counters

2. Scintillation counters

3. Autoradiograph or photographic exposure


Geiger-Muller CounterGeiger-Muller Counter• The most common devise to detect radiation,

particularly -particles.

• Geiger counter is simply an argon-filled tube with two electrodes. When radiation

(orcollide with gas atoms ejection of electrons ions formation.

– Geiger counter produces electrical current in proportion to the amount of ionizing radiation.

• Radiation produces a clicking sound in this devise. The more radiation that enters the tube,

the more frequent the clicks. Intensity of radiation can also be registered on a meter


Geiger-Muller CounterGeiger-Muller Counter

current ( i )

• This devise is seldom used for accurate measurements, but extremely useful as a survey meter to detect contamination,

exposure, and rough estimation of radioactivity.


Scintillation CounterScintillation Counter• The most versatile method for measuring radiation in the

laboratory.

• In this devise, a radioactive substance is placed in a vial, mixed with scintillation cocktail, and placed in the counter

– scintillation cocktail contains a solvent, usually aromatic, plus fluorescent substances, usually PPO (2,5-diphenyloxazole)

and POPOP (1,4-bis-PPO)

• When radiation strike the solvent, a serious of reaction take place that emit a flash of light. The number of flashes

are counted electronically.


Scintillation CounterScintillation Counter


AutoradiographyAutoradiography

• The simplest devise for detecting radiation is a photographic film.

• If the film is protected from light, any radiation striking the film will trigger the

formation photon- & electron-dense location.


AutoradiographyAutoradiography

• Extremely useful for all kinds of blots (southern, northern, etc.), hybridization studies, localization of biomolecules in

cell or organelles, monitoring the fate of metabolites, plus many other

applications.

Dr Gihan Gawish. Atomic structure A = Z + N (N) (Z)

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