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

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Presentation on theme: "Dr Gihan Gawish. Atomic structure A = Z + N (N) (Z)"— Presentation transcript:

1 Dr Gihan Gawish

2 Atomic structure A = Z + N (N) (Z)

3 Dr Gihan Gawish Isotope 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.

4 Dr Gihan Gawish Isotope 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

5 Dr Gihan Gawish Isotope Elements are composed of one or more naturally occurring isotopes, which are normally stable. Some elements have unstable (radioactive) isotopes

6 Dr Gihan Gawish Isotope

7 Isotope 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)

8 Dr Gihan Gawish Nuclear 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.

9 Dr Gihan Gawish Nuclear 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.

10 Dr Gihan Gawish Radioactive 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.

11 Dr Gihan Gawish Radioactivity Nuclear 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.

12 Dr Gihan Gawish Radioactivity The radiation or radioactivity was later shown to be separable by electric (and magnetic) fields into three different types: 1. Alpha (  ); a helium nucleus, He 2+, emitted as alpha particle. 2. Beta (  ); an electron emitted from the nucleus 3. Gamma (  ); radioactivity consisting of high-energy light waves.

13 Dr Gihan Gawish Alpha emission When 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

14 Dr Gihan Gawish Beta 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

15 Dr Gihan Gawish Gamma 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:

16 Dr Gihan Gawish Ionizing 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. 32 P releases high-energy  particles, while tritium 3 H 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.

17 Dr Gihan Gawish Ionizing 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.

18 Dr Gihan Gawish Radioactivity Half-Life Nuclear decay is a first order process  Rates of nuclear decay are measured in units of half life (t 1/2 ), defined as the time required for one half of the radioactive sample to decay. IsotopeParticle typeHalf life 3H3H -- 12.3 yr 14 C -- 5570 yr 32 P -- 14 days 22 Na  - &  15 hr 125 I  60 days 131 I -- 8 days 238 U  >billions yrs

19 Dr Gihan Gawish Units of Radiation The SI unit of radioactive decay (the phenomenon of natural and artificial radioactivity) is the becquerel (Bq).SIbecquerel One Bq is defined as one transformation (or decay) per second.

20 Dr Gihan Gawish Units 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 10 10 Bp or dps.

21 Dr Gihan Gawish Units of Radiation Instruments (  or  counters) report radiation as cpm or (count per minute). cpm = dpm * (counting efficiency of machine) dpm= disintegration per minute

22 Dr Gihan Gawish Detecting Radiation Three methods are commonly employed in Biochemistry to detect radiation: 1.Geiger-Muller counters 2.Scintillation counters 3.Autoradiograph or photographic exposure

23 Dr Gihan Gawish Geiger-Muller Counter The most common devise to detect radiation, particularly  -particles. Geiger counter is simply an argon-filled tube with two electrodes. When radiation (  or  collide 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

24 Dr Gihan Gawish Geiger-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.

25 Dr Gihan Gawish Scintillation 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.

26 Dr Gihan Gawish Scintillation Counter

27 Dr Gihan Gawish Autoradiography 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.

28 Dr Gihan Gawish Autoradiography 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.


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