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Atomic structure (N) (Z) A=Z+N 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. 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 Isotope • Elements are composed of one or more naturally occurring isotopes, which are normally stable. • Some elements (radioactive) isotopes have unstable Isotope Elements in chemistry are represented by a symbol with two numbers * Atomic number = number of protons or electrons * Atomic mass = number of nucleons (protons + neutrons). 12 6 C Change the atomic number = a different element. Isotopes are atoms with identical atomic numbers but different mass numbers 1 2 3 12 13 14 32 31 131 127 1 1 1 6 6 6 15 15 53 53 H HH C C C P P I I 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) 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. 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. 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. 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. Radioactivity • The radiation or radioactivity was later shown to be separable by electric (and magnetic) fields into three different types: 1. Alpha (a); a helium nucleus, He2+, emitted as alpha particle. 2. Beta (b); an electron emitted from the nucleus 3. Gamma (g); radioactivity consisting of high-energy light waves. Radioactivity Half-Life • Nuclear decay is a first order process dN N 0.693 N Ln t t1 / 2 dt N0 • 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. Isotope Particle type Half life 3H b- 12.3 yr 14C b- 5570 yr 32P b- 14 days 22Na b- & g 15 hr 125I g 60 days 131I b- 8 days 238U a >billions yrs Units 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 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, mCi •The two units can be interconverted: – 1 Ci= 3.7 x 1010 Bp or dps. Units of Radiation • Instruments (b or g counters) report radiation as cpm or (count per minute). cpm = dpm * (counting efficiency of machine) dpm= disintegration per minute