Chapter Nineteen

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Chapter Nineteen

• Nuclear Chemistry
• sections 19.1 19.7

2

• Review
• Atoms consist electrons, protons, and neutrons.
• Atoms of elements are distinguished by number of
  protons in nucleus ( atomic number).
• Isotopes of an element have different numbers of
  neutrons but same number of p and e-.
• Isotopes of element react identically, in most
  instances.
• Traditional chemical reactions focus primarily on
  interactions in the outer valence electrons of
  atoms.

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Now we look at reactions that take place in the
nucleus radioactivity Stable nuclides are not
radioactive and are not the subject of this
chapter. Only the less common naturally
occurring and man-made isotopes that are
radioactive 14C, 32P, 3H (tritium), 86Rn, 60Co,
238U Review of isotopes, p, n, e Chapter 2
4

• A nucleus with a specified number of protons and
  neutrons is a nuclide.
• E
• A mass number (number of P number of n)
• Z atomic number
• Together, protons and neutrons are called
  nucleons.

A Z
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Introduction

• Radioactivity, or radioactive decay, is the
  spontaneous change of the nuclei of certain
  atoms, accompanied by the emission of subatomic
  particles and/or high-frequency electromagnetic
  radiation.
• There are five principal ways in which atomic
  nuclei may display radioactivity
• 1. Alpha emission
• 2. Beta emission
• 3. Gamma emission
• 4. Positron emission
• 5. Electron capture

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Types Of Radioactive Decay

• An alpha (?) particle has the same composition as
  a helium nucleus (42He) two protons and two
  neutrons.
• Beta (?-) particles are electrons (-10e).
• Gamma (g) rays are a highly penetrating form of
  electromagnetic radiation (00g).
• Positrons are particles having the same mass as
  electrons but carrying a charge of 1 (10e).
• Electron capture (EC) is a process in which the
  nucleus absorbs an electron from an inner
  electron shell, usually the first or second, with
  the release of an X-ray.

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Nuclear Equations

• Basic principle in writing a nuclear equation
• charge, mass number and atomic number must be
  conserved in a nuclear reaction.
• The two sides of a nuclear equation must have
  the same totals of atomic numbers and mass
  numbers.

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Three Types of Radiation
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Summary Of Decay Types
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Radioactive Decay Series

• Most of the naturally occurring nuclides of the
  lighter elements have stable nuclei they are not
  radioactive.
• Many naturally occurring nuclides with high
  atomic numbers are are radioactive.
• A series of radioactive decays beginning with a
  long-lived radioactive nuclide and ending with a
  non-radioactive one is called a radioactive decay
  series.

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Radioactive Decay Series For Uranium-238
238U start
238U ? 234Th ? 234Pa? 238U ? 234Th ? 226Rn ?
222Ra ? 218Po ? ? ?
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Radioactive Decay Rates

• The radioactive decay law states that the rate of
  disintegration of a radioactive nuclide, called
  the decay rate or activity, A, is directly
  proportional to the number of atoms present.
• Rate of radioactivity decay (A) ? N
• - Radioactive decay is a first-order process.
• - A activity, measured in atom/time or
  disintegration/time, A lt-gt rate
• - ? - decay constant, ? lt-gt k in the rate of
  reaction.
• - N number of atoms of radioisotope present, N
  lt-gt conc.
• Integrated rate law expression
• Nt 0.690
• ln ___ - ? t t1/2 ________
• No ?

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HalfLife (t1/2)

• The half-life (t1/2) of a radioactive nuclide is
  the time required for one-half the nuclei in a
  sample of the nuclide to decay.
• The shorter the half-life t1/2, the larger the
  value of ? (decay constant) and the faster the
  decay proceeds.

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Selected Nuclide Half-lives
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An Example

• Example 19.2
• The nuclide sodium-24 is used to detect
  constrictions and obstructions in the human
  circulatory system. It emits beta particles.
• (a) What is the decay constant, in s-1, for
  sodium-24?
• What is the activity of a freshly synthesized
  1.00-mg (1.00x10-3 g) sample of sodium-24?
• What will be the rate of decay of the 1.00-mg
  sample after one week (168 h)?

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Radiocarbon Dating

• Carbon-14 is formed at a nearly constant rate in
  the upper atmosphere by the bombardment of
  nitrogen-14 with neutrons from cosmic radiation.
  The carbon-14 is eventually incorporated into
  atmospheric carbon dioxide.
• Carbon-14 in living matter decays by ? emissions
  at a rate of about 15 disintegrations per minute
  per gram of carbon.
• When the organism dies, no more carbon-14 is
  integrated into the system.
• The half-life for carbon-14 is 5,730 years. This
  dating method works well if an object is between
  5,000 and 50,000 years old.

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Synthetic Nuclides

• For centuries, alchemists tried - without success
  - to change one element into another alchemy
  turn lead into gold.
• The process of changing one element into another
  is called transmutation.
• Modern scientists have learned to do this.
• Rutherford, in 1919, was able to convert
  nitrogen-14 into oxygen-17 plus some extra
  protons by bombarding the nitrogen atoms with a
  particles. This is a naturally occurring isotope
  of oxygen and is not radioactive.
• 147N 42He ? 178O 11H
• Phosphorous-30 was the first synthetic
  radioactive nuclide.
• Since its discovery, scientists have synthesized
  over a thousand others.

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Transuranium Elements

• In 1940, the first of the transuranium elements
  - elements with a Z gt 92 - was synthesized by
  bombarding uranium-238 nuclei with neutrons. This
  first element is plutonium.
• 23892U 10n ? 23992U
• 23992U ? 23993Np 0-1e
• 23993Np ? 23994Pu 0-1e

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Nuclear Stability

• About 160 stable nuclides have an even number of
  protons and an even number of neutrons.
• About 50 stable nuclides have an even number of
  protons and an odd number neutrons.
• About 50 stable nuclides have an odd number of
  protons and an even number neutrons
• Only four stable nuclides have an odd number of
  protons and an odd number of neutrons.
• The magic numbers of protons or neutrons for
  nuclear stability are 2, 8, 20, 28, 50, 82, and
  126.

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Stability of Nuclides
All the stable nuclides lie within the belt of
stability (as do some radioactive ones).
Nuclides outside the belt are radioactive. Their
modes of radioactive decay are indicated.
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Energetics Of Nuclear Reactions

• While working out the details of the theory of
  special relativity, Einstein derived the equation
  for the equivalence of mass and energy E mc2.
• In a typical spontaneous nuclear reaction, a
  small quantity of matter is transformed into a
  corresponding quantity of energy.
• Nuclear energies are normally expressed in the
  unit MeV (megaelectronvolt).
• 1 u 931.5 MeV one atomic mass unit contains
  energy equivalent to 931.5 megaelectronvolts.
• 1 amu 1 u

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Nuclear Binding Energy

• The energy released in forming a nucleus from its
  protons and neutrons is called the nuclear
  binding energy and is expressed as a positive
  quantity.
• Alternatively, nuclear binding energy is the
  quantity of energy necessary to separate a
  nucleus into individual protons and neutrons.
• This explains why there is a mass loss of 0.0304
  u in the formation of a helium nucleus from the
  two protons and two neutrons which comprise it.
  This quantity is called the mass defect of the
  nucleus.

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Nuclear Binding Energy For Helium
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Average Binding Energies
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Summary

• The five types of radioactive nuclides involve
  emission of alpha (?) particles, beta (?)
  particles, gamma (?) rays, positrons, and
  electron capture.
• All known nuclides with Z gt 83 are radioactive,
  and many of them occur naturally as member of
  four radioactive decay series.
• In the formation of an atomic nucleus from its
  protons and neutrons, a quantity of mass is
  converted into energy.