Chapter 37. Nuclear Chemistry. Copyright (c) 2011 by Michael A. Janusa, PhD. All rights reserved.

37. Radioactivity Radioactive decay is the process in which a nucleus spontaneously disintegrates, giving off radiation. Radiation are the particles or rays emitted. Radiation comes from the nucleus as a result of an alteration in nuclear composition or structure. This occurs in a nucleus that is unstable and hence radioactive. Nuclear symbols are used to designate the nucleus and consists of - Atomic symbol (element symbol) - Atomic number (Z : #protons) - Mass number (A : #protons + #neutrons) A Z E 2

mass number A number of protons and neutrons 5 protons, 6 neutrons 5 B atomic number Z number of protons atomic symbol This symbol is the same as writing boron- and defines an isotope of boron. In nuclear chemistry this is often called a nuclide. This is not the only isotope (nuclide) of boron. 3

Some isotopes are stable The unstable isotopes are the ones that produce radioactivity (emit particles); the actual process of radioactive decay. Radioactive decay is the process in which nucleus spontaneously disintegrates, giving off radiation. 4

Radioactivity Radioactivity was discovered by Antoine Henri Becquerel in 896. The work involved uranium salts which lead to the conclusion that the minerals gave off some sort of radiation. This radiation was later shown to be separable by electric (and magnetic) fields into three types; alpha ( ), beta ( ), and gamma ( ) rays. 5

Radioactivity Alpha rays ( ) bend away from a positive plate indicating they are positively charged. They are known to consist of helium-4 nuclei (nuclei with two protons and two neutrons). Slow moving (relatively large mass compared to other nuclear particles; therefore, moves slow - % speed of light) Stopped by small barriers as thin as few pages of paper. Symbolized in the following ways: 4 2 He 2 4 2 He α 4 2 α 2p, 2n 6

Radioactivity Beta rays ( ) bend in the opposite direction indicating they have a negative charge. They are known to consist of high speed electrons (9% speed of light). Emitted from the nucleus by conversion of neutron into a proton. Higher speed particles; therefore, more penetrating than alpha particles (stopped by only more dense materials such as wood, metal, or several layers of clothing). The symbol is basically equivalent to electron e β β pure electron - 7

Radioactivity Gamma rays ( ) are unaffected by electric and magnetic fields. They have been shown to be a form of electromagnetic radiation (pure energy) similar to x rays, but higher in energy and shorter in wavelength. Alpha and beta radiation are matter; contains p, n, or e while gamma is pure energy (no p, n, e). Highly energetic, the most penetrating form of radiation (barriers of lead, concrete, or more often, a combination is required for protection). Symbol is or 8

37. 2 Nuclear Equations A nuclear equation is a symbolic representation of a nuclear reaction using nuclide symbols. For example, the nuclide symbol for uranium-238 is 238 U 92 92 p, 46 n 9

Nuclear Equations Reactant and product nuclei are represented in nuclear equations by their nuclide symbol. 238 The radioactive decay of 92 by alpha-particle 4 emission (loss of a nucleus) is written 2 He U 238 92 U 234 9 lost 2p & 2n Th 4 2 He

Nuclear Equations Other particles are given the following symbols. Proton Neutron Electron Positron Gamma photon H p n or or e or e

Nuclear Equations In a nuclear equation, you do not balance the elements, instead... the total mass on each side of the reaction arrow must be identical (this means that the sum of the superscripts for the products must equal the sum of the superscripts for the reactants). the sum of the atomic numbers on each side of the reaction arrow must be identical (this means that the sum of the subscripts for the products must equal the sum of the subscripts for the reactants). 2

Alpha Decay 238 92 U 234 9 Th 4 2 He 238 = 234 + 4 239 94 239 mass number 92 = 9 + 2 atomic number Ex. Plutonium 239 emits an alpha particle when it decays, write the balanced nuclear equation. Pu Pu A Z 235 E? U 4 2 4 He He mass A: 239 = 4 + A A = 239 4 = 235 94 = 2 + Z 94 92 2 Z = 94 2 = 92 U Z: 3

Beta Decay 6 7 N 6 8 O - e n p Ex. Protactinium 234 undergoes beta decay. Write the balanced nuclear equation. 234 9 Pa A Z E? - e mass A: 234 = + A A = 234 = 234 234 9 Pa 234 92 U - e Z: 9 = - + Z Z = 9 + = 92 U 4

A Problem To Consider Technetium-99 is a long-lived radioactive isotope of technetium. Each nucleus decays by emitting one beta particle. What is the product nucleus? The nuclear equation is 99 Tc A X 43 Z mass A: 99 = + A A = 99 = 99 5

37.4 Nuclear Structure and Stability Binding Energy - the energy that holds the protons, neutrons, and other particles together in the nucleus. Binding energy is very large for unstable isotopes. When isotopes decay (forming more stable isotopes,) binding energy is released (go to lower E state; more stable arrangement). 8

Important factors for stable isotopesnuclear stability correlates with: Ratio of neutrons to protons in the isotope. Nuclei with large number of protons (84 or more) tend to be unstable. The magic numbers of 2, 8, 2, 5, 82, or 26 help determine stability. These numbers of protons or neutrons are stable. These numbers, called magic numbers, are the numbers of nuclear particles in a completed shell of protons or neutrons. Even numbers of protons or neutrons are generally more stable than those with odd numbers. All isotopes (except H) with more protons than neutrons are unstable. 9

Nuclear Stability Several factors appear to contribute the stability of a nucleus. when you plot each stable nuclide on a graph of neutrons vs. protons, these stable nuclei fall in a certain region, or band. The band of stability is the region in which stable nuclides lie in a plot of number of neutrons against number of protons. 2

Figure : Band of stability. n p more neutrons than needed for stability p n more protons than needed for stability Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, 25. 2

Predicting the Type of Radioactive Decay Nuclides outside the band of stability are generally radioactive. Nuclides to the left of the band have more neutrons than that needed for a stable nucleus. These nuclides tend to decay by beta emission because it reduces the neutron-to-proton ratio. n p emit electron in process more neutrons than needed for stability 22

Predicting the Type of Radioactive Decay Nuclides outside the band of stability are generally radioactive. In contrast, nuclides to the right of the band of stability have a neutron-to-proton ratio smaller than that needed for a stable nucleus. These nuclides tend to decay by positron emission or electron capture because it increases the neutron to proton ratio. p n more protons than needed for stability 23

Predicting the Type of Radioactive Decay Nuclides outside the band of stability are generally radioactive. In the very heavy elements, especially those with Z greater than 83, radioactive decay is often by alpha emission. Lose 2p and 2n - emit alpha particle 24

Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, 25. 25

37.3 Types of Radioactive Decay There are six common types of radioactive decay. Alpha emission (abbreviated ): emission of a 4 nucleus, or alpha particle, from an 2He unstable nucleus. An example is the radioactive decay of radium-226. 226 88 Ra 222 86 Rn Lost 2p & 2n 4 2 He 26

Types of Radioactive Decay There are six common types of radioactive decay. Beta emission (abbreviated or - ): emission of a high speed electron from a unstable nucleus. This is equivalent to the conversion of a neutron to a proton. n p An example is the radioactive decay of carbon-4. 4 4 7 C N 6 e n p 27

Types of Radioactive Decay There are six common types of radioactive decay. Positron emission (abbreviated + ): emission of a positron from an unstable nucleus. This is equivalent to the conversion of a proton to a neutron. p The radioactive decay of technetium-95 is an example of positron emission. p n 95 Tc 95 Mo e 43 42 n e 28

Types of Radioactive Decay There are six common types of radioactive decay. Electron capture (abbreviated EC): the decay of an unstable nucleus by capturing, or picking up, an electron from an inner orbital of an atom. In effect, a proton is changed to a neutron, as in positron emission. p e n An example is the radioactive decay of potassium-4. 4 9 K e 4 8 Ar p n 29

Types of Radioactive Decay There are six common types of radioactive decay. Gamma emission (abbreviated ): emission from an excited nucleus of a gamma photon, corresponding to radiation with a wavelength of about -2 m. In many cases, radioactive decay produces a product nuclide in a metastable excited state. The excited state is unstable and emits a gamma photon and goes to a lower energy state (more stable). The atomic mass and number do not change. An example is metastable technetium-99. 99m Tc 99 Tc 43 43 3

Types of Radioactive Decay There are six common types of radioactive decay. Spontaneous fission: the spontaneous decay of an unstable nucleus in which a heavy nucleus of mass number greater than 89 splits into lighter nuclei and energy is released. For example, uranium-236 undergoes spontaneous fission. 236 92 U 96 39 Y 36 53 I 4 n 3

37.5 Rate of Radioactive Decay The rate of radioactive decay, that is the number of disintegrations per unit time, is proportional to the number of radioactive nuclei in the sample. You can express this rate mathematically as Rate kn t where N t is the number of radioactive nuclei at time t, and k is the radioactive decay constant. 32

Rate of Radioactive Decay All radioactive decay follows first order kinetics as outlined in kinetics chapter. Therefore, the half-life of a radioactive sample is related only to the radioactive decay constant. The half-life, t½,of a radioactive nucleus is the time required for one-half of the nuclei in a sample to decay. The first-order relationship between t ½ and the decay constant k is.693 t 2 k 33

Rate of Radioactive Decay Once you know the decay constant, you can calculate the fraction of radioactive nuclei remaining (N t/ N o ) after a given period of time. Recall the first-order time-concentration equation is ln N N t o kt Or if we don t know k we can substitute k =.693/t ½ and get ln N N t o.693 t t 2 34

A Problem To Consider Phosphorus-32 has a half-life of 4.3 days. What fraction of a sample of phosphorus-32 would remain after 5.5 days? N ln N N ln N t o Fraction or t o 77% nuclei.693 t t 2.693 (5.5d) (4.3 d) remaining remaining N N.267 t o e.267 or Fraction n t t / 2.77 nuclei remaining HW 5 35 2 n

37.4. Nuclear Fission and Nuclear Fusion Nuclear fission is a nuclear reaction in which a heavy nucleus splits into lighter nuclei and energy is released. For example, one of the possible mechanisms for the decay of californium-252 is 252 98 Cf 42 56 Ba 6 42 Mo 4 n 36

Nuclear Fission and Nuclear Fusion In some cases a nucleus can be induced to undergo fission by bombardment with neutrons. n 235 92 U 236 92 U 92 36 Kr 4 56 Ba 3 n energy When uranium-235 undergoes fission, more neutrons are released creating the possibility of a chain reaction. A chain reaction is a self-sustaining series of nuclear fissions caused by the absorption of neutrons released from previous nuclear fissions. 37

Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, 25. Chain reaction - the reaction sustains 38 itself by producing more neutrons

Nuclear Fission and Nuclear Fusion Nuclear fusion is a nuclear reaction in which light nuclei combine to give a stable heavy nucleus plus possibly several neutrons, and energy is released. Such fusion reactions have been observed in the laboratory using particle accelerators. Sustainable fusion reactions require temperatures of about million o C. 39

Nuclear Fission and Nuclear Fusion Fusion (to join together) - combination of two small nuclei to form a larger nucleus. Large amounts of energy is released. Best example is the sun. An Example: 2 H 3 H 4 2 He n energy 4