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Home , Beta decay, Positron emission

Chapter 37

Nuclear Chemistry

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Copyright (c) 2011 by Michael A. Janusa, PhD. All rights reserved. 37.1 Radioactivity is the process in which a nucleus spontaneously disintegrates, giving off . 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 A - Atomic symbol (element symbol) Z E - (Z : #) - number (A : #protons + #)

2 A number of protons and neutrons 11 B 5 protons , 6 neutrons 5

atomic symbol atomic number Z number of protons

• This symbol is the same as writing -11 and defines an of boron. • In this is often called a . • This is not the only isotope (nuclide) of boron. 3

• Some 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 in 1896.

– The work involved salts which 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 -4 nuclei (nuclei with two protons and two neutrons). – Slow moving (relatively large mass compared to other nuclear particles; therefore, moves slow - 10% ) – Stopped by small barriers as thin as few pages of paper. – Symbolized in the following ways: 2p, 2n 4 2 4 4 2 He 2He α 2α

6 Radioactivity – Beta rays ( ) bend in the opposite direction indicating they have a negative charge. • They are known to consist of high speed (90% speed of light). • Emitted from the nucleus by conversion of into a . • Higher speed particles; therefore, more penetrating than alpha particles (stopped by only more dense materials such as wood, , or several layers of clothing). • The symbol is basically equivalent to 0 0 1e -1β 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 0 or 0 8 37. 2 Nuclear Equations

• A nuclear equation is a symbolic representation of a using nuclide symbols.

– For example, the nuclide symbol for uranium-238 is

238 92 U 92 p, 146 n

9 Nuclear Equations

– Reactant and product nuclei are represented in nuclear equations by their nuclide symbol.

238 – The radioactive decay of 92 U by alpha-particle 4 emission (loss of a 2 He nucleus) is written

238 234 4 92 U 90Th 2He lost 2p & 2n

10 Nuclear Equations

– Other particles are given the following symbols.

1 1 Proton 1 H or 1p 1 Neutron 0 n 0 0 Electron 1 or 1e 0 0 1 or 1e 0 Gamma photon 0

11 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).

12 238 234 4 92 U 90Th 2He 238 = 234 + 4 mass number 92 = 90 + 2 atomic number Ex. 239 emits an when it decays, write the balanced nuclear equation. mass A: 239 A 4 239 = 4 + A 94 Pu ZE? 2He A = 239 – 4 = 235 Z: 239 Pu 235U 4He 94 = 2 + Z 94 92 2 Z = 94 – 2 = 92  U 13

16 16 0 n  p 7 N 8 O -1e

Ex. 234 undergoes beta decay. Write the balanced nuclear equation.

234 A 0 mass A: 91 Pa ZE? -1e 234 = 0 + A A = 234 – 0 = 234 234 234 0 Z: 91 Pa 92 U -1e 91 = -1 + Z Z = 91 + 1 = 92  U

14 A Problem To Consider

• -99 is a long-lived radioactive isotope of technetium. Each nucleus decays by emitting one . What is the product nucleus? – The nuclear equation is 99 A 0 43Tc Z X 1 mass A: 99 = 0 + A A = 99 – 0 = 99 15 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 A 0 43Tc Z X 1 Z: 43 = -1 + Z Z = 43 + 1 = 44  Ru

16 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 A 0 43Tc Z X 1 – Hence A = 99 and Z = 44 (), so the product is 99 44 Ru

17 37.4 and Stability

• - 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).

18 • Important factors for stable isotopes- nuclear 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, 20, 50, 82, or 126 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 1H) with more protons than neutrons are unstable. 19 Nuclear Stability

• Several factors appear to contribute the stability of a nucleus.

– when you plot each 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 lie in a plot of number of neutrons against number of protons.

20 n  p Figure : more neutrons than Band of stability. needed for stability

p  n more protons than needed for stability

21 Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, 2005. 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 or 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, 2005.

25 37.3 Types of Radioactive Decay

• There are six common types of radioactive decay.

– Alpha emission (abbreviated ): emission of a 4 He nucleus, or alpha particle, from an unstable2 nucleus.

– An example is the radioactive decay of -226. 226 222 4 88 Ra 86 Rn 2He Lost 2p & 2n 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. 1 1 0 0 n 1p 1e

– An example is the radioactive decay of carbon-14. 14 14 0 n p 6 C 7 N 1

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. 1 1 0 1p 0 n 1e

– The radioactive decay of technetium-95 is an example of positron emission.  95 95 0 p n 43Tc 42 Mo 1e

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 . – In effect, a proton is changed to a neutron, as in positron emission. 1 0 1 1p 1e 0 n – An example is the radioactive decay of -40. p n 40 0 40 19 K 1e 18 Ar 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 10-12 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 and number do not change. – An example is metastable technetium-99. 99m 99 0 43Tc 43Tc 0 30 Types of Radioactive Decay

• There are six common types of radioactive decay. – : 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 96 136 1 92 U 39Y 53I 40 n

31 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 kNt

where Nt 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 t½ and the decay constant k is 0.693 t 1 2 k 33 Rate of Radioactive Decay • Once you know the decay constant, you can calculate the fraction of radioactive nuclei

remaining (Nt/No) after a given period of time.

– Recall the first-order time-concentration equation is N ln t kt No – Or if we don’t know k we can substitute k = 0.693/t½ and get N 0.693 t ln t N t 1 o 2 34 A Problem To Consider • Phosphorus-32 has a half-life of 14.3 days. What fraction of a sample of phosphorus-32 would remain after 5.5 days? or 1 N 0.693 t Fraction nuclei remaining ln t 2n N t 1 t o 2 n t N 0.693 (5.5d) 1/ 2 ln t 0.267 No (14.3 d) N Fraction nuclei remaining t e 0.267 0.77 No or 77% remaining HW 51 35 37.4.1 and

• 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 -252 is

252 142 106 1 98Cf 56 Ba 42Mo 40 n

36 Nuclear Fission and Nuclear Fusion

– In some cases a nucleus can be induced to undergo fission by bombardment with neutrons.

1 235 236 92 141 1 0 n 92 U 92 U 36 Kr 56 Ba 30 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, 2005.

• Chain reaction - the reaction sustains itself by producing more neutrons 38 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 100 million oC.

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 3 4 1 1 H 1H 2He 0n energy

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