Radioactivity & Nuclear Chemistry

OAKTON Community College 101 • Introductory Chemistry Dr. Maria Yermolina Radioactivity & Nuclear Chemistry

Depicting Atoms Depicting Atom Basic Chemistry I Lecture 3, September 14

Isotopes Isotopes Basic Chemistry I Lecture 3, September 14

Skillbuilder Skillbuilder A. Basic Chemistry I Lecture 3, September 14 5

Skillbuilder Skillbuilder A. Basic Chemistry I Lecture 3, September 14

Skillbuilder Skillbuilder How do protons and neutrons are held together so tightly in the nucleus? The strong nuclear force acts between two nucleons: Four Fundamental Forces: the gravitational and electromagnetic interactions, which produce significant long-range forces whose effects can be seen directly in everyday life, and the strong and weak interactions, which produce forces at subatomic distances and govern nuclear interactions.

The Strong Nuclear Force Skillbuilder How do protons and neutrons are held together so tightly in the nucleus? The strong nuclear force acts between two nucleons: > 10-14 m

A Multinucleon Nucleus Skillbuilder • Protons and neutrons are held together less tightly in large nuclei. The circle shows the range of the attractive strong force. • Small nuclei have few protons, so the repulsive force on a proton due to the other protons is small. (A) A proton on the surface is attracted by the six or seven nearest nucleons. • In large nuclei, the attractive strong force is exerted only by the nearest neighbors, but all the protons exert repulsive forces. The total repulsive force is large. (B)

Radioactivity Skillbuilder If a strong force is not large enough (atoms with more than 83 protons) to hold a nucleus together tightly, the nucleus can decay and give off matter and energy to adjust the neutron-proton imbalance. Radioactivity (or radioactive decay): a spontaneous process of nuclei undergoing a change by emitting particles or rays. Nuclide (or radionuclide): nuclei that undergo spontaneous decay (=disintegration). Example: 238U or 14C. Discovered by Henri Becquerel in 1896. Pier and Marie Curie discovered Po, U and Ra. Nobel Prize in 1903.

Three Components of Radiation Skillbuilder • An electric field separates the rays from a radioactive source or a heavy nuclides, such as uranium, into: • alpha (a) particles = positively charged helium nuclei, 42He; • beta (b) particles = negatively charged electrons; • neutral gamma (g) rays = high –energy electromagnetic radiation A ➝ B + b General Nuclear Equation: A = the parent nucleus, B = the daughter nucleus, b = the emitted particle or ray

General Terms • Nuclide: a nucleus with a specified number of neutrons (almost synonymous with “isotope”, “isotone”, “isobar”, etc) • Refers more to the “thing” rather than the “type of matter” • Radioactive Nuclide: a nuclide that undergoes a spontaneous nuclear decay process • With a corresponding release of some energetic particle (or photon) • Radiation: general (historic) term for the kind of energetic particles (or photons) that are emitted from a sample containing radioactive nuclides. Many kinds: • Alpha, beta, gamma, positron • Stable Nuclide*: a nuclide that does not undergo any spontaneous nuclear decay process. *more on “stable” later

Kinds of Nuclear Reactions Not all nuclear reactions are decay! • Spontaneous Nuclear Decay (discussed first) • Radioactive nuclides (only) • one (“reactant”) nuclide turns into another nuclide • not initiated (just happens) • Other nuclear reactions (later) • Generally involve initiation & more than one nuclide as “reactants”. • Fission and Fusion • Transmutation (bombardment) reactions

a Emission Skillbuilder Alpha emission: the expulsion of an alpha particle from an unstable, disintegrating nucleus; travels from 2 to 12 cm through the air, depending on the NRG source; easily stopped by a sheet of paper close to the nucleus. Notice: • The total charge is conserved during a nuclear reaction. • This means that the sum of the subscripts for the products must equal the sum of the subscripts for the reactants.

b Emission Skillbuilder 2. Beta emission: ejecting electron at a high speed; increasing the amount of protons in a nucleus; more penetrating than a particles; may travel several hundreds cm thought the air; stopped by a thin layer of metal. + Transmutation: the process of changing one element to another through nuclear decay.

g Emission Skillbuilder 3. Gamma emission: a high-NRG burst of electromagnetic radiation (photon) from an excited nucleus so it can return to a lower NRG state; can completely pass a person but can be stopped by a 5 cm thick piece of lead or concrete close to the source; no change in the number of nucleons. or

Positron Emission Skillbuilder 4. Positron emission: occurs when an unstable nucleus emits a positron; positron is the antiparticle of the electron: the same mass but opposite charge. If a positron collides with an electron, the two particles destroy each other, releasing energy in the form of gamma rays. In positron emission, a proton is converted into a neutron and emits a positron:

Electron Capture Skillbuilder 5. Electron Capture: involves a particle being absorbed by instead of emitted from an unstable nucleus. Electron capture occurs when a nucleus assimilates an electron from an inner orbital of its electron cloud. Like positron emission, the net effect of electron capture is the conversion of a proton into a neutron.

Summary: Modes of Radioactive Decays Skillbuilder

Summary: Modes of Radioactive Decays Overview: Table 20.1 in Tro (2nd part)

Skillbuilder What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued ? + = proton 5 p+ 4 n0 = neutron = positron Positron emission giving a daughter nuclide with four protons and five neutrons

Skillbuilder What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued ? + = proton 9 p+ 12 n0 = neutron = electron b emission giving a daughter nuclide with 10 protons and 11 neutrons

Skillbuilder What Kind of Decay and How Many Protons and Neutrons Are in the Daughter? ? + = proton 11 p+ 9 n0 = neutron a emission giving a daughter nuclide with nine protons and seven neutrons

Skillbuilder Write a nuclear equation for each of the following: alpha emission from U–238 beta emission from Ne–24 positron emission from N–13 electron capture by Be–7 Tro: Chemistry: A Molecular Approach

Band of Stability Skillbuilder What does stable mean? Stable means atom does not undergo radioactive decay. a-emission As the number of protons increase, the neutron-to proton ratio of the stable nuclei also increases in a band of stability: the region in which stable nuclides lie in a plot of number of protons against number of neutrons. When you plot each stable nuclide on a graph of protons vs. neutrons, stable nuclei fall in a certain region, or band. Nuclei outside this band of stability are radioactive. b-emission

Band of Stability Skillbuilder a-emission for Z = 1 -20, stable N/Z ≈ 1 b-emission for Z = 20 -40, stable N/Z approaches 1.25 for Z = 40 -80, stable N/Z approaches 1.5 for Z > 83, there are no stable nuclei

Band of Stability Skillbuilder Number of Stable Nuclides with Even and Odd Numbers of Nucleons Even numbers (of nucleons) appear to correlate with stability. Theory of nucleon energy levels is beyond the scope of this course.

The stable nuclides lie in a very narrow band of neutron-to-proton ratios. Band of Stability • The ratio of neutrons to protons in stable nuclides gradually increases as the number of protons in the nucleus increases. • Light, stable nuclides, such as 12C, contain about the same number of neutrons and protons. • Heavy, stable nuclides, such as 204Hg, contain up to 1.55 times as many n’s as p’s. • There are no stable nuclides with atomic numbers larger than 83 (209Bi is last stable one). • This narrow band (valley) of stable nuclei is surrounded by a sea of instability. • Nuclei that lie below the valley don't have enough neutrons and are therefore neutron-poor. They tend to decay via P.E. or E.C. • Nuclei that lie above the valley have too many neutrons and are therefore neutron-rich. They tend to decay via b-decay • Nuclei that lie “beyond” the valley have too much of both. They tend to decay by a-decay

The stable nuclides lie in a very narrow band of neutron-to-proton ratios. Radioactive Decay Series • The ratio of neutrons to protons in stable nuclides gradually increases as the number of protons in the nucleus increases. • Light, stable nuclides, such as 12C, contain about the same number of neutrons and protons. • Heavy, stable nuclides, such as 204Hg, contain up to 1.55 times as many n’s as p’s. • There are no stable nuclides with atomic numbers larger than 83 (209Bi is last stable one). • This narrow band (valley) of stable nuclei is surrounded by a sea of instability. • Nuclei that lie below the valley don't have enough neutrons and are therefore neutron-poor. They tend to decay via P.E. or E.C. • Nuclei that lie above the valley have too many neutrons and are therefore neutron-rich. They tend to decay via b-decay • Nuclei that lie “beyond” the valley have too much of both. They tend to decay by a-decay

Band of Stability 40K half-life 1.3 x 109yr NOTE: The farther away a nuclide is from the valley of stability, the shorter its half life. “Farther = less (kinetically) stable” 261Db half-life 27 s 15C half-life 2.449 s 214Po half-life 164 ms http://en.wikipedia.org/wiki/File:Isotopes_and_half-life.svg

Rules of Nuclear Stability Skillbuilder All isotopes with an atomic number greater than 83 have an unstable nucleus. Isotopes that contain 2, 8, 20, 28, 50, 82, or 126 protons or neutrons in their nuclei are more stable than those with other numbers of protons and neutrons. Pairs of protons and pairs of neutrons have increased stability, so isotopes that have nuclei with even numbers of both protons and neutrons are generally more stable than those that have nuclei with odd numbers of both protons and neutrons. Isotopes with an atomic number less than 83 are stable when the ratio of neutrons to protons in the nucleus is about 1:1 in isotopes with up to 20 protons, but the ratio increases in larger nuclei in a band of stability. Isotopes with a ratio to the left or right of this band are unstable and thus will undergo radioactive decay. Hg 200 80

Rules of Nuclear Stability Skillbuilder All isotopes with an atomic number greater than 83 have an unstable nucleus. Isotopes that contain 2, 8, 20, 28, 50, 82, or 126 protons or neutrons in their nuclei are more stable than those with other numbers of protons and neutrons. Pairs of protons and pairs of neutrons have increased stability, so isotopes that have nuclei with even numbers of both protons and neutrons are generally more stable than those that have nuclei with odd numbers of both protons and neutrons. Isotopes with an atomic number less than 83 are stable when the ratio of neutrons to protons in the nucleus is about 1:1 in isotopes with up to 20 protons, but the ratio increases in larger nuclei in a band of stability. Isotopes with a ratio to the left or right of this band are unstable and thus will undergo radioactive decay. O He Ni 58 28 16 8 4 2 118 50 Sn

Rules of Nuclear Stability Skillbuilder All isotopes with an atomic number greater than 83 have an unstable nucleus. Isotopes that contain 2, 8, 20, 28, 50, 82, or 126 protons or neutrons in their nuclei are more stable than those with other numbers of protons and neutrons. Pairs of protons and pairs of neutrons have increased stability, so isotopes that have nuclei with even numbers of both protons and neutrons are generally more stable than those that have nuclei with odd numbers of both protons and neutrons. Isotopes with an atomic number less than 83 are stable when the ratio of neutrons to protons in the nucleus is about 1:1 in isotopes with up to 20 protons, but the ratio increases in larger nuclei in a band of stability. Isotopes with a ratio to the left or right of this band are unstable and thus will undergo radioactive decay. Co 60 27

Rules of Nuclear Stability Skillbuilder All isotopes with an atomic number greater than 83 have an unstable nucleus. Isotopes that contain 2, 8, 20, 28, 50, 82, or 126 protons or neutrons in their nuclei are more stable than those with other numbers of protons and neutrons. Pairs of protons and pairs of neutrons have increased stability, so isotopes that have nuclei with even numbers of both protons and neutrons are generally more stable than those that have nuclei with odd numbers of both protons and neutrons. Isotopes with an atomic number less than 83 are stable when the ratio of neutrons to protons in the nucleus is about 1:1 in isotopes with up to 20 protons, but the ratio increases in larger nuclei in a band of stability. Isotopes with a ratio to the left or right of this band are unstable and thus will undergo radioactive decay. Ca C 40 20 14 6 T 3 1 radioactive not radioactive

Skillbuilder Skillbuilder Predict if the the following nuclei are radioactive or stable. Give your reasoning behind each prediction. 40 K Radioactive, having an odd number of protons (19) and an odd number of neutrons (21). 24 Ne Stable, because even numbers of protons and neutrons are usually stable. 128 Pb Stable, because there are even numbers of protons and neutrons and because 82 is a particularly stable number of nucleons 214 Pu Radioactive, an atomic number greater than 83.

Skillbuilder Skillbuilder Predict if the the following nuclei are radioactive or stable. Give your reasoning behind each prediction. 25 Al Radioactive, have an odd number of protons (13) and an even number of neutrons (12), not 1:1ratio. 95 Tc Stable, have an odd number of protons (43) and an even number of neutrons (52) in 1:1.25 ratio, falls into a band of stability. 120 Sn Stable, because there are even numbers of protons and neutrons and because 50 is a particularly stable number of nucleons 200 Hg Stable, because even numbers of protons and neutrons are usually stable.

Summary of Strategy for Predicting Decay Type • First determine if “above, below, or beyond” the valley of stability: • If Z > 83, it is “Beyond” • Not always “correct”, but correct prediction • If Z ≤ 83, Figure out if the nuclide has: • “too many neutrons” (“Above”) OR • “too few neutrons” (“Below”) • (NOTE: long way or shortcut way*; even if you use shortcut, be able to relate it to the n/p ratio!) • Then make conclusion by noting which process makes daughter closer to the “valley” *Discussed later

Summary of Strategy for Predicting Decay Type • It turns out that… A radioactive nuclide tends to decay in such a way that its daughter nuclide is closer to the valley of stability • b decay: turns n to p  used by nuclides abovevalley (“neutron rich”) • PE or EC: turns p to n  used by nuclides below valley (“neutron poor”) • a decay: lose both n and p  used by nuclides beyondvalley (too many of both)

Summary of Strategy for Predicting Decay Type How to determine if a nuclide is “above”, “below”, or “beyond”? • Long way: • Calculate n/p (=N/Z) ratio • Compare actual n/p ratio to ~stable n/p ratio: • Know that for Z = 1-20, n/p = ~1 is stable • Know that for Z = ~80, n/p = ~1.5 is stable • Know that for Z = ~40, n/p ~1.25 is stable Example: 114Ag, 60Co, 21Na, 226Ra, 59Fe? • Short way: (next slide) (if you don’t have a valley of stability table)

Half -Life Skillbuilder The rate of radioactive decay is usually described in term of its half-life: time required for one-half of the unstable nuclei to decay. - different for different isotopes - measured in fractions of seconds or minutes, hours, days, months, years, or billions of years.

Half -Life Skillbuilder • The rate of radioactive decay is usually described in term of its half-life: time required for one-half of the unstable nuclei to decay. • - different for different isotopes • - measured in fractions of seconds or min, hrs, days, months, yrs, or billions of yrs. • Rate of Radioactive Decay • first-order kinetics, so the rate of decay in a particular sample is directly proportional to the number of nuclei present: • rate =kN • Nis the number of radioactive nuclei and kis the rate constant. Therefore, t1⁄2 t1/2 = 0.693/k

Skillbuilder Skillbuilder The half-life of iodided-131 is 8 days. How much of a 1.0 oz sample of iodine-131 will remain after 32 days? Solution: 1. 32 days is four half-lives. 32/8 = 4 2. After the first half life (8 days), 1/2 oz will remain. 3. After the second half-life (8 + 8, or 16 days), 1/4 oz will remain. 4. After the third half-life (8 + 8 + 8, or 24 days), 1/8 oz will remain. 5. After the fourth half-life (8 + 8 + 8 + 8, or 32 days), 1/16 fraction will remain. 6. What is 1/16 of 1.0 oz? It’s 1.0 oz x 1/16 = 6.3 × 10–2 oz.

Skillbuilder Skillbuilder The half-life of zinc-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed? Solution: 1. 7.2 / 2.4 = 3 half-lives 2. (1/2)3 = 0.125 fraction (the amount remaining after 3 half-lives) 3. 100.0 g x 0.125 = 12.5 g remaining

Skillbuilder Skillbuilder Osmium-182 has a half-life of 21.5 hours. How many grams of a 10.0 gram sample would have decayed after exactly three half-lives? Solution: (1/2)3 = 0.125 fraction (the amount remaining after 3 half-lives) 10.0 g x 0.125 = 1.25 g remain 3. 10.0 g - 1.25 g = 8.75 g have decayed Note that the length of the half-life played no role in this calculation. In addition, note that the question asked for the amount that decayed, not the amount that remaining!

The Integrated Law Skillbuilder

t1/2 k + m0, t mt Skillbuilder If you have a 1.35 mg sample of Pu–236, calculate the mass that will remain after 5.00 yrs Given: Find: mass Pu–236 = 1.35 mg, t = 5.00 yr, t1/2 = 2.86 yr mass remaining, mg Conceptual Plan: Relationships: Solve:

t1/2 k + m0, t mt Skillbuilder Rn–222 is a gas that is suspected of causing lung cancer as it leaks into houses. It is produced by uranium decay. Assuming no loss or gain from leakage, if there is 10.24 g of Rn–222 in the house today, how much will there be in 5.4 weeks? ( Rn–222 half-life is 3.8 Days) Given: Find: mass Rn–222 = 10.24 g, t = 5.4 wks, t1/2 = 3.8 d mass remaining, g Conceptual Plan: Relationships: Solve:

Background Radioactivity Skillbuilder Background radioactivity: the ionization radiation present in the environment, to which we all are exposed, even in the absence of an actual radioactive source. Originates from a variety of natural sources: soil, rocks, water, and vegetation from which it is inhaled or ingested into the body - internal exposure. Or external exposure: radioactive materials that remains outside the body and from cosmic radiation from space. For example, about one out of every trillion carbon atoms is 14C, which emits a b particle when it decays. With each breath, you inhale about 3 million 14C atoms.

Radioactive Dating Skillbuilder Isotopes = nuclear “clocks” Radiocarbon dating (also referred to as carbon dating or C-14 dating) is a method for determining the age of an object containing organic material by using the properties of radioacarbon, a radioactive isotope (half-life = 5730 yr) 14C is constantly formed: 14C is constantly decays: Basic Chemistry I Lecture 3, September 14

Radioactive Dating Skillbuilder

Radioactivity & Nuclear Chemistry