Chapter 23 Nuclear Chemistry Objectives: Determine the half-life of a radioactive isotope from a decay graph. Recall the different radiation types and their penetration abilities. Carry out balancing of nuclear reactions. Interpret the N/Z ratio importance and how an atom might decay to reach a stable configuration from the band of stability chart. Determine the age of a sample given the half-life and the activity % of an isotope.

Atomic Composition Protons Electrons Neutrons + electrical charge mass = 1.672623 x 10-24 g relative mass = 1.007 atomic mass units (amu) Electrons negative electrical charge relative mass = 0.0005 amu Neutrons no electrical charge mass = 1.009 amu

Isotopes Atoms of the same element (same Z) but different mass number (A). Boron-10 (10B) has 5 p and 5 n: 105B Boron-11 (11B) has 5 p and 6 n: 115B 10B 11B

Radioactivity One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of ______________(1876-1934). She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces. Ernest Rutherford found Ra forms Rn gas when emitting an alpha particle. 1902—Rutherford and Soddy proposed radioactivity is the result of the natural change of the isotope of one element into an isotope of a different element.

Types of radiation

Penetrating ability

Nuclear Reactions Alpha emission Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2. Nucleons are rearranged but conserved

Nuclear Reactions Beta emission Note that mass number (A) is unchanged and atomic number (Z) goes up by 1. How does this happen?

Radioactive Decay Series A radioactive isotope (parent) is found to decay to form a product that is also radiactive (daughter).

Other types of nuclear reactions Positron (0+1b): a positive electron 207 K-capture: the capture of an electron from the first (inermost) or K shell An electron and proton combine to form a neutron. 0-1e + 11p --> 10n

Origin of the Elements The Big Bang Theory In the first moments there were only 2 elements: _______________ and __________________. H is 88.6% of all atoms He is 11.3% of all atoms H + He = 99.9% of all atom & 99% of mass of the universe. This tells us about the origin of the elements, and so does the existence of isotopes.

Stability of Nuclei Very few isotopes are stable. Up to calcium (Z=20), stable isotopes often have equal numbers of _____________ and _______________. As the mass increases, elements have larger number of ___________. Beyond bismuth (Z=83 with 126 neutrons) all isotopes are unstable and __________________.

Isotopes Iron Hydrogen: 11H, protium 21H, deuterium 5426Fe, 5.82% abundant 5626Fe, 91.66% abundant 5726Fe, 2.19% abundant 5826Fe, 0.33% abundant Hydrogen: 11H, protium 21H, deuterium 31H, tritium (radioactive) Helium, 42He Lithium, 63Li and 73Li Boron, 105B and 115B

Isotopes Except for 11H the mass number is always at least 2 x atomic number. Repulsive forces between protons must be moderated by ____________. Heaviest naturally occurring non-radioactive isotope is 209Bi with _________ protons and _________neutrons There are 83 x 126 = 10,458 possible isotopes. Why so few actually exist?

Isotopes Up to Z = 20 (Ca), n = p (except for 73Li, 115B, 199F) Beyond Ca, n > p (A > 2 Z) Above Bi all isotopes are radioactive. Fission leads to smaller particles, the heavier the nucleus the greater the rate. Above Ca: elements of EVEN Z have more isotopes and most stable isotope has EVEN N.

Stability of Nuclei Out of > 300 stable isotopes: N Even Odd Z 157 52 3115P Even Odd 50 5 21H, 63Li, 105B, 147N, 18073Ta 199F

Stability of Nuclei Even Odd Z N 157 52 50 5 Suggests some PAIRING of NUCLEONS There are “nuclear magic numbers” 2 He 28 Ni 8 O 50 Sn 20 Ca 82 Pb

Band of Stability and Radioctive Decay 24395Am --> 42a + 23993Np a emission reduces Z b emission increases Z 6027Co --> 0-1b + 6028Ni Isotopes with low n/p ratio, below band of stability decay, decay by _____________________or ______________________

Binding Energy Eb is the energy required to separate the nucleus of an atom into _____________ and _____________. For deuterium, 21H ---> 11p + 10n Eb per nucleon = Eb/2 nucleons Mass of 21H = 2.01410 g/mol Mass of proton = 1.007825 g/mol Mass of neutron = 1.008665 g/mol ∆m = 0.00239 g/mol From Einstein’s equation: Eb = (∆m)c2 = 2.15 x 108 kJ/mol Eb per nucleon = Eb/2 nucleons = 1.08 x 108 kJ/mol

Binding Energy

Half-Life HALF-LIFE is the time it takes for _______________________________________. The rate of a nuclear transformation depends only on the “reactant” concentration. Concept of HALF-LIFE is especially useful for 1st order reactions.

Half-Life Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives? Do: mass * (t1/2)^number of half-lives

Kinetics Activity (A) = Disintegrations/time = (k)(N) where N is the number of atoms Decay is first order, and so ln (A/Ao) = -kt The half-life of radioactive decay is t1/2 = 0.693/k

Copper acetate containing 64Cu is used to study brain tumors Copper acetate containing 64Cu is used to study brain tumors. This isotope has a half-life of 12.7 h. If you begin with 25.0 mg of 64Cu , what mass in micrograms remains after 64 h?

Radiocarbon dating Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation 14N + 1on ---> 14C + 1H The C-14 is oxidized to CO2, which circulates through the biosphere. When a plant dies, the C-14 is not replenished. But the C-14 continues to decay with t1/2 = 5730 years. Activity of a sample can be used to date the sample.

A piece of charred bone found in the ruins of a Native American village has a 14C:12C ratio that is 72% of the radio found in living organisms. Calculate the age of the bone fragment.

Artificial Nuclear Reactions New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B. Reactions using neutrons are called n, g reactions because a g ray is usually emitted. Radioisotopes used in medicine are often made by n, g reactions. Example of a n,g reaction is production of radioactive 31P for use in studies of P uptake in the body. 3115P + 10n ---> 3215P + g

Balance the equations below (both mass and charge to be conserved in a nuclear reaction):

Transuranium Elements Elements beyond 92 (transuranium) made starting with an n,g reaction 23892U + 10n ---> 23992U + g 23992U ---> 23993Np + 0-1b 23993Np ---> 23994Np + 0-1b 106Sg

109Mt Nuclear Fission Fission chain has three general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron) 2. Propagation. 236U fission releases neutrons that initiate other fissions 3. Termination. 109Mt

Nuclear Fission and Power Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. 17% of the world’s energy comes from nuclear.

Radiation Units Curie: 1 Ci = ___________ distintegrations/s SI unit is the becquerel: 1 Bq = __________ Rad: measures amount of energy absorbed 1 rad = __________J absorbed/kg tissue Rem: based on rad and type of radiation. Quantifies biological tissue damage Usually use “millirem”

Effects of Radiation

Nuclear Medicine: Imaging Technetium-99m is used in more than 85% of the diagnostic scans done in hospitals each year. Synthesized on-site from ___________. 9942Mo ---> 99m43Tc + 0-1b 99m43Tc decays to 9943Tc giving off g ray. Tc-99m contributes in sites of high activity.

Nuclear Medicine: Imaging Imaging of a heart using Tc-99m before and after exercise.

BNCT: Boron Neutron Caption Therapy 10B isotope (not 11B) has the ability to capture slow neutrons In BNCT, tumor cells preferentially take up a boron compound, and subsequent irradiation by slow neutrons kills the cells via the energetic 10B --> 7Li neutron capture reaction (that produces a photon and an alpha particle) 10B + 1n ---> 7Li + 4He + photon

Food Irradiation Food can be irradiated with g rays from 60Co or 137Cs. Irradiated milk has a shelf life of 3 mo. without refrigeration. USDA has approved irradiation of meats and eggs.

End of Chapter Go over all the contents of your textbook. Practice with examples and with problems at the end of the chapter. Practice with OWL tutor. Practice with the quiz on CD of Chemistry Now. Work on your OWL assignment for Chapter 23.