NUCLEAR CHEMISTRY NUCLEONS – The particles found in the nucleus Protons (+) Neutrons (0) Mass Number (A)  Atomic Number (Z)  Hg 80 196 Protons: 80 Neutrons: 196 - 80 = 116 2C-1 (of 15)

STABILITY SERIES STABLE ISOTOPES – Atoms with nuclei that last forever RADIOACTIVE ISOTOPES – Atoms with nuclei that eventually break down to more stable nuclei Isotopes are stable when their nuclei have enough neutrons to minimize proton-proton repulsion (a) For Z < 20 Stable nuclei need 1 neutron per proton (b) For Z > 20 Stable nuclei approach needing 1.5 neutrons per proton 2C-2

Stable 200Hg atom: 120 n, 80 p 1.5:1 ratio Stable 16O atom: 8 n, 8 p (1:1 ratio) 2C-3

NUCLEAR DECAY Each radioactive isotope undergoes nuclear decay at its own unique rate HALF-LIFE (t1/2) – The time required for half of the radioactive isotopes in a sample to decay The shorter the half-life, the more unstable the isotope Half-life for 131I = 60 days At 0 days: At 60 days: At 120 days: At 180 days: At 240 days: 16 125I atoms 8 125I atoms 4 125I atoms 2 125I atoms 1 125I atom 2C-4

Half-life for 14C = 5,730 years At 0 yrs: At 5,730 yrs: At 11,460 yrs: At 17,190 yrs: At 22,920 yrs: 16 14C atoms 8 14C atoms 4 14C atoms 2 14C atoms 1 14C atom How old is an axe with an elk antler sleeve if it has 25% the 14C content of antlers in living elks? 11,460 years old 2C-5

NUCLEAR DECAY (1) ALPHA DECAY (α) – The release of a helium-4 nucleus (4He2+) from a nucleus to become more stable α’s are emitted from radioactive isotopes with A > 200 U → 92 238 He + 2 4 Th 90 234 A and Z are always conserved in nuclear changes Po → 84 207 He + 2 4 Pb 82 203 Alpha particles can be stopped by the outermost layer of skin 2C-6

(2) BETA MINUS DECAY (β-) – The release of an electron from a nucleus to become more stable β-’s are emitted from radioactive isotopes with too many neutrons Essentially a neutron is decaying into a proton plus an electron C → 6 14 e + -1 N 7 14 K → 19 40 e + -1 Ca 20 40 Beta particles penetrate about 1 cm into the body 2C-7

(3) POSITRON DECAY (β+) – The release of an electron with a positive charge from a nucleus to become more stable β+’s are emitted from radioactive isotopes with too many protons Essentially a proton decays into a neutron and an antimatter electron C → 6 11 e + 1 B 5 11 Ca → 20 38 e + 1 K 19 38 2C-8

(4) GAMMA DECAY (γ) – The release of any high energy photon of electromagnetic radiation γ-’s are emitted along with other forms of decay Gamma rays are deeply penetrating 2C-9

Usually very radioactive (6) SPONTANEOUS FISSION – When a large nucleus (Z > 80) breaks into two, approximately equal halves Several neutrons, and lots of energy are released when nuclei fission U → 92 239 Ru + 44 116 Cd + 48 120 3 n 1 U → 92 239 Rh + 45 118 Ag + 47 119 2 n 1 Daughter Products Usually very radioactive 2C-10

U + n → U 235 1 236 92 92 NUCLEAR REACTORS Nuclear reactions release over 100 times more energy than chemical reactions 235U is used as a fuel U + 92 235 n → 1 U 92 236 236U decays by spontaneous fission CHAIN REACTION – When at least one neutron per fission produces a new 236U 2C-11

Not enough neutrons captured for a chain reaction Just enough neutrons captured to maintain a chain reaction So many neutrons captured the chain reaction is an explosion CRITICAL MASS – The minimum amount of 235U needed to support a chain reaction 2C-12

Water – Acts as a MODERATOR to slow down the neutrons, as a COOLANT to keep the reactor core from overheating, and as PROTECTION because it absorbs radiation Cd or B Control Rods – Absorb neutrons to control the rate of the chain reaction Fuel Elements – Metal casings containing 235U Reactor Core 2C-13

San Onofre Nuclear Generating Station Heat from the nuclear fission boils water, and steam turns a turbine, which produces electricity Used up full elements contain radioactive daughter products, which must be disposed of safely 2C-14

FUSION – The combining of small nuclei to produce large nuclei Fusion occurs in stars 4 H → 1 He 2 4 Very high temperatures or pressure are needed to overcome the repulsion of the positive hydrogen nuclei Fusion releases much more energy than fission 2C-15