Nuclear Reactions Nuclear Reactions involve the nucleus of atoms When a nuclear reaction occurs, the element is changed completely into another element As a result, nuclear reactions are also known as Transmutations

The Nucleus d  cm (d  cm for an atom)

The Nucleons mass (AMU)charge (esu) proton1+1 neutron10

In case you forgot... many elements have several isotopes U U protons 143 neutrons 92 protons 146 neutrons Identical chemistries, different nuclear reactions

Chemical vs. Nuclear Energies CH 4(g) + 2 O 2(g)  CO 2(g) + 2 H 2 O (g)  H = -896 kJ/mol = -56 kJ/g 235 U nuclear fission other nuclei  H = -8.2 x 10 7 kJ/g

Why do nuclear reactions occur? Naturally, some atoms are stable while others are unstable Usually, when the ratio of neutron:proton is greater than 1.3, the nucleus is unstable Examples, 236 U, 209 Po, 14 C, 230 Th An unstable nucleus is radioactive and naturally emits certain radiations and is converted to a more stable isotope

proton (p) neutron (n) the nuclear force Forces in the Nucleus electrostatic repulsion

Nuclear Stability protons (Z) neutrons (N) N/Z= “Island of Stability”   decay    EC decay  decay neutrons needed for stability

Determine if any of the following isotopes is stable or unstable Isotope Protons (P) Neutrons (N) N/P ratioStable or unstable 218 Po 14 C 214 Pb 206 Pb

Types of Radiation There are 4 main types of radiation: RadiationChargeMassSymbol Alpha rays (  -particles) He Beta rays (  -particles)  Gamma rays (  -rays) 00 0000 Positrons+10 O +1 

The Discovery of Radioactivity U - +    ++

 Particles positively charged massive accurate measurements  4 He nuclei 2 protons 2 neutrons

Emission of  particles Some unstable nuclide decay by emitting only  -particles Examples: Ra → Rn He Po → Pb He Th → Ra He Using Table N, identify 4 nuclide that undergo alpha decay and write the nuclear equation

 Decay U He Th nucleons are conserved (238) charge is conserved (92) identities of atoms are not!

Emission of  particles Some unstable nuclide decay by emitting only  -particles Examples: U → Np  Np → Pu  14 6 C → 14 7 N  Using Table N, identify 4 nuclide that undergo beta decay and write the nuclear equation

TABLE N Selected Radioisotopes

  Particles negatively charged small mass accurate measurements  electrons

  Decay Th e Pa i.e. a neutron is turned into a proton + an electron 90 p 144 n 91 p 143 n

Electron Capture Fe + e Mn 26 p 29 n 25 p 30 n i.e. a proton captures an electron and is turned into a neutron

Electron Capture (EC)

Decay Series A  B  C  D ...  Ž non-radioactive nuclide There are three such series: A = 238 U, A = 232 Th, A = 235 U and

238 U Decay Series 238 U Ž 234 Th Ž 234 Pa Ž 234 U Ž 230 Th     226 Ra  222 Rn (g) Œ  218 Po Œ  214 Pb Œ   214 Bi Ž 214 Po Ž 210 Pb Ž 210 Bi     210 Po 206 Pb Œ  non-radioactive 

Nuclear Reactions A + B Ž C [mass (A) + mass (B)]  mass (C) e.g. Fe p + 30 n ŽFe find mass before and after reaction

Nuclear Reactions mass before = 26 m p + 30 m n = 26( amu) + 30( amu) = amu mass after = mass 56 Fe atom - 26 m e = amu - 26( amu) = amu “mass defect” = amu

Nuclear Reactions The mass is converted to energy!

"It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing -- a somewhat unfamiliar conception for the average mind. Furthermore, the equation E is equal to m c-squared, in which energy is put equal to mass, multiplied by the square of the velocity of light, showed that very small amounts of mass may be converted into a very large amount of energy and vice versa. The mass and energy were in fact equivalent, according to the formula mentioned before. This was demonstrated by Cockcroft and Walton in 1932, experimentally."

Nuclear Reactions The mass is converted to energy!  E =  mc 2  m = amu / 56 Fe nucleus = g/mol = x kg/mol c = 3.00 x 10 8 m/s

Nuclear Reactions  E =  mc 2 = x kg/mol (3.00 x 10 8 m/s) 2 = 4.75 x (kg m 2 s -2 ) / mol = 4.75 x J/mol = 4.75 x kJ/mol = the “binding energy” of 56 Fe

Binding Energy is a maximum at 56 Fe lighter elements become more stable upon fusion heavier elements become more stable upon fission

Nuclear Fission n 235 U unstable 236 U 92 Kr Ba n 0

Nuclear Fission 1 n U  141 Ba + 92 Kr n Many other fission “pathways” exist: 1 n U  137 Te + 97 Zr+ 2 1 n

Exercises Page 814: questions 8, 9, 10, 11 and 12

Artificial Transmutations Aim: What is Artificial Transmutation and how do Artificial Transmutations occur? Do Now Write Nuclear Equations for the following natural transmutations: (a)U-235 → Th-234 (b) Th-230 → Ra-226 (c) Po-214 → Pb-210 (d) Pb-214 → Bi-214 (e) U-234 → Th-230 (e) Pa-234 → U-234

What is Artificial Transmutation? Artificial Transmutation is where a stable isotope is made to disintegrate This is usually done by bombardment with high speed particles Examples: 4 2 He N → 17 8 O p Al He → P n P  → Si

Individual Practice Page 816: Problems 15 and 16 Page 837: Problems Group Practice Examine the Figure on page 814. Write a series of nuclear equations showing the transmutations from U-238 to Po-214