Nuclear Reactions

The Nucleus X Zone of Stability Nuclide: a unique atom represented by the symbol: A X Z Zone of Stability

Stability Radioactive Decay: the probability that a nucleus will undergo decomposition to form a different nucleus Isotopes: Atoms that have identical atomic numbers but different mass number values (neutrons have changed)

Important Observations All nuclides with 84 or more protons are unstable Light nuclides (P to N ration =1) are stable For stable heavy nuclides P to N ratio is >1 Special Stability: certain combinations confer special stability (even vs odd) Magic Numbers: certain combinations confer especially stable nucleus 2,8,20,28,50,82,126 (parallels behavior for stable electrons for chemical stability)

Decay The atomic mass number (A) and atomic number (Z) must remain constant across the reaction! A: 238 = 234 +4 Z: 92 = 90 + 2

Types of Decay Alpha Decay () : A very common mode of decay for heavy nuclides. (helium nucleus)

Types of Decay Beta Decay (-) : Most common, The mass number of the decaying remains constant, the net effect is the change of a neutron to a proton!

Types of Decay Positron Production (+) : Occurs for nuclides below the zone of stability, a particle with the same mass as an electron but opposite charge. The net effect is a change of a proton to a neutron.

Types of Decay Gamma ray () : Release of a high energy photon, frequently accompanies other reactions

Types of Decay Electron Capture : process in which one of the inner orbital electrons is captured by the nucleus Interesting, and why do we not just turn mercury into gold?

Types of Decay Summary of Decay reactions

Decay Series Decay Series: a sequence of nuclear reactions that ultimately result in the formation of a stable isotope.

Half-life Half-life (t½) Time required for half the atoms of a radioactive nuclide to decay. Shorter half-life = less stable.

Half-life mf: final mass mi: initial mass n: # of half-lives

Mass Defect When a system gains or loses energy it also gains or loses a quantity of mass. E = mc2 m = mass defect E = change in energy If E =  (exothermic), mass is lost from the system. 7

Nuclear Transformations Nuclear Transformation: the change of one element to another.

F ission splitting a nucleus into two or more smaller nuclei 1 g of 235U = 3 tons of coal

F ission Chain reaction - self-propagating reaction Critical mass - mass required to sustain a chain reaction Subcritical: When the process dies out Supercritical: rapid escalation of reaction

F ission

Fusion combining of two nuclei to form one nucleus of larger mass thermonuclear reaction – requires temp of 40,000,000 K to sustain 1 g of fusion fuel = 20 tons of coal occurs naturally in stars

Fission vs. Fusion 235U is limited danger of meltdown toxic waste thermal pollution fuel is abundant no danger of meltdown no toxic waste not yet sustainable