chemistry 100
September 24 (Monday)
Exam on Wednesday
- bring pencils, ID
- 50 minutes only
- Friday notes (Web)
Review, Q/A Tonight
- 6 -(8) pm
- room: Math Sci 210 (our regular classroom)
Office Hours
- Overman 152
- Monday 2:30-4:00
- Tuesday 10:30 - 12:00 and 4:30 -5:00
- Wednesday 11:00-12:00
(first, finish notes labeled Friday 9/21)
Isotopes
- atomic number = # protons
- mass number = protons + neutrons= integer
- notation: atomic number is prefix, subscript
- atomic mass is prefix, superscript
- example 92238U
- of course symbol and atomic number redundant, optional 238U
- the 92 and 238 should line up, but that's hard to type
- I may use 92U238 notation which I can generate easier
- # neutrons
- for small atoms -- about equal to # protons
- 6C12, 7N14 , 11Na23
- (6+6, 7+7, 11p+12n)
- for larger atoms, rises to about 2 x
- 53I127 is 53p, 127-53 = 74 neutrons
- 92U238 is 92 protons 238-92 = 146 neutrons
- some elements have several masses-- isotopes
- then apparent weight is an average
- atomic weight of Cl = 35.5
- atomic weight is not an integer
- many elements have only one common isotope
- then, atomic weight is nearly integer
- So why worry about isotopes?
- a) chemically-- isotopes of the same element behave identically
- b) nuisance-- atomic weights aren't simple numbers
- c) radioactivity-- some isotopes are unstable
- nuclear fuel, explosives
- medical tracers (can track radioactive iodine in the body)
- carbon 14 dating (below)
- d) a is a slight lie... very tiny differences based on mass
- deuterium (below and p 97 textbook)
Carbon dating
- C 99% 12C,1% C13, 1 part in 1012 14C
average = 12.01
- 14C forms from N (7N14) + comic rays
- 14C unstable, decays
- half life 5730 years
- half one in one half life
- 3/4 gone in two half lives
- 7/8. 15/16 in 3,4 half-lives
-
- leads to CO2 with trace of 14C
- gradually taken up by plants
- carbon is slightly radioactive
-
- old carbon (buried wood, charcoal, cloth)
- loses radioactivity
- no fresh source of C14
- can date to about 30,000 yr. old
Another Web site with
some nice carbon dating cartoons
Deuterium (1H2 or heavy hydrogen)
- H isotopes given names deuterium, tritium
- tritium is radioactive (smoke detectors)
- deuterium stable
- big relative difference compared to other elements
- no obvious effect on electrons
- but D atoms are a bit slower
- evaporate a little more slowly
- less in vapor and rain
- more in oceans
- depends on temperature
- less effect when warm
- more effect when cold
- can use D/H ratio in ice (old rain/snow)
- ice layers can trap that information
Another interesting application
Honey and Isotopes (click here)
- C, CO2 and sugar had normal C13 ratio
- honey has slightly different C13 ratio
- analysis of honey can detect adulteration
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