Body

chemistry 100

September 24 (Monday)

(first, finish notes labeled Friday 9/21)

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
- ₁₇Cl³⁵ and ₁₇Cl³⁷
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

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

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
- same chemistry
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)
  - to estimate temperature
- ice layers can trap that information