atomic structure
molecular structure and bonding
electromagnetic radiation
absorption of light
atmospheric chemistry and the ozone hole
chlorofluorocarbons
neutrons - neutral (uncharged)
protons - positively charged - same mass as a neutron
electrons - negatively charged - small mass (1/1838 as much as proton or neutron)
nucleus - contains protons and neutrons (essentially all the mass of an atom)
electrons occupy space around the nucleus (essentially all the volume of an atom)
atomic number - number of protons in the nucleus
atomic number uniquely defines the element
typically # of protons = # of electrons (charges balance, atom is neutral)
(if charges don't balance, the atom is charged, an ion )
arrangement based on similarity of chemical and physical properties (Mendeleev)
energy levels - quantum theory - energy is quantized
electrons can exist only in specific energy states with fixed capacities
first energy level can only hold 2 electrons, next 8 electrons, etc.
valence electrons - electrons in the outermost (highest) energy state of an atom
the chemistry of an atom is primarily determined by the number of valence electrons
compare hydrogen (1 valence electron) and sodium (1 valence electron)
atomic weight sum of protons plus neutrons (electrons negligible)
isotopes - different forms of the same element with different atomic weights
e.g., hydrogen and deuterium, carbon-12, carbon-13, carbon-14
bonding - atoms like to have a full outermost shell of electrons
options: give away valence electrons, get extra electrons, share electrons
examples - H2 (sharing of two electrons), H2O (sharing of 8 electrons),
Na+ Cl- (transfer of one electron)
Lewis structures - represent electrons as dots about atoms
a bond is a pair of bonding (shared) electrons, represented as a line
octet rule - arrange electrons around atoms so as to fulfill an octet
including shared electrons (bonds) and unshared electrons
hydrogen does not follow the octet rule, only wants two electrons
examples - N2, O2, O3
includes all forms of oscillating electromagnetic fields: light, x-rays, radiowaves, etc.
wavelength ( lambda ) - distance between equivalent spots on the wave
frequency ( nu ) - number of waves moving past a given point per second
electromagnetic radiation moves at the speed of light ( c ) c = lambda x nu
the human eye can only detect a small part of the electromagnetic spectrum
visible light lambda ~ 400 - 700 nm
photons - particles of light energy (zero mass, moving at speed of light)
energy of a photon is related to its frequency E = h n = h c / l
when the energy of a photon exactly matches the energy levels available in a molecule,
the photon can be absorbed and its energy imparted to the molecule
radiowaves - very low photon energies, detection requires special equipment
microwaves - low photon energies, correspond to molecular rotational energies
infrared - intermediate energy, correspond to molecular vibrations
visible - high energy photons, correspond to electronic energy levels
ultraviolet - very high energy, correspond to electronic bond strengths
x-rays - extremely high energy, correspond to nuclear energy levels
oxygen (O2) absorbs UV at wavelengths below 242 nm and dissociates
O2 ----(hn)----> 2 O
ozone (O3) absorbs UV at wavelengths below 320 nm and dissociates
O3 ----(hn)----> O2 + O
between the above two processes, most of the UV from sunlight is absorbed
a small amount of UV above 320 nm comes through, but it is the least harmful
part of the UV spectrum
biological effects of UV are generally measured by DNA modification
compare Figs 2.4 and 2.5 - calculate where maximum danger of UV is
at 340 nm, high intensity but low sensitivity
at 300 nm, low intensity but high sensitivity
ozone concentration reaches a steady-state through the Chapman cycle of reactions:
(1) O2 ----(hn)----> 2 O
(2) O + O2 ------> O3
(3) O3 ----(hn)----> O2 + O
(4) O + O3 ------> 2 O2
O atoms are highly reactive and react almost immediately ( << 1 sec)
ozone is more stable and a typical molecule exists for about 100 - 200 seconds
note that steps 1 & 3 create O atoms, steps 2 & 4 remove O atoms
step 2 creates O3 , steps 3 & 4 remove O3
overall the maximum concentration of ozone occurs in the stratosphere (10 - 50 km up)
this is where step 1 is significant
other reactions that affect the ozone concentration:
uv photolysis of water H2O ----(hn)----> H + OH
H + O3 --------> OH + O2
OH + O3 --------> H + 2 O2
note that H and OH regenerate one another while they convert O3 to O2
i.e., they are catalysts for the removal of ozone
a more accurate description of the previous chain reaction is:
uv photolysis of water H2O ----(hn)----> H + OH
H + O2 --------> HO2
HO2 + O3 --------> OH + 2 O2
OH + O3 --------> HO2 + O2
in this case, HO2 and OH regenerate one another while they convert O3 to O2
the difference is that H atoms are reactive enough that they combine with O2 first
then HO2 carries out the catalytic decomposition (not H atoms)
free radicals - molecules with an odd number of electrons
can't follow octet rule, generally reactive and unstable
other free radicals also tend to affect ozone concentrations
NO + O3 --------> NO2 + O2
NO2 + sunlight --------> NO + O
O + O2 --------> O3
(the SST flies in the stratosphere and generates excess NO and NO2 there)
compounds composed of only carbon, fluorine, and chlorine (no hydrogen)
used as refrigerants, propellants, solvents, etc. trade-name Freon (DuPont)
useful because they are stable, nontoxic, nonflammable, almost inert
after use, they are released to the atmosphere
in 1985 ~ 0.6 ppb atmospheric concentration, increasing ~ 0.4% per year
only way to destroy CFCs is by UV photolysis in upper atmosphere
but this also provides a catalytic pathway for removal of more ozone
CF2Cl2 -----(hn)-----> CF2Cl + Cl
Cl + O3 --------> ClO + O2
ClO + O3 --------> Cl + 2 O2
note Figure 2.11 - where ozone is low, ClO is unusually high and vice versa
1987 Montreal Protocol - cut in CFC production worldwide (U.S. phase-out 1995)
graph on page 35 - measurements of total ozone over South Pole in October
winter in Antarctica (June - Sept) is coldest place on earth (-90°) and has no sunlight
water is frozen into ice clouds in the stratosphere
these ice crystals catalyze additional chemistry
return of the sun in October melts ice crystals, releases burst of reactive molecules that can serve as catalysts for ozone removal (e.g., Cl atoms)
weather conditions at the North Pole are similar but not as severe
Solving the Problem
replacements for CFCs
HFCs - hydrofluorocarbons
have no Cl atoms to interact with ozone
C-H bond is easily broken, HFCs can be intentionally degraded readily
adding more ozone (see the cartoon, p 65)
removing chlorine atoms (and their sources)