Chapter 2: Protecting the Ozone Layer
Ozone Formation
3 O2
2 O3
Energy must be absorbed for this reaction
Ozone is an allotropic form of oxygen.
Element
oxygen
carbon
Allotropes
O2, O3
graphite, diamond, buckminister fullerenes
2.1
Atomic number (A) -The number of protons
8
O
16.00
Mass number (Z) -The sum of the protons
and neutrons
2.2
The electrons in the outermost energy levels are called
valence electrons.
The group number (of the representative elements) on the
periodic table tells you the number of valence electrons.
Group 1A: 1 valence electron
1A
8A
Group 3A: 3 valence electrons
2A
3A 4A 5A 6A 7A
2.2
Isotopes are two or more forms of the same element
(same number of protons) whose atoms differ in
number of neutrons, and hence in mass.
Isotopes of carbon: C-12, C-13, C-14
also written as: 12C 13C 14C
2.2
Representing molecules with Lewis structures:
Consider water, H2O: H
O
H
1. Find sum of valence electrons:
1 O atom x 6 valence electrons per atom = 6
+ 2 H atoms x 1 valence electron per atom = 2
8 valence electrons
2. Arrange the electrons in pairs; distribute them
so that the octet rule is satisfied:
lone pair
O
H
H
bonding pair
2.3
Representing molecules with Lewis structures:
Typical valence for selected atoms
Element
Typical
valence
1
Classification
O
2
divalent
N
3
trivalent
C
4
tetravalent
H,
X (X= F, Cl, Br, I)
monovalent
2.3
Representing molecules with Lewis structures:
Multiple bonds
H
O
C
C
H
Triple bond
O
Double bond
Occasionally a single Lewis structure does not adequately represent
the true structure of a molecule; so we use resonance forms:
O
O
O
O
N
N
N
O
O
O
O
O
2.3
The Nature of Light
Low E
High E
Wavelength (l) = distance traveled between successive peaks (nm)
Frequency (n) = number of waves passing a fixed point in one second
(waves/s or 1/s or s-1 or Hz)
2.4
The Electromagnetic Spectrum
The various types seem different to our senses, yet
they differ only in their respective l and n.
2.4
Visible: l = 700- 400 nm
ROY G BIV
Decreasing wavelength
Infrared (IR) : longest of the visible spectrum,
heat ray absorptions cause molecules to bend and stretch
Microwaves: cause molecules to rotate
At short l range: UV (ultraviolet), X-rays, gamma rays
2.4
The wavelength and frequency of electromagnetic
radiation are related by: C = ln
where C = 3 x108 m/s (the speed of light).
The energy of a photon of electromagnetic radiation
is calculated by: E = h n
where h = 6.63 x 10-34 J.s (Plank’s constant).
Energy and frequency are directly relatedhigher frequency means higher energy.
2.5
UV radiation is of sufficient energy to cause
molecular bonds to break.
2.5
2.6
The Chapman Cycle
A steady state
condition
2.6
Biological Effects of Ultraviolet Radiation
The consequences depend primarily on:
1. The energy associated with the radiation, and
2. The sensitivity of the organism to that radiation.
2.7
How CFCs Interact with Ozone
First, UV radiation breaks a carbon-halogen bond:
.CClF + Cl. (free radicals)
Photon (l < 220 nm) + CCl2F2
2
2.9
The chlorine radical attacks an O3 molecule:
2Cl. + 2O3
2ClO. + 2O2
Then two chlorine monoxide radicals combine:
2 ClO.
ClOOCl
The ClOOCl molecule then decomposes:
UV photon + ClOOCl
ClOO.
The net reaction is: 2 O3
ClOO. + Cl.
Cl. + O2
The Cl. Radicals
are free to attack
more O3.
3O2
2.9
As ClO. concentrations increase,
ozone concentration decreases.
2.9
HCFCs are alternatives to CFCs: they decompose more
readily in troposphere so they will not accumulate to
the same extent in stratosphere.
2.9