Chapter 2: Protecting the Ozone Layer

Ozone Formation 3 O 2 2 O 3 Ozone is an allotropic form of oxygen. Energy must be absorbed for this reaction ElementAllotropes oxygenO 2, O 3 carbongraphite, diamond, buckminister fullerenes 2.1

O Atomic number (A) Mass number (Z) -The number of protons -The sum of the protons and neutrons 2.2

The electrons in the outermost energy levels are called valence electrons. 2.2 The group number (of the representative elements) on the periodic table tells you the number of valence electrons. 1A 2A 3A 4A5A 6A7A 8A Group 1A: 1 valence electron Group 3A: 3 valence electrons

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: 12 C 13 C 14 C

Representing molecules with Lewis structures : 2.3 Consider water, H 2 O: 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:

2.3 Representing molecules with Lewis structures: Typical valence for selected atoms ElementTypical valence Classification H, X (X= F, Cl, Br, I) 1monovalent O2divalent N3trivalent C4tetravalent

2.3 Representing molecules with Lewis structures: Multiple bonds Triple bond Double bond Occasionally a single Lewis structure does not adequately represent the true structure of a molecule; so we use resonance forms:

2.4 The Nature of Light Low E High E Wavelength ( ) = distance traveled between successive peaks (nm) Frequency ( ) = number of waves passing a fixed point in one second (waves/s or 1/s or s -1 or Hz)

The Electromagnetic Spectrum 2.4 The various types seem different to our senses, yet they differ only in their respective and 

Visible: = nm R O Y G B I V Infrared (IR) : longest of the visible spectrum, heat ray absorptions cause molecules to bend and stretch Microwaves: cause molecules to rotate At short  range: UV (ultraviolet), X-rays, gamma rays Decreasing wavelength 2.4

2.5 The energy of a photon of electromagnetic radiation is calculated by: E = h  where h = 6.63 x J. s (Plank’s constant). The wavelength and frequency of electromagnetic radiation are related by: C = where C = 3 x10 8 m/s (the speed of light). Energy and frequency are directly related- higher frequency means higher energy.

UV radiation is of sufficient energy to cause molecular bonds to break. 2.5

2.6

The Chapman Cycle 2.6 A steady state condition

2.7 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.

First, UV radiation breaks a carbon-halogen bond: Photon  < 220 nm) + CCl 2 F 2. CClF 2 + Cl. (free radicals) 2.9 How CFCs Interact with Ozone

2.9 2Cl. + 2O 3 2ClO. + 2O 2 2 ClO. ClOOCl The chlorine radical attacks an O 3 molecule: Then two chlorine monoxide radicals combine: UV photon + ClOOCl ClOO. + Cl. ClOO. Cl. + O 2 The ClOOCl molecule then decomposes: The net reaction is: 2 O 3 3O 2 The Cl. Radicals are free to attack more O 3.

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.