The ozone story

• The atomsphere is a relatively thin layer of gas surrounding teh earth, it has four layers to it.
• The troposphere is from 0km-15km and 90% of all the molecules in the atmosphere are in the troposphere, the atmosphere becomes less dense as you go higher up.
• The stratosphere is from 15km-50km
• The mesosphere is from 50km-80km
• The thermosphere is from 80km and above
• Gas concentrations can be expressed by percentages (nitrogen=78%) but when the concentration becomes smaller than 0.1% then it is easier to express it in parts per million (ppm)
• To convert ppm into % = divide by 1,000,000 then times by 100 (or divide by 10,000)
• To convert % into ppm = times by 10,000

• The most damaging part of the spectrum to the skin is ultraviolet
• There are some chemicals which absorb much of the uv light given of by the sun. For example a window will let visbale light in but absorbs the high-energy unltraviolet radiation so if you are sat inside the UV radiation will never reach your skin.
• it is possible though to burn under water as water does let some uv through.
• light can be thought of as waves or particles                  
• speed of light = 3 x 10^8 m/s                                                                            
• speed of light C (m/s) = wavelength λ (m) x frequency v (s^-1)      C= λ x v    
• Planck constant = 6.63 x 10^-34 js                                                                          
• Energy of a photon E (j) = planck constant h (Js) x frequency v (s^-1)   E=h x v   

•  Electromagnetic radiation can interact with amtter, transferring enrgy to the chemicals involved.
• Energy is associated with different types of behaviour:
• Translation (molecule moving as a whole)
• Rotation ( of the molecule as a whole)
• vibration of the bonds
• Electron energy
• All the types of energy are quantised(it has fixed levels) and the electronic energy cahnges when an elctron moves from one energy level to another
• The vibrational energy corresponds to infrared radiation. I.R causes chemicals in your skin to vibrate more, increasing their kinetic energy making you feel warmer
• Rotational energy corresponds to the microwave region
• Electronic changes require higher energy than for vibrational changes. so these energy changes correspond to the visable and ultraviolet parts of the spectrum.

Electronic changes that occur when molecules absorb ultraviolet radiatio (chlorine):

•  Electrons may be excited to a higher energy level. This is how Chlorine gets its colour,Cl₂  absorbs visable light of such a frequency that the remaining unabsorbed light looks green as the energy is released
• sometimes the molecule can absorb so much energy that the bonding electrons can no longer bond the atoms together. This is photodissociation and radicals are formed, radicals are atoms or molecules with at least one unpaired electron and are very reactive. There formation may lead to further chemical reactions
• With very high energy photons, the molecules may acquire so much energy that an electron is able to leave it - the molecuel is ionised

Heterolytic fission:

• Both the shared electrons go to just one of the atoms when the bond breaks.
• This atom becomes negatively charged as it gains electrons
• The other atoms becomes positively charged as it loses electrons
• This type of fission is common for polar bonds

Homolytic fission:

• One of the two shared electrons goes to each atom
• The atoms have no overall charge as they have the same electronic structure they had before they formed the bond
• It forms species with an unpaired electron ( Radicals)
• They are very reactive
• Mostly occur when the bond being broken is non-polar

Biradicals are when the species have two unpaired electrons e.g. Oxygen atoms and molecules

• Radicals are very reactive because they want to fill their outer shell by grabbing an electron from another atom or molecule.
• In an equation the curley arrow indicates the movement of an electron
• The tail show where the electron starts and the head shows where it finishes.
• A single headed arrow represents one electron.
• A double headed arrow represents a pair of electrons.
• A radical will combine with another molecule to create a new radical and so it goes on

It is a radical chain reaction and has three stages:

• Initiation: Radicals are formed by photodissociation, these radicals start teh chain reaction
• Propagation: The radicals react with other molecules. These reactions produce new radicals which will keep the reaction going
• Termination: Occasionally two radicals collode thsi removes teh radicals so the reaction is terminated.