Elements and Molecules
Earth's primordial atmosphere was probably similar to the gas cloud that created the sun and planets. It consisted of hydrogen and helium, along with methane, ammonia, and water. This was a reducing atmosphere. There was no molecular oxygen or other reactive oxides. Over time, some of this first atmosphere, particularly the lighter gases, outgassed and was lost. More water may have arrived with comets colliding on the surface of the planet. Volcanic activity in the early, Earth created major changes with release of water vapor, carbon dioxide, and ammonia along with small quantities of SO2, H2S, HCl, N2, NO2, He, Ar, and other noble gases. This produced the second atmosphere.Comet impacts may have increased the amount of water. Water vapor formed clouds. These produced rain. Over a period of thousands of years, the liquid water accumulated as rivers, lakes, and oceans on the Earth's surface. Bodies of liquid water acted as sinks for carbon dioxide. Chemical and biological processes transformed CO2 gas to carbonate rocks. The nitrogen and argon accumulated in the atmosphere. They do not react with water or other atmospheric components. Oxygen existed in only trace quantities before life began.
Living things created much of the third atmosphere, the one that now exists on Earth. Cyanobacteria were responsible for the rise in the atmospheric concentration of oxygen beginning 2.3 billion years ago. These bacteria, algae, and other plants produce oxygen by photosynthesis. Although most of this oxygen is used in respiration (biological oxidation) or in the atmospheric oxidation of the carbon-containing products, approximately 0.1 % of the organic matter is sequestered in sediments and that quantity of oxygen is added to the atmosphere. Over time, the excess oxygen has built up so that it is now makes up nearly 20% of the gases close to Earth.
| Composition of Earth's Atmosphere | |
|---|---|
| Nitrogen | 78.1% |
| Oxygen | 20.9% |
| Argon | 0.9% |
| Carbon dioxide, Methane, Rare (inert) gases | 0.1% |
Nitrogen and oxygen are the most common gases in today's atmosphere. Others are present in small concentrations. The other more common gases are shown in the table below. There is a remarkable difference between the original, reducing atmosphere and the current oxidizing atmosphere.
Nitrogen
Nitrogen is present in the atmosphere mainly as the dimeric molecule N2. See below for a summary of information on atomic nitrogen. There are two primary isotopes of atomic nitrogen with a ratio of 14N/ 15N = 272. Dinitrogen is extremely robust with a very high heat of dissociation (944 kJ mol-1 ). It is a colorless gas with a boiling point of -196 deg C.
The Haber Process is a method for producing ammonia developed by Germany during World War I. The Germans used the ammonia as a source of nitrogen for making explosives. The process is still used by industrial chemists.
2 NH3Certain plants are able to transform dinitrogen into ammonia at ambient temperature through a biochemical process involving the enzyme nitrogenase. Molecular nitrogen also reacts with molecular oxygen during lightning storms to produce nitric oxide, NO. Other than that, there is virtually no reaction chemistry of N2 in the atmosphere. It is an unreactive gas.
Bonding in Molecular Nitrogen
Why is N2 so unreactive? This is due to the strong, triple bond between the nitrogen atoms.
Oxygen
Living things created much of the atmosphere that now exists on Earth. Cyanobacteria were responsible for the rise in the atmospheric concentration of oxygen beginning 2.3 billion years ago. These bacteria, algae, and other plants produce oxygen by photosynthesis. Although most of this oxygen is used in respiration (biological oxidation) or in the atmospheric oxidation of the carbon-containing products, approximately 0.1 % of the organic matter is sequestered in sediments and that quantity of oxygen is added to the atmosphere. Over time, the excess oxygen has built up so that it is now makes up nearly 20% of the gases close to Earth.Information on oxygen is summarized in the chart below.
Atomic oxygen has three isotopes, 16O (99.759 %), 17O (0.0374 %), 18O (0.2039 %). The most common oxygen species in the atmosphere is triplet dioxygen, a diradical (boiling point = -183 deg C). Singlet dioxygen is diamagnetic with two electrons in one of the pi antibonding orbitals. This is higher in energy than triplet dioxygen and acts as an electrophile in its chemical reactions.
The singlet/triplet designations arise from the number of transitions between spin states when the molecule is placed in an external magnetic field. Where S is the sum of all electron spins, the spin state = (2S + 1): 1 unpaired electron, doublet state 2 unpaired electrons, triplet state0 unpaired electrons, singlet state
Molecules with no upaired spins are diamagnetic and are weakly repelled by magnetic fields. Molecules with one or more unpaired spins are paramagnetic and are attracted by magnetic fields.
Molecular oxygen, O2, is much more reactive than molecular nitrogen, N2. The bond between the atoms of O2 is significantly weaker than that between the atoms of N2, so bond homolysis can occur at a lower energy. Also, while dinitrogen is diamagnetic, dioxygen is paramagnetic and reacts with other radical species in the atmosphere.
Ozone or O3 is the other allotrope of oxygen (boiling point = -112 deg C). This is a bent, diamagnetic species. Ozone is a reactive molecule and plays an important role in the chemistry and photochemistry of the upper atmosphere. Photolysis of ozone in the stratosphere helps to protect life on Earth from highly energetic UV radiation and its depletion in the ozone layer by reactions with halogenated compounds is a serious concern. The reactivity of ozone makes it toxic to humans. The reactions of nitrogen oxides, hydrocarbons, and oxygen near the Earth's surface produce ozone and the concentration of this compound is frequently used as a measure of air pollution.
Bonding in O2
Lewis structures give us an approximation of the bonding in molecules. This approximation isn't very good for molecular oxygen. The best Lewis structure doesn't indicate the ground state properties of the molecule: its diradical nature.
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The molecular orbital diagrams for all homonuclear diatomic molecules is similar. The 2s atomic orbitals combine to form bonding and antibonding molecular orbitals (just like the 1s orbitals on hydrogen). Similarly, 2pz orbitals on both atoms combine to form sigma bonding and antibonding orbitals. The 2px orbitals combine to form bonding and antibonding orbitals with pi rotational symmetry about the bond axis. The 2py orbitals combine to form another set of pi orbitals at 90 degrees to the first set. The two pi bonding orbitals and the two pi antibonding orbitals form degenerate sets. |
Water, H2O
Water is present in variable amounts in the atmosphere, from 0 % to 4 %. Unlike oxygen and nitrogen, the concentration of oxygen depends on local weather conditions and changes greatly from place to place on Earth. The water in the atmosphere makes up only a very small percentage of the total water on Earth.
In the atmosphere, water exists as a gas (water vapor from evaporation), as a liquid (droplets of rain and liquid water that coats solid particles), and as a solid (snow and ice). Its structure depends on its state.
Water in the gas phase has a bent structure with an H-O-H angle of 104.5 degrees. In the liquid and solid forms, there are hydrogen bonds between the hydrogen atoms of one molecule of H2O and oxygen atoms of other molecules. This gives a 3-dimensional structure in which each oxygen atom is surrounded by a tetrahedral array of 4 hydrogen atoms.
Argon, Ar
Argon is one of the inert or noble gases. Because it has a filled shell electronic configuration, argon in unreactive and is present in the atmosphere as a monoatomic gas. Argon is the product of radioactive decay processes within the Earth. | Atomic number: 18 Melting Point: -189.3 deg C Boiling Point: -186.0 deg C Electronic Configuration: [He]2s22p6 Isotope Ar-36: Stable Isotope Ar-37: halflife= 35.0 days Isotope Ar-38: Stable Isotope Ar-39: halflife= 269.0 years Isotope Ar-40: Stable Isotope Ar-41: halflife= 1.8 hours |