Oxygen

History
At the begining, when the sunshine was incidenting on the earth surface, it was impossible to breath atmosphere without the oxygen. But the oxygen which was releasing from the plants, has accumulated in the atmosphere. 2.5 mililards years ago was in atmosphere 1000 times less of oxygen like in the present. 500 milions year ago was in atmosphere equal amount of oxygen like in the present.

The subsequent evolution of the atmosphere has been dominated by biological processes. During the microbial stage, organisms were gradually developing their metabolic capabilities through natural selection. Atmospheric composition was evolving in response to the introduction of one new metabolic process after another. This stage culminated in the development of green-plant photosynthesis and the rise of atmospheric oxygen. This stage ended with the Precambrian era.

When the first prokaryotic cells developed an ability to capture free energy from sunlight through photosynthesis, a fundamental change took place. Molecular oxygen was liberated into the environment at a very slow but steady pace. The oxygen did not immediately accumulate in the atmosphere since it sought out electron-rich rocks and minerals to bind with (in a process known today as rusting).

The Biological Era - The Curious Case of Atmospheric Oxygen
The biological era was marked by the simultaneous decrease in atmospheric carbon dioxide and the increase in oxygen due to life processes. We need to understand how photosynthesis could have led to maintenance of the ~20% present-day level of O2.

The build up of oxygen had three major consequences
Firstly, Eukaryotic metabolism could only have begun once the level of oxygen had built up to about 0.2%, or ~1% of its present abundance. This must have occurred by ~2 billion years ago, according to the fossil record. So, the eukaryotes came about as a consequence of the long, steady, but less efficient earlier photosynthesis carried out by Prokaryotes.

Secondly, once sufficient oxygen had accumulated in the stratosphere, it was acted on by sunlight to form ozone, which allowed colonization of the land. The first evidence for vascular plant colonization of the land dates back to ~400 million years ago.

Thirdly, the availability of oxygen enabled a diversification of metabolic pathways, leading to a great increase in efficiency


While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. The oxygen did not build up in the atmosphere for a long time, since it was absorbed by rocks that could be easily oxidized (rusted). To this day, most of the oxygen produced over time is locked up in the ancient "banded rock" and "red bed" rock formations found in ancient sedimentary rock. It was not until ~1 billion years ago that the reservoirs of oxidizable rock became saturated and the free oxygen stayed in the air.


For many centuries people think that air is composed of more than one component. The behavior of oxygen and nitrogen as components of air led to the advancement of the phlogiston theory of combustion, which captured the minds of chemists for a century. Oxygen was prepared by several chemists, including Bayen and Borch, but they did not know how to collect it, they did not study its properties, and they did not recognize it as an elementary substance.

Milions of years ago­
Priestly as first gave an evidence of ogygen, although and Scheele also discovered it independently.
Its atomic weight was used as a standard of comparison for each of the other elements until 1961 when the International Union of Pure and Applied Chemistry adopted carbon 12 as the new basis.

The oxygen concentration Problem.
Why does present-day oxygen sit at 20%? This is not a trivial question since significantly lower or higher levels would be damaging to life. If we had less than 15% oxygen, fires would not burn, yet at more than 25% oxygen, even wet organic matter would burn freely.

Sources
Oxygen is the third most abundant element found in the sun, and it plays a part in the carbon-nitrogen cycle, the process which give the sun and stars their energy. Oxygen under some conditions is responsible for the bright red and yellow-green colors of the Aurora.
Oxygen forms 21% of the atmosphere by volume and is obtained in fractional distillation. The atmosphere of Mars contains about 0.15% oxygen. The element and its compounds make up 49.2%, by weight, of the earth's crust. About two thirds of the human body and nine tenths of water is oxygen.
In the laboratory it can be prepared by the electrolysis of water or by heating potassium chlorate with manganese dioxide as a catalyst.

Properties
The gas is colorless, odorless, and tasteless. The liquid and solid forms are a pale blue color and are strongly paramagnetic.

Forms
Ozone, O3 - a highly active compound, is formed by the action of an electrical discharge or ultraviolet light on oxygen.
Ozone in the atmosphere (amounting to the equivalent of a layer 3 mm thick under ordinary pressures and temperatures) helps prevent harmful ultraviolet rays of the sun from reaching the earth's surface.
There are three oxygen atoms in ozone. Consider that there are two possible ways to connect three atoms: in a "line"(1)
and in a "ring".(2)
One pair of electrons must be brought over from an attached oxygen to form a double bond and satisfy all oxygen's desire for an eight electrons on the outer shell. Since either of the attached oxygens can do this, there are two possible, equivalent, distributions of electrons.
The above calculations have shown us the most stable structure and multiplicity for ozone. Advanced students may be interested in exploring the MOs of ground state ozone. The MOs of the ring structure may also prove interesting.

Compounds
Oxygen is making compounds which are called oxides. There are two main groups of oxides. Except these two groups there are three other groups so we know five groups of oxides.
1. Oxides which make alkalies - they have negative charge on oxygen. Oxides which have an abbility to dissolve in water react according to this equation: O2- + H2O 2OH-


To this group almost belong the alkali metals. Because there is a bond with difference in electronegativity we can say that the bond between a metal and oxygen is ionic with little deal of covalention. In pure ionic bond, has oxygen 2 negative charges. But they are reduced because of covalention. With rising proton number of alkali metals is decreasing the deal of covalention, proportional to rising of the difference in electronegativity between the metal and oxygen.
According to this reactions we can say that fastest reaction between water and oxide will have the oxide which bond is nearliest to pure ionic bond. Because of this is from alkali oxides most stable Li2O.

2. Oxides which make acids – When these oxides react with water a new acid is formed. They decay temperatures are lower than it was in the 1st group of oxides.



Let’s write some reaction of these oxides. For example: SO3 + H2O - H2SO4
About this reaction we know that it’s process is very forceful. It is nearly explosive reaction.If we want to talk about this reaction we have to know electron structure of SO3 – Sulphure trioxide:
According to this scheme is clear that all three atoms of oxygen are equal. Between one atom of oxygen we don’t put double bond, because we want to have in sulphure eight electrons on the outer shell.
And also sulphure doesn’t have the abbility to make double bonds. Because of that is the charge of the sulphure 3+. Than we can show the reaction with water like this.SO3 + H20 - H2SO4,H2SO4 + H2O - HSO4– + H3O+



3. Oxides of elements in III. and IV.group – When some element from III. or IV. group react with water a weak acid or a weak alkali is formed. So in the middle of the periodic table the elements will have both type of oxides. This is called amphoteric character. This character have especially Al, Pb, Ge, Be, Zn etc.
In many cases we can see that oxygen can has different oxidant number.For example: PBO, PBO2 ; SnO, SnO2 ; SO2, SO3
In these examples the oxides with grater oxidant number are more acid making oxides, than the oxides with smaller oxidant number.
For example when SO2 react with water the week H2SO3 is formed.