Oxygen-the gas that we inhale..

The gas that we inhale daily is oxygen,without oxygen, we can only survive for just a few minutes. In atmosphere, there is 21% of oxygen. Under ordinary conditions (STP) on Earth, oxygen is a colorless, odorless, tasteless gas that is only slightly soluble in water. Oxygen has a pale blue color in the liquid and the solid phases. Ordinary oxygen gas (O2) exists as diatomic molecules. It also exists in another allotropic form, the triatomic molecule ozone (O3). Although eight isotopes of oxygen are known, atmospheric oxygen is a mixture of only three: those having mass numbers 16, 17, and 18. Let's have a briefly introduce about oxygen...
Name: Oxygen
Symbol: O
Atomic Number: 8
Atomic Mass: 15.9994 amu
Melting Point: -218.4 °C (54.750008 K, -361.12 °F)
Boiling Point: -183.0 °C (90.15 K, -297.4 °F)
Number of Protons/Electrons: 8
Number of Neutrons: 8
Classification: Non-metal
Density @ 293 K: 1.429 g/cm3
Color: colorless
Group: 16
Period: 2
Electron Configuration: 1s2 2s 2p4
Most Common Ions: OH−, OH2−, O2−

Oxygen is very reactive. Its reaction with another substance to form an oxide is called oxidation. It is a constituent of a number of compound groups, such as acids, hydroxides, carbonates, chlorates, nitrates and nitrites, and phosphates and phosphites—as well as carbohydrates, proteins, fats, and oils. The respiration of animals and plants is actually a form of oxidation, essential to the production of energy within these organisms.

Due to its electronegativity, oxygen forms chemical bonds with almost all other elements hence the origin of the original definition of oxidation. The only elements known to escape the possibility of oxidation are a few of the noble gases, and fluorine. The most famous of these oxides is water (H2O). Other well known examples include compounds of carbon and oxygen, such as carbon dioxide (CO2), alcohols (R-OH), carbonyls, (R-CO-H or R-CO-R)), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO3−), perchlorates (ClO4−), chromates (CrO42−), dichromates (Cr2O72−), permanganates (MnO4−), and nitrates (NO3−) are strong oxidizing agents in and of themselves. Many metals such as iron bond with oxygen atoms, iron(III) oxide (Fe2O3). Ozone (O3) is formed by electrostatic discharge in the presence of molecular oxygen.

Oxygen not only required in respiration, it is also important is other processes.
Combustion
The burning of substances in air is a rapid form of oxidation called combustion. Complete combustion(with sufficient of oxygen) will produce carbon dioxode, water and heat energy.

For example, the burning of propane is:
C3H8 + 5O2 --> 3CO2 + 4H2O


Rusting
Rusting is a chemical process, it occurs when the iron or steel is exposed to moist air, it reacts with the oxygen in the air to create Iron (III) oxide. The formation of rust can occur at some distance away from the actual pitting or erosion of iron as illustrated below. This is possible because the electrons produced via the initial oxidation of iron can be conducted through the metal and the iron ions can diffuse through the water layer to another point on the metal surface where oxygen is available. This process results in an electrochemical cell in which iron serves as the anode, oxygen gas as the cathode, and the aqueous solution of ions serving as a "salt bridge" as shown below.