Oxygen - ₈O: the essentials

• Name: oxygen
• Symbol: O
• Atomic number: 8
• Relative atomic mass (A_r): 15.999 range: [15.99903, 15.99977] [see notes g r]
• Standard state: gas at 298 K
• Appearance: colourless as a gas, liquid is pale blue
• Classification: Non-metallic

• Group in periodic table: 16
• Group name: Chalcogen
• Period in periodic table: 2
• Block in periodic table: p
• Shell structure: 2.6
• CAS Registry: 7782-44-7

Oxygen is a Group 16 element. While about one fifth of the atmosphere is oxygen gas, the atmosphere of Mars contains only about 0.15% oxygen. Oxygen is the third most abundant element found in the sun, and it plays a part in the carbon-nitrogen cycle, one process responsible for stellar energy production. Oxygen in excited states is responsible for the bright red and yellow-green colours of the aurora. About two thirds of the human body, and nine tenths of water, is oxygen. The gas is colourless, odourless, and tasteless. Liquid and solid oxygen are pale blue (see picture above) and strongly paramagnetic (contains unpaired electrons).

Ozone (O₃) is the second allotrope of oxygen. It is formed from electrical discharges or ultraviolet light acting on O₂. It is an important component of the atmosphere (in total amounting to the equivalent of a layer about 3 mm thick at ordinary pressures and temperatures) which is vital in preventing harmful ultraviolet rays of the sun from reaching the earth's surface. Aerosols in the atmosphere have a detrimental effect on the ozone layer. Large holes in the ozone layer are forming over the polar regions and these are increasing in size annually. Paradoxically, ozone is toxic! Undiluted ozone is bluish in colour. Liquid ozone is bluish-black, and solid ozone is violet-black.

Oxygen is very reactive and oxides of most elements are known. It is essential for respiration of all plants and animals and for most types of combustion.

Oxygen occurs as two allotropes, dioxygen and trioxygen (ozone). Dioxygen, O₂, is a diatomic colourless oxidizing gas but a pale blue liquid upon condensation. It is paramagnetic (that is, possesses unpaired electrons) as a result of two unpaired electrons in the two O₂ π* orbitals. It makes up about a fifth of the earth's atmosphere.

Ozone, O₃, is a very reactive blue gas characterized by a pungent "electrical" smell (the name ozone derives from the Greek ozein, to smell). Ozone molecules are bent with a bond angle of about 117° in the gas phase. Ozone condenses to a blue-black liquid (boiling point -111.9°C and further cooling results in a dark violet-black solid (melting point -192.5°C.

Ozone is unstable thermodynamcally with respect to dioxygen but only converts to O₂ slowly in the absence of a catalyst.

2O₃(g) → 3O₂(g)

Arrangements of ozone molecules, O₃, in the solid state.

Ozone is present in the atmosphere as well, but at amuch lower level than O₂. Ozone is a key component of the upper atmosphere because it has the ability to absorb ultraviolet light in the 220-290 nm range, so preventing harmful rays at those wavelengths reaching the earth's surface. These days, various factors deplete the ozone layer in the upper atmosphere and the "ozone holes" in the northern and southern polar regions are of particular concern.