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Molecular Oxygen (O2)

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Introduction

Physical Forms (Allotropes)

oxygen periodic table Oxygen can exist in a number of physical forms. Most commonly it is found as a diatomic O-O gas and is called molecular oxygen (O2) or dioxygen. The gas exists as a triplet state but the singlet oxygen can also be formed and is more reactive. This gas can be liquefied at low temperatures to produce a blue liquid oxygen (LOX) and cooled further produces a blue solid. After electrical discharge or photolysis Oxygen may also form the triatomic ozone (O3) and under very extreme conditions atomic oxygen (O•) is produced.

 

The Gas

Molecular Oxygen (O2) is a colourless, odorless and tasteless gas. It is denser than air and is only slightly soluble in water. It is a poor conductor of heat and readily supports combustion. Normal atmospheric oxygen (O2) is diatomic with a molecular weight of 31.9988. Oxygen is only slightly soluble in aqueous solution, dissolving to about 0.04 g/L at 25°C. Molecular oxygen is also special in that it exists as a triplet ground state with two unpaired electrons (biradical).

 

 

 oxygen cylinder

The Liquid

When cooled below its boiling point (-183°C) O2 becomes a pale blue liquid (LOX); when cooled still further the liquid solidifies, retaining its colour (ozone is also blue). The blue colour is due to the electronic properties of O2 which also impart its paramagnetic properties. Three stable isotopes of oxygen exist with mass numbers 16, 17, and 18. Liquid oxygen is also potentially hazardous about flames and sparks as it will greatly accelerate combustion.

 

 

 

 

 liquid oxygen

The Oxide

Oxygen is most commonly reacted as an oxide forming dihydrogen oxide: H2O (i.e. water). The formation of water is a thermodynamic endpoint (no further reactions proceed) and water has accumulated on the planet in vast quantities. The water-oxygen geochemical cycle is one of the most important cycles for life on this planet.

Other oxides are found in the earth along with silicates and make up about 45% of the earths crust.

 

 water oxide

Ozone

Ozone gas (O3) is often generated as an undesirable pollutant in smog and industrial emissions. It is also produced for sterilisation and for cleaning and detoxification purposes and is s a "green" alternative to chlorine. Ozone in the atmosphere is a critical component for filtering damaging UV radiation. Pure ozone gas is a bluish colour. At low temperatures (-111.9°C) liquid ozone forms as a blue-black liquid and if generated is a potentially explosive and unstable oxidant. At lower temperatures (-193°C) the solid ozone is formed as a bluish-black solid.

 

 

ozone molecule

© RKM

 

Table of Physical Properties for Oxygen

Oxygen is a second row element, atomic number = 8 (protons = 8, neutrons = 8, electrons = 8)

CAS (chemical abstract number)
7782-44-7
 
Atomic Number
8
  
Valence
2
  
Molecular Weight
15.9994(3)
 amu
Oxidation State
-2
  
Orbital Electrons
[He]2S2 2P4
 
Boiling Point
-182.97
 C
 
90.18
 K
Melting Point
-218.40
C
 
54.75
 K
Density (gas) @ OC
1.429
 g/L
Density (liquid) -183 C
1.14
 g/L
Molar Volume
17.36x10-6
 m3/mol
Heat of Vaporisation
3.4099
kj/mol
Heat of Fusion
0.22259
 kj/mol
Specific Heat Capacity
920
 J/(kg•K)
Paramagnetism
strong
  
Atomic Radii
0.66
 Å
Ionic Radii (crystal)
1.26
 Å
van de Waals Radii
1.52
 Å
Electronegativity
3.44
 (Pauling)
Electrical Conductivity
0.02674
 W/(m• K)
Crystal Structure
cubic
  
Origin of Name
oxy genes = "acid forming"
[Greek]

Ref CRC Handbook of Chemistry and Physics <on-line>

 

Comparison of Physical Properties for Dioxygen and Ozone

 

  Dioxygen
Ozone
Structure
 O2
O3
Molecular Weight
31.9988
47.9982
CAS (chemical abstract number)
7782-44-7
10028-15-6
Melting Point (1 atm.)
-182.97°C
-111.9°C
Boiling Point
-218.4°C
-197.7°C
Critical Temperature
-118.6°C
-12.1°C
Critical Pressure
49.77 atm.
54.6 atm.
Density (gas) @ 0°C
1.429 g/L
2.144 g/L
Density (liquid) -183°C
1.14 g/L
1.571 g/L
Viscosity of Liquid -183°C
0.1958 cp
1.57 cp

Ref CRC Handbook of Chemistry and Physics <on-line>

emissionline

Capsule Story Oxygen properties also found at these sites <site1>, <site2>, <site3>

MSDS by Air Liquide <pdf> (see their site for conditions of use).

Atomic Oxygen Oxygen gas in the upper atmosphere is subject to intense cosmic bombardment . Following particular outburst from the sun colourful bright red and yellow-green emissions from atomic oxygen can be seen from aurora. The emissions are from the 558 nm and 630 nm OI line in oxygen.

Liquid oxygen LOX as a fuel Throughout the 50-60's remarkable breakthroughs in the aviation world were made. Speed and height records were broken and central to this were the X-series rocket planes tested by NASA. The most famous was perhaps the X-1 flown by Gen. Chuck Yeager on October 14, 1947 to beyond Mach 1 for the first time.

Ozone Electrical discharge through O2 will produce the blue ozone O3 gas. Ozone is a highly corrosive pollutant, that is poisonous <MSDS> and is a constituent of "smog" pollution. It has many useful commercial applications.

Mineral Oxides Vast quantities of oxygen are tied up in the earth in the form of mineral oxides , carbonates, silicates and sulphates.

Ozone Hole O3 in the upper atmosphere forms an important protective layer reducing the UV exposure on the surface of the earth.
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