The Elements : Oxygen


Name Symbol Atomic Number Electronic Structure Melting Point (°C) Boiling Point (°C)
 Oxygen  O 8 2 - 6 -218.79 -182.95

Oxygen is element number 8 and has the symbol O.

Oxygen is incorrectly named. The name comes from the Greek for producer of sourness because it was believed that Oxygen atoms occurred in all acids (which taste sour). Although Oxygen is present in many acids, it is not present in all acids (Hydrochloric Acid contains only atoms of Hydrogen and Chlorine). Acids are usually compounds of non-metals with Hydrogen.

The Oxygen atom has its 8 electrons arranged in two shells: 2, 6.

Oxygen is a very common atom in the Universe.

It is the most common element in the Earth's crust, 46% of the atoms in the Earth's crust are Oxygen atoms.

Almost all rocks and minerals contain Oxygen. Examples are sand, quartz and glass (Silicon Dioxide, SiO2), limestone and chalk (Calcium Carbonate, CaCO3), gypsum (Calcium Sulphate, CaSO4).

Many rocks are Silicates which contain different metals combined with Silicon and Oxygen.

Clays are Aluminium Silicates. Marble and Granite are both Calcium and Aluminium Silicates. Olivine (which also occurs in meteorites) is an Iron and Magnesium Silicate. Basalt is a Silicate of several metals (Calcium, Iron, Magnesium). Other Silicates are Chlorites, Micas and Feldspars.

Many ores are combinations of metals with Oxygen. Examples are Hematite (an Iron Oxide, Fe2O3), Magnetite (another Iron Oxide, Fe3O4). The surface of Mars is covered with red coloured oxides of Iron.

Many gemstones contain Oxygen. Examples are Topaz (an Aluminium Silicate), Rubies and Sapphires, both Aluminium Oxide, Al2O3, with the different colours due to different metal impurities. Emeralds are a Silicate of Beryllium and Aluminium. Opal is a non crystalline form of Silicon Dioxide (SiO2). Amethyst is also Silicon Dioxide.

Two thirds of the surface of the Earth is water, an Oxide of Hydrogen (H2O), and a very common molecule in the Universe.

The water molecule is "bent". The Oxygen atom has two Hydrogen atoms attached to it. The angle H-O-H is 109 degrees (unlike Carbon Dioxide where the O-C-O angle is 180 degrees).

The bending of the water molecule causes the electric charges in the outer electrons to be bunched up. The molecule is neutral overall but parts of it are slightly positive and other parts are slightly negative. The water molecule is said to be Polar. This Polarity makes the water molecule "sticky". Water has a much higher boiling point than other similar chemicals and is a liquid on the Earth's surface. The Polarity also causes surface tension which allows small insects to "walk" on the surface. Another unusual effect of the polarity of the water molecule is that ice is less dense than water and therefore floats. Most solids are denser than their liquids.

All life on Earth requires water to exist.

Oxygen occurs as an element in the Earth's atmosphere where it makes up 20% of the air.

Oxygen in the atmosphere occurs normally as diatomic molecules (O2). This is a colourless and odourless gas that supports combustion (it allows burning). It becomes a pale blue liquid at -183C.

Elemental Oxygen is one of the most reactive elements. Many substances burn in air due to the presence of Oxygen. In pure Oxygen, even powdered metals will burn and a glowing piece of wood bursts into flame. Oxygen is produced by plants during photosynthesis. If it wasn't being replenished, free Oxygen in the atmosphere would eventually react and disappear.

There exists another form of elemental Oxygen with three atoms in its molecule (O3). This is called Ozone and is a pale blue pungent (strong smelling) gas. It has the interesting and important property of absorbing ultra violet light. Most of the Earth's Ozone is found in a layer several kilometres above sea level. Its presence there makes life possible on the Earth's land surface.

Compounds containing oxygen with one other element are called Oxides.

Different Oxides of the same element can exist and these may have different properties depending on the composition. For example, Hydrogen has two oxides: water (H2O) and Hydrogen Peroxide (H2O2, a liquid used for bleaching hair). Carbon has two (common) oxides: The poisonous Carbon Monoxide (CO) and the product of respiration, Carbon Dioxide (CO2). Carbon Dioxide makes up 1% of the Earth's atmosphere as well as the majority of the atmospheres of Mars and Venus.

Nitrogen has several oxides including these: N2O - laughing gas; NO2 - nitrogen dioxide (an exhaust gas that causes acid rain).

Rust and Iron Ore are oxides of Iron (FeO, Fe2O3 or Fe3O4). Sand and quartz are an oxide of Silicon (SiO2). Several gemstones are impure oxides of metals.

Oxides can be divided into four classes depending on their chemical properties.

Acidic Oxides are the oxides of non-metals which can react with water to form acids. Carbon Dioxide is an acidic oxide. It dissolves in water to give a weak acid called Carbonic Acid (H2CO3) which gives fizzy drinks their fizz. Sulphur Trioxide (SO3) is another acidic oxide. This reacts with water (violently and giving out a lot of heat) to yield Sulphuric Acid (H2SO4).

Basic Oxides are the oxides of most metals. These react with water to form alkalis (or bases). Sodium Oxide (Na2O) forms Sodium Hydroxide (NaOH) which is better known as Caustic Soda. Acids and Alkalis neutralise each other.

Neutral Oxides are either insoluble in water or do not form acids or alkalis. Examples are Carbon Monoxide (CO) and Nitrous Oxide (N2O).

Amphoteric Oxides can produce either acids or bases depending on circumstances. Aluminium Oxide (Al2O3) is a well known example.

Acidic and basic reactions in water are another result of the polarity of the water molecule.

In organic chemistry Oxygen is found in Carbohydrates. Examples are fats, alcohols, ketones, aldehydes, fatty acids, esters, starches and sugars.

Oxygen is a very important element. If forms much of the Earth we stand on, the liquids we drink, the air we breathe and the cells of our bodies. It is a very reactive element which would not exist in its elemental state if it wasn't being replenished by plants.

Oxygen is, in fact, the second most reactive of the elements.

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