The atom phosphorus is located below nitrogen in the periodic table. It has similar electronic configuration to nitrogen except that it can form an extended octet. As a result of this fact phosphorus can form more bonds than nitrogen. It can form up to five bonds, while nitrogen can form maximally three bonds only.
Its outer shell electronic configuration is 3s2-3p3. Hybridization using one 3d orbital and one 3s orbital and three 3p orbitals give five equivalent hybrid orbitals of similar energy that is designated dsp3 hybridization. It can accommodate five bonds.
An example of a compound of phosphorus with five bonds is PCl5. PCl5 is used in organic chemistry as a chlorinating reagent for alcohols. PCl5 is electronically saturated while it is coordinately unsaturated. This means that it can bind another ligand to form a compound with the coordination number six.
This is manifested in the compound hexa fluorophosphate or PF6-. PCl5 does not form a sixth bond due to the larger space the chlorine atoms occupy around phosphorus so there is a steric problem for the approach of the sixth chlorine atom.
Phosphorus is biologically important as it forms the phosphate group which is incorporated in the ATP molecule or adenosine triphosphate. ATP is the energy currency of the cell. It is needed for driving metabolic reactions which are under normal conditions are not favourable and which require energy to proceed.
Enzymes are catalysts that use ATP to catalyze metabolic reactions inside our body such as Na+, K+, ATPase. Besides its existence in ATP the phosphate group is incorporated also into DNA and RNA molecules.
Elemental phosphorus is toxic to human tissue. It has three allotropic forms: white, red and black. The most reactive allotrope is the white form. It is the thermodynamically least stable. It burns spontaneously in the presence of oxygen or air.
Phosphorus occurs in the form of a molecule which unlike nitrogen which occurs as a diatomic molecule. It occurs as a tetrahedral molecule of four atoms of phosphorus P4, that are bound by a single bond.
Phosphorus compounds have many applications due to the variety of compounds it can form. Phosphorus resemble carbon in its chemistry because it has similar electro-negativity.
Organophosphates and organophosphonates are organophosphorus compounds which have activity against acetylcholine esterase ( the enzyme that is responsible for degrading acetylcholine at the synaptic cleft). Namely they inhibit the activity of this enzyme.
This potential use of these groups of compounds have found use for them in chemistry and industry. They are used in idustry as pesticide in addition to their use in chemical warfare as nerve agents. An example of an organophosphate that is used as nerve gas is Sarine and Tabum.
Organophosphorus compounds have wide use in organic chemistry. There is a whole branch in organic chemistry that is called organophosphorus chemistry. Phosphorus is used in NMR technology to identify chemicals that have phosphorus within them.
Phosphines are used in the organic labs in the famous Wittig reaction which is used to convert a ketone to a double bond. The specific phosphine used in the Wittig reaction is triphenylphosphine. This is a convenient compound to use because it is solid and easy to handle. An ylide is formed in this reaction and which disintegrates to give the thermodynamically stable double bond.
As was mentioned previously PCl5 and PCl3 are used a chlorinating reagents which convert the alcohol ( A bad leaving group) to the better leaving group ( the chloride).
Phosphides are inorganic phosphorus compounds which are compounds of metals with phosphorus. In these compounds the phosphorus atom has a formally negative charge while the metal has a positive formal charge.
An example of a phosphide is Ca3P2 or calcium phosphide. This compound is toxic and is actually used as a poison for rodents. It reacts under the stomach acidic environment with acids to liberate the toxic phosphine molecule which can kill the rodent.