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Element Fact
In 1961, the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 as the basis of atomic weight, which has a mass of exactly 12.000 atomic mass unit.

Phosphorus, P

phosphorus sample

Atomic number: 15
Atomic mass: 30.973761(2)
Natural abundance: 1120 ppm
31P (30.9737615) - 100.0%

History and Production

In Greek, phos means light, phorus means bringing. It ws first isolated by H. Brandt in 1669 by allowing urine to putrify before boiling it down to a paste. The industrial production normally derived from the mineral family apatites, containing tricalcium phosphate, of which the Africa continent contains the largest phosphate rock deposits. The rock is heated with sand and coke (carbon) in an electric furnace. The white phosphorus thus form as a vapour is collected under the water. It can be oxidized in moisture to produce phosphoric acid, an important ingrediant for making fertilizer. Red phosphorus is used in the manufacture matches, pyrotechnics, pesticides etc. Phosphorus is an important element for the growth and development of plants and animals. Its compounds also play important part in photosynthesis, metabolism, nerve functions etc.

Physical Data

It is eleventh element in order of abundance in earth's crust, often in the form of orthophosphate minerals. Phosphorus exists in many alloptropic form. The most common form being the waxy white phosphorus. It is, nevertheless, the most reactive and volatile and is normally kept in the water. On the other hand, the amorphous red phosphorus (as shown above) is denser but is less reactive than the white counterpart. It is essentially less toxic and safer to handle.

Interatomic distance: 221 pm (white)
Melting point: 44.15C (white)
Boiling point: 280.5C (white)
Thermal conductivity/Wm-1K-1: 0.235 (27C)
Density/kgm-3: 1820 (white, 20C), 2200 (red, 20C)

Standard Thermodynamic Data (atomic gas, white)

Enthalpy of formation: 316.5 kJ/mol
Gibbs free energy of formation: 280.1 kJ/mol
Entropy: 163.2 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Ne] 3s2 3p3
Term symbol: 4S3/2
Electron affinity: 72.0264 kJ/mol   Electronegativity (Pauline): 2.19
Ionization energy (first, second, third): 1011.81, 1907.46, 2914.06 kJ/mol

Chemical properties

White phosphorus is very reactive and can spontaneously ignite in air at 35C. However, the reactivity decreases for polymeric allotrope, the red and the black forms. On exposure to air, white phosphorus emits a faint green light visible in the dark. If carbon dioxide gas is passed over warm phosphorus in the flask, the gas emerging from the flask appears to burn with a green flame. However, the flame is cold and will not ignite a match.

Test for orthophosphate ions (PO43-):
(1) Heat with ammonium molybdate and nitric acid gives yellow precipitate.
(2) Ammoniacal solution of magnesium and ammonium chlorides give white precipitate.
(3) Neutral silver nitrate solution gives yelllow precipitate.

Test for pyrophosphate ions (P2O74-):
(1) Neutral silver nitrate gives a white gelatinous precipitate.
(2) Pyrophosphate solution coagulates albumen. Such effect is not observed in ortho- and meta- phosphates.

Test for metaphosphate ions (PO3-):
(1) Neutral silver nitrate gives a white crystalline precipitate.

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