A-level Chemistry/OCR (Salters)/Phosphorus

Phosphorus is the fifteenth element in the periodic table. The phosphorus nucleus therefore contains fifteen protons.

Isotopes

Only phosphorus-31, ³¹P, is stable, so all naturally-occurring phosphorus is this isotope, which contains 16 neutrons. The ³¹P nucleus is NMR-active, and ³¹P NMR is widely used in chemistry.

³²P is radioactive and emits beta particles; it is used in biological experiments.

Electron configuration

The electron configuration of the neutral phosphorus atom, H, in the gas phase is

$1{\text{s}}^{2}2{\text{s}}^{2}2{\text{p}}^{6}3{\text{s}}^{2}3{\text{p}}^3$

which is more concisely written as

$[\text{Ne}]3{\text{s}}^{2}3{\text{p}}^3$

This simplifies to 2.8.5 in GCSE-style electron shell notation.

Allotropes

In its standard elemental state, phosphorus is known to exist in several infinite covalent network structures. The most important allotropes of P are:

• White phosphorus: contains discrete P₄ tetrahedra (these also exist in liquid P)
• Black phosphorus: sheets of phosphorus atoms stacked on top of each other - each sheet is puckered
• Red phosphorus: amorphous, like glass

• 
  ball-and-stick model of a sheet of P atoms in black phosphorus

Compounds

Phosphorus forms compounds with most elements in the periodic table. Due to its larger size, phosphorus is much worse at forming double bonds than nitrogen is, so it mostly forms single bonds. One key exception is the very strong P=O bond is phosphoric acid.

Phosphates

Salts of phosphoric acid, contain the phosphate ion, PO₄³⁻.

• sodium phosphate, Na₃PO₄
• magnesium phosphate, Mg₃(PO₄)₂
• aluminium phosphate, AlPO₄

Phosphides

In compounds with very electropositive elements, as in sodium phosphide, Na₃P, phosphorus can exist as the phosphide ion, P³⁻. P³⁻ does not exist in solution because it is exceptionally reactive. For example, if you try to dissolve Na₃P in water, the following reaction occurs:

Na₃P + 3H₂O(l) → PH₃(g) + 3NaOH(aq)

In compounds with elements whose electronegativity is similar to or greater than phosphorus, such as phosphorus trifluoride, PF₃, phosphorus is covalently bonded to the other element.

Organophosphorus compounds

• Phosphines: phosphorus analogues of amines
• Organophosphates: esters of phosphoric acid
• Organophosphites: esters of phosphorous acid