THE PHOSPHORUS FAMILY

	SYMBOL	ATOMIC WEIGHT	DENSITY	MELTING POINT
Phosphorus	P	31.0	1.8	43.3°
Arsenic	As	75.0	5.73	—
Antimony	Sb	120.2	6.7	432°
Bismuth	Bi	208.5	9.8	270°

The family. The elements constituting this family belong in the same group with nitrogen and therefore resemble it in a general way. They exhibit a regular gradation of physical properties, as is shown in the above table. The same general gradation is also found in their chemical properties, phosphorus being an acid-forming element, while bismuth is essentially a metal. The other two elements are intermediate in properties.

Compounds. In general the elements of the family form compounds having similar composition, as is shown in the following table:

PH₃	PCl₃	PCl₅	P₂O₃	P₂O₅
AsH₃	AsCl₃	AsCl₅	As₂O₃	As₂O₅
SbH₃	SbCl₃	SbCl₅	Sb₂O₃	Sb₂O₅
	BiCl₃	BiCl₅	Bi₂O₃	Bi₂O₅

In the case of phosphorus, arsenic, and antimony the oxides are acid anhydrides. Salts of at least four acids of each of these three elements are known, the free acid in some instances being unstable. The relation of these acids to the corresponding anhydrides may be illustrated as follows, phosphorus being taken as an example:

P₂O₃ + 3H₂O = 2H₃PO₃ (phosphorous acid).

P₂O₅ + 3H₂O = 2H₃PO₄ (phosphoric acid).

P₂O₅ + 2H₂O = H₄P₂O₇ (pyrophosphoric acid).

P₂O₅ + H₂O = 2HPO₃ (metaphosphoric acid).