Introduction:
Phosphorus is a chemical element that belongs to group 15 of the periodic table. It has an atomic number of 15 and is represented by the symbol P. Phosphorus has two valencies, +3 and -3, which allow it to form different types of chemical bonds.

Valencies:
Valency is the combining capacity of an element, which determines the number of electrons an atom can gain, lose, or share to form a stable compound. The valency of an element is determined by the number of valence electrons it has.

Electronic configuration:
Phosphorus has an electronic configuration of 1s2 2s2 2p6 3s2 3p3, with five valence electrons in its outermost energy level (3s and 3p orbitals). These valence electrons are involved in the formation of chemical bonds.

Valency of +3:
Phosphorus can achieve a stable configuration with 8 valence electrons by losing 3 electrons from its outermost energy level. This results in the formation of a cation with a charge of +3. The loss of electrons creates a positive charge, allowing phosphorus to combine with elements that have a valency of -3, such as chlorine. This valency is characteristic of phosphorus in many of its compounds, such as phosphorus trichloride (PCl3) and phosphorus pentachloride (PCl5).

Valency of -3:
Phosphorus can also achieve a stable configuration by gaining 3 electrons to fill its outermost energy level. This results in the formation of an anion with a charge of -3. The gain of electrons creates a negative charge, allowing phosphorus to combine with elements that have a valency of +3, such as aluminum. This valency is characteristic of phosphorus in compounds such as aluminum phosphate (AlPO4) and calcium phosphate (Ca3(PO4)2).

Summary:
Phosphorus has two valencies, +3 and -3, which allow it to form different types of chemical bonds. The valency of +3 is achieved by losing 3 electrons, resulting in a positive charge, while the valency of -3 is achieved by gaining 3 electrons, resulting in a negative charge. These valencies enable phosphorus to form compounds with various elements, contributing to its wide range of chemical properties and applications.