Question

how are the elements with variable valency named

Answer 1

Valency is the number of hydrogen atoms that can combine with (or displace) one atom of the element to form a compound. For example, two atoms of hydrogen combine with one atom of oxygen to form water (H2O); so, the valency of oxygen is two.

For elements that do not combine with hydrogen, the valency is the combining power of the element with another element whose valency is known. Valency may also be defined as the number of electrons that an atom donates or accepts to form the duplet state (i.e., 2 electrons in outermost shell) or octet state (i.e., 8 electrons in outermost shell).

The valency of an element is always a whole number. Some elements exhibit more than one valency, i.e., they have variable valency. In Iron [II] sulfate or ferrous sulfate, i.e., FeSO4, the valency of iron is two. In Iron [III] sulfate or ferric sulfate, i.e., Fe2(SO4)3, the valency of iron is three. Generally, the Latin / Greek name for the element (e.g., Ferrun) is modified to end in 'ous' for the lower valency (e.g., Ferrous) and to end in 'ic' for the higher valency (e.g., Ferric). The table below lists some elements that exhibit variable valencies.

Answer 2

Valency of an element is decided by numer of outermost shell electrons. ... (d-orbital) : some of the elements have vacant sub-shell in same energy level. so they expand their valency and exhibit more than 1 valency in its compound. Phosphorus form PCl3 and PCl5 with chlorine. 3 is the normal valency of phosphorus.