Question

Some elements show variable valency. ( like copper = 1,2 ) how to know that which valency will applied while making a reaction ?
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Answer 1

Answer:

Valency is the combining capacity of an element. The number of electrons donated or accepted by an atom of an element so as to have electronic configuration of nearest noble gas is called its valency.

Answer 2

Answer:

Explanation:

Valency of copper like other transition metals is not immediately obvious without further information.

First since metals do not form covalent bonds, it is better to refer to it as oxidation state rather than valency. It is +1 for cuprous CU+ and +2 for cupric CU++.

Now looking at the electronic configuration of copper you may get [Ar] 4s1 3d10 which has one electron in outer shell or you may get [Ar] 4s2 3d9 which is very close in energy to the other state and has 2 electrons in the outer shell. The d electrons are tightly bound and do not participate. Copper has variable valency and you really do not know which until you see the salt or have some other chemical information. For example if it is Copper(I)chloride, CuCl, oxidation state is +1 and the solid is green and little soluble in water. If it is Copper(II) chloride, CuCl2, oxidation state is +2 and the solid is brown in anhydrous and blue in hydrated form and extremely soluble in water. You can get more info at Wiki about the chemical reactions of these compounds.