Question

I find very difficult to learn valancy in chemistry. Please help.

Answer 1

valency is nothing but the number of electrons present in outermost shell of an atom.

we can find it by electronic configuration.

ex :--

Na:-

1s²2s²2p63s¹

valency is +1

Answer 2

Valency is the combining capacity of an element.

For metals, it is equal to the number of electrons in the outermost shell of their atoms, and for nonmetals it is eight minus the number of electrons in the outermost shell. Transition metals like iron and copper show more than one valency in compound formation.

The valency of hydrogen is conventionally taken as 1 (one). Based on this, valency of an element can be defined as the number of hydrogen atoms with which an atom of the element is capable of combining.

The alkali metals combine with hydrogen in 1:1 ratio to form hydrides like LiH, NaH etc., and so their valency is 1. Similarly, the halogens also combine in 1:1 ratio to form the hydrogen halides like HCl, and so they are also monovalent. But, oxygen and nitrogen combine with hydrogen in 1:2 and 1:3 atomic ratios to form H2O and NH3 respectively. So, oxygen's valency is 2, and for nitrogen it is 3.

For metals, it is more convenient to define valency in terms of the number of chlorine atoms with which one metal atom combines to form their normal chlorides. Thus, the valencies of sodium, magnesium and aluminium are 1, 2 and 3 as they form NaCl, MgCl2 and AlCl3 in the atomic ratios 1:1, 1:2 and 1:3 respectively. Iron’s valencies are 2 and 3 as it forms two normal chlorides- FeCl2 and FeCl3.

Valency is different from oxidation number, and it has NO SIGN. Thus, the valency of nitrogen is 3, whereas it can have oxidation numbers from -3 to +5. Oxidation number is the hypothetical charge of an atom in a molecule or ion, and it is a measure of its apparent capacity to gain or lose electrons within that species.

It can be seen that valency of an element is numerically equal to its most common oxidation number, without any positive or negative sign. Thus, though sulphur can have oxidation numbers from -2 to +6, its valency is 2, as it forms simple compounds like H2S, Na2S, PbS, ZnS and Al2S3 in the -2 state.