CBSE Class 9 Answered

In chemistry, valence, also known as valency or valency number, is a measure of the number of chemical bonds formed by the atoms of a given element . The above definition Suggested in problem is not sufficient as many elements form bonds in a covalent fashion or co-ordinate bonds.The number of bonds formed by a given element was originally thought to be a fixed chemical property and in fact, in many cases, this is a good approximation. For example, in many of their compounds, carbon forms four bonds, oxygen two and hydrogen one. However it soon became apparent that, for many elements, the valence could vary between different compounds. One of the first examples to be identified was phosphorus, which sometimes behaves as if it has a valence of three and sometimes as if it has a valence of five.