why is Halogens known as the reactive nonmetals?

Answer 1

Well obviously because there are reactive (because they only have one-electron in their outer shell) and because they are nonmetals.

Answer 2

Halogens have one electron less in their outer orbit to complete the stable state configuration. Thus they react with metals which have one free electron. Thus Na (Sodium) and Chlorine react to form NaCl. Hydrogen and Chlorine form HCl etc. Hydrogen is classified as a metalloid and is rather abnormal in its behavior.

The classification of metals and non-metals is primarily linked to the availability of FREE electrons which give the metals their good electrical and thermal conductivity. Since Halogens are deficient in electrons, they lack the properties of metals and behave as non-metals. Since they are reactive (they are eager to reach a stable state by forming stable compounds with the metals), they are reactive non-metals.