Non-metals react by gaining electrons to become negative ions. A univalent non-metal accepts one electron while a divalent one accepts two electrons. The ion formed carries the corresponding number of negative charges, but they rarely exceed two and never exceed three.
When a substance loses electron(s), it becomes oxidized, i.e. its oxidation number increases. This is called oxidation. Due to the fact that non-metals accept the electrons(s) donated by other substances, particularly metals, they are, therefore, termed as oxidizing agents. This is because by accepting the electrons, they help oxidize the electron donors.
Those substances or metals which donated the electrons are called reducing agents. This is because the electrons they donate reduce the oxidation number of non-metals. This process is called reduction. In this respect, non-metals act as oxidizing agents while metals act as reducing agents.
Strong and weak oxidants.
As we have already seen, non-metals ionize by electron gain. In all cases, the extra electron(s) accepted lead to the formation of negative ions. The easiness of formation of negative ions depends on the ability of an element to accept the electrons. The ability of accepting electrons is called electronegativity of an element. Some elements are more electronegative than others.
The order of electronegativity of some non-metals is as follows: Fluorine < Chlorine > Bromine > Iodine > Nitrogen > Carbon. The degree of electronegativity indicates reactivity and hence oxidizing power of the element. Elements with higher electronegativity will displace those elements with lower electronegativity from their compounds.
Referring to the series above, fluorine will displace all the rest of the elements from their compounds as it is more electronegative than any other element in the series. Likewise, chlorine can displace bromine, iodine and nitrogen from their compounds. The higher the electronegativity the stronger the oxidant. For example, bromine is a stronger oxidant than iodine, nitrogen and carbon.