AS Level Chemistry Practice Exam

The oxidation number of an element in its most basic form, known as its natural or elemental state, is always zero. This applies to all elements when they are not combined with other elements or in a different form. For example, oxygen as O2, nitrogen as N2, and hydrogen as H2 all have an oxidation number of zero.

The question specifically asks about natural elements, highlighting that any element in its elemental form will exhibit this property. Therefore, choosing natural elements aligns perfectly with the concept that their oxidation number is indeed zero.

In contrast, individually selected elements like hydrogen, fluorine, or oxygen can have different oxidation states depending on their compounds or molecular forms. For instance, molecular hydrogen (H2) has an oxidation number of zero, but in compounds it may have a positive oxidation state. Similarly, fluorine typically has a -1 oxidation state in compounds. Oxygen often has a -2 oxidation state when bonded with metals or in oxides. Hence, the choice of natural elements encapsulates the correct fundamental principle that all of them have an oxidation number of zero when in their natural forms.