ICSE Class 9 Answered

Oxidation state of an atom is always defined in the context of what else that atom combines with. For example, the oxidation number of hydrogen in hydrogen chloride(HCl) is +1, since chlorine is more electronegative, and is most likely to hold both of the bonding electrons pretty close to itself. Similarly, oxidation number of hydrogen in sodium hydride molecule (NaH) is -1, since hydrogen is more electronegative than sodium, and it does hold on to both the bonding electrons quite closely to itself (Na+H- is an ionic structure).
Oxidation number of hydrogen in hydrogen molecule (H₂) is zero, since both hydrogens equally share the bonding electrons.
So, the same atom can have three different oxidation numbers, depending on what else it combines with.
Don’t get so hung up on memorizing oxidation numbers of atoms. Just learn how to figure it out, based on what other elements are present in the compound. Usually, bonding pair follows the octet rule, but there are situations (e.g. BF₃) where octet rule is not followed.