Elements in the s- or p-blocks generally have only one common oxidation number, while d-block elements can have more than one possible oxidation number.

This statement is generally true due to the nature of the electron configurations of these elements.

1. s-block elements: These elements (Groups 1 and 2, including hydrogen and helium) have their outermost electrons in the s orbital. Group 1 elements (alkali metals) commonly have an oxidation number of +1, while Group 2 elements (alkaline earth metals) commonly have an oxidation number of +2. This is because they tend to lose electrons in order to achieve a stable electron configuration, and the oxidation number represents the charge that the atom would have if all bonds were ionic.

2. p-block elements: These elements (Groups 13 to 18) have their outermost electrons in the p orbital. The common oxidation numbers for these elements are usually the group number minus ten (for Groups 13 to 15) or the negative of the group number minus eighteen (for Groups 16 to 18). This is because they tend to gain, lose, or share electrons to achieve a stable electron configuration.

3. d-block elements: These elements (transition metals) have their outermost electrons in the d orbital. They can have more than one possible oxidation number because they can lose different numbers of s and d electrons. For example, iron can have oxidation numbers of +2 or +3, and copper can have oxidation numbers of +1 or +2. This ability to have multiple oxidation states is one of the characteristic properties of transition metals.