Which one of the following statements about the oxidation number of hydrogen in its compounds is correct, taking into account its behaviour with metals and non-metals?

Introduction / Context:
Oxidation numbers are useful for understanding redox reactions and the distribution of electrons in compounds. Hydrogen is a special element that can show different oxidation numbers depending on what it is bonded to. This question asks which statement correctly describes the possible oxidation numbers of hydrogen, connecting basic rules of oxidation states with examples of common compounds.

Given Data / Assumptions:

• We consider hydrogen in typical compounds with metals and non metals.
• Oxidation numbers are assigned using standard rules: elements in free state have zero oxidation number, etc.
• The options suggest that hydrogen might have a fixed or variable oxidation number.
• No unusual or exotic compounds are considered, just standard inorganic examples.

Concept / Approach:
In most compounds with non metals, such as water (H2O), hydrogen chloride (HCl), and methane (CH4), hydrogen has an oxidation number of +1. However, when hydrogen forms hydrides with metals, such as sodium hydride (NaH), calcium hydride (CaH2) or lithium aluminium hydride (LiAlH4), hydrogen has an oxidation number of -1. In its elemental form as H2, the oxidation number of hydrogen is zero. Therefore, hydrogen can have more than one oxidation number, usually +1 or -1, depending on the chemical environment.

Step-by-Step Solution:

Verification / Alternative check:
Checking common textbooks on redox chemistry confirms these rules. They state that hydrogen is usually +1 in compounds but is -1 in metal hydrides and zero in elemental form. Any statement that hydrogen always has the same oxidation number, regardless of compound, is described as incorrect. This confirms that the most accurate general statement is that hydrogen can have more than one oxidation number.

Why Other Options Are Wrong:

• The oxidation number for hydrogen is always zero: This is only true for hydrogen gas, not for its compounds.
• The oxidation number for hydrogen is always +1: This ignores metal hydrides where hydrogen is -1.
• The oxidation number for hydrogen is always -1: This only applies to hydrides with metals and is not the general case.
• Hydrogen never shows a negative oxidation number: This contradicts known examples like NaH and CaH2, in which hydrogen has oxidation number -1.

Common Pitfalls:
Students often memorise that hydrogen is +1 in all compounds and forget about metal hydrides. Another common mistake is to think that hydrogen behaves exactly like alkali metals in all cases. The best way to avoid errors is to recall that oxidation numbers are formal charges based on electronegativity and compound type. In hydrides with metals, the metal is less electronegative, so hydrogen takes the negative oxidation state.

Final Answer:
The correct statement is that Hydrogen can have more than one oxidation number.