The oxidation state of hydrogen is most commonly +1.
Hydrogen typically exhibits an oxidation state of +1 when it forms compounds with more electronegative elements, which include most nonmetals. In these situations, hydrogen tends to lose its single valence electron, effectively becoming a H$^+$ ion.
However, there is an important exception to this general rule:
- In metal hydrides, hydrogen has an oxidation state of -1. This occurs when hydrogen bonds with a metal, which is less electronegative than hydrogen. Here, hydrogen gains an electron to form a hydride ion (H$^-$).
Understanding Oxidation States
Oxidation states (also known as oxidation numbers) are assigned to atoms in molecules or ions to help track electron distribution and identify which atoms are being oxidized or reduced in a chemical reaction. They represent the hypothetical charge an atom would have if all its bonds were purely ionic.
Common Oxidation States for Key Elements
The following table summarizes the usual oxidation states for some common elements, including hydrogen:
| Element | Usual Oxidation State | Exceptions |
|---|---|---|
| Hydrogen | +1 | Metal hydrides (-1) |
| Oxygen | -2 | Peroxides (e.g., H₂O₂ = -1) and F₂O (e.g., +2) |
| Fluorine | -1 | Always |
| Chlorine | Usually -1 | Compounds with O or F (e.g., HClO = +1, ClF₃ = +3) |
Examples of Hydrogen's Oxidation States
Understanding the context of the compound helps determine hydrogen's oxidation state.
-
+1 Oxidation State (Most Common):
- Water (H₂O): Oxygen is significantly more electronegative than hydrogen. Each hydrogen atom has an oxidation state of +1.
- Hydrochloric Acid (HCl): Chlorine is more electronegative. Hydrogen has an oxidation state of +1.
- Methane (CH₄): Carbon is more electronegative than hydrogen. Each hydrogen atom has an oxidation state of +1.
-
-1 Oxidation State (Metal Hydrides):
- Sodium Hydride (NaH): Sodium is an alkali metal and is less electronegative than hydrogen. Sodium has an oxidation state of +1, while hydrogen has an oxidation state of -1.
- Calcium Hydride (CaH₂): Calcium is an alkaline earth metal. Calcium has an oxidation state of +2, and each hydrogen atom has an oxidation state of -1.
These examples clearly demonstrate how the nature of the bonding partner (its electronegativity) dictates hydrogen's specific oxidation state within a compound.
Importance in Chemistry
Knowing the oxidation states of elements is fundamental for comprehending redox reactions, which are crucial in many chemical and biological processes. These reactions involve the transfer of electrons, and changes in oxidation states indicate which atoms are being oxidized (losing electrons) and which are being reduced (gaining electrons).
For a deeper dive into the rules for assigning oxidation states, you can explore resources like Chemistry LibreTexts.
