What is the oxidation number of hydrogen?
Introduction to Oxidation Numbers
In chemistry, an oxidation number (also known as an oxidation state) is a theoretical indicator of the degree of oxidation of an atom in a chemical compound. It tells us how many electrons an atom has gained, lost, or seemingly shared when it bonds with other elements.
When it comes to hydrogen, determining its oxidation number is usually straightforward, but there are a few fascinating exceptions you need to know to master chemistry. Let's dive into the rules, exceptions, and practical examples!
The Golden Rules for Hydrogen
Most of the time, hydrogen plays by a predictable set of rules. As the first element on the periodic table, hydrogen has just one electron, which heavily dictates its bonding behavior.
1. In Most Compounds: +1
When hydrogen bonds with nonmetals (such as water, $H_2O$, or hydrochloric acid, $HCl$), it has an oxidation number of +1. This happens because nonmetals are more electronegative than hydrogen, meaning they pull the bonding electrons closer to themselves.
2. In Pure Elements: 0
Like all uncombined elements, pure hydrogen gas ($H_2$) has an oxidation number of 0 because it is bonded to an identical atom with the exact same electronegativity.
3. In Metal Hydrides: -1
When hydrogen bonds with active metals (such as sodium hydride, $NaH$, or calcium hydride, $CaH_2$), its oxidation number flips to -1. Because metals are less electronegative than hydrogen, hydrogen actually pulls the electron density toward itself!
Quick Reference Table
Here is a handy comparison of hydrogen's oxidation states depending on its chemical bonding partner:
| Bonding Partner | Example Compound | Chemical Formula | Oxidation Number of Hydrogen |
|---|---|---|---|
| Pure Element | Hydrogen Gas | $H_2$ | 0 |
| Nonmetal | Water | $H_2O$ | +1 |
| Metal | Sodium Hydride | $NaH$ | -1 |
Real-World Examples
Let's look at how we calculate the oxidation number of hydrogen in everyday chemical contexts.
- Water ($H_2O$): Oxygen almost always has an oxidation number of -2. To keep the overall molecule neutral (sum equals 0), the two hydrogen atoms must split a total charge of +2, meaning each hydrogen is +1.
- Ammonia ($NH_3$): Nitrogen usually takes a -3 oxidation state in simple compounds. Therefore, the three hydrogens must balance this out with a total of +3, making each hydrogen +1.
- Lithium Hydride ($LiH$): Lithium is an alkali metal and strictly maintains a +1 oxidation state in compounds. To balance this, the hydrogen atom must take an oxidation state of -1.
Common Pitfalls to Avoid
Students often make mistakes when calculating oxidation numbers because they memorize rules too rigidly. Keep these tips in mind:
- Don't confuse oxidation numbers with ionic charges: While they look similar (e.g., +1 or -1), oxidation numbers are a bookkeeping tool for all bonds (including covalent ones), not just ionic ones.
- Watch out for the metal vs. nonmetal distinction: Always check what hydrogen is bonded to. If it is attached to the left side of the periodic table (metals), think -1. If it is attached to the right side (nonmetals), think +1.