Usual Hydrogen Oxidation Number and Hydrides

Introduction

Hydrogen is commonly +1 when combined with nonmetals, but it is −1 in many simple metal hydrides. In elemental H₂ it is zero. These values matter when using hydrogen transfer as a redox clue: count the actual chemical environment rather than applying +1 automatically.

Core explanation

In water, H₂O, each hydrogen is +1 and oxygen is −2, giving 2(+1) − 2 = 0. In HCl, hydrogen is +1 and chlorine −1. In ammonia, NH₃, three hydrogens each at +1 require nitrogen −3. These examples match the common rule for hydrogen with nonmetals.

Elemental H₂ is different: each atom has oxidation number zero. During H₂ + Cl₂ → 2HCl, hydrogen changes 0 → +1 and is oxidised, while chlorine changes 0 → −1 and is reduced. During C₂H₄ + H₂ → C₂H₆, hydrogen from H₂ also changes from 0 to its usual positive assignment in C–H bonds; the carbon-containing substrate is reduced.

Sodium hydride, NaH, illustrates the metal-hydride exception. Sodium as a group 1 metal is +1 in the neutral compound, so hydrogen must be −1. Calcium hydride, CaH₂, has calcium +2 and two hydrogens at −1 each. Calling hydrogen +1 in these formulas would make the oxidation-number sums +2 or +4 rather than zero. The metal's known value and the sum rule expose the correct assignment.

Hydride chemistry has variety, so a formula and bonding context are needed. The school rule “hydrogen is −1 with metals” is a useful guide for simple ionic hydrides of highly electropositive metals, but it should not be stretched to every complex metal–hydrogen compound without examination. The introductory NaH and CaH₂ cases clearly show the contrast.

A compound's formula may contain hydrogen in more than one role. For an introductory calculation, use the specified species and standard rules rather than guessing from an everyday name. If a reaction converts H⁻ in a metal hydride to H₂, hydrogen rises from −1 to 0 and is oxidised. If H₂ becomes H⁻, it falls from 0 to −1 and is reduced. Signed comparisons, not “hydrogen gained” language alone, determine the formal direction.

When hydrogen is +1 in both a reactant acid and product water, hydrogen itself has not changed oxidation number, even if a proton moved. For example, HCl + NaOH → NaCl + H₂O involves acid–base transfer but not hydrogen redox. This separates proton movement from electron transfer and prevents a common overclassification.

Step-by-step reasoning

1. Identify whether hydrogen is in free H₂, a compound with a nonmetal or a simple metal hydride. 2. Assign 0, usually +1 or usually −1, respectively. 3. Check the whole formula sum against its charge. 4. Assign hydrogen in its product form independently. 5. Compare signed values and identify any paired element change.

Visual explanation

Draw three columns: H₂ with H 0, H₂O with H +1, and NaH with H −1. Below each write the sum check: elemental 0; 2(+1) + (−2) = 0; (+1) + (−1) = 0. This makes the environment dependence visible.

Real-world analogy

The same person can have different roles in different teams. Hydrogen's formal number similarly depends on its bonding partner: with oxygen it is commonly +1, while in simple sodium hydride it is −1. The analogy does not substitute for the charge-sum calculation.

Real-world example

Some metal hydrides react with water to release hydrogen gas. The reaction can involve hydrogen atoms from both hydride and water, so a balanced equation and separate oxidation-number assignments are needed to track which atoms change. One should not describe the process from the word “hydride” alone.

Why?

Why does sodium hydride give hydrogen −1? Sodium is assigned +1 in its common compound, and neutral NaH must sum to zero. Hydrogen therefore receives −1 in the formal accounting, reflecting its different electron-allocation environment from hydrogen bonded to nonmetals.

Common misconception

“Hydrogen is always +1 except when it is H₂.” NaH and CaH₂ show another important case: hydrogen is −1 in these simple metal hydrides. Apply the species-specific rule and check the total.

Worked example

Find hydrogen's number in CaH₂ and compare with H₂. Calcium is +2 in the neutral hydride. Let each H be x; +2 + 2x = 0 gives x = −1. In elemental H₂, each H is 0. Thus a formal change from hydride H to H₂ would raise hydrogen's oxidation number, an oxidation for those hydrogen atoms. A complete reaction must show the other species and the matched reduction.

Quick check

1. What oxidation number has hydrogen in neutral NaH when sodium is +1? Answer: Hydrogen is −1, making the sodium and hydrogen assignments sum to zero.

Exam focus

Distinguish H₂, hydrogen with nonmetals and simple metal hydrides. Use the sum rule as a check. Proton movement in an acid–base equation is not by itself proof of a hydrogen oxidation-number change.

Advanced insight

Hydride can mean different structural classes, including covalent compounds and metal complexes. The simple −1 assignment is most transparent in highly ionic hydrides. More complex bonding may require a careful formal convention rather than an unqualified shortcut.

Summary

Hydrogen is zero in H₂, usually +1 with nonmetals and −1 in simple metal hydrides such as NaH and CaH₂. The complete formula's charge provides a check. Compare the actual starting and final environments before calling a hydrogen change oxidation or reduction.

Practice questions

1. What is hydrogen's number in HCl? Answer: +1, balanced by chlorine at −1. 2. What is hydrogen's number in H₂? Answer: Zero for each atom because H₂ is elemental hydrogen. 3. What is hydrogen's number in CaH₂? Answer: −1 for each hydrogen, balancing calcium at +2. 4. Does proton transfer in HCl + NaOH → NaCl + H₂O prove redox? Answer: No. Hydrogen remains at its usual +1 assignment, so proton transfer alone is not an oxidation-number change.