Oxidation Number and Monatomic Ion Charge

Introduction

For a monatomic ion, oxidation number equals the ion's charge. Fe³⁺ has +3, Cu⁺ has +1 and Cl⁻ has −1. This simple rule connects the earlier electron-counting half-reactions to the broader oxidation-number system used for compounds. It has a precise boundary.

Core explanation

A single Zn²⁺ ion consists of one zinc atom and has total charge +2. Its zinc oxidation number is therefore +2. In Zn → Zn²⁺ + 2e⁻, zinc moves from zero as a free element to +2 as a monatomic ion. That increase marks oxidation and agrees with the loss of two electrons. The same direct reading works for Na⁺ at +1, Mg²⁺ at +2 and Al³⁺ at +3.

Negative monatomic ions carry negative oxidation numbers equal to their charge. Cl⁻ is −1, O²⁻ is −2 and S²⁻ is −2. When Cl₂ is reduced to 2Cl⁻, each chlorine atom changes from 0 to −1. Two chlorine atoms accept a total of two electrons: Cl₂ + 2e⁻ → 2Cl⁻. The left charge is −2 after including electrons, and the right is two times −1, also −2.

Monatomic ions can change charge without becoming elemental substances. Fe²⁺ → Fe³⁺ + e⁻ is oxidation because iron goes +2 → +3. Fe³⁺ + e⁻ → Fe²⁺ is reduction because iron goes +3 → +2. The names iron(II) and iron(III) report the oxidation states in these ions, but the Roman numerals are not coefficients. One Fe³⁺ ion is still one iron atom.

The rule should not be extended carelessly to atoms within polyatomic ions. In SO₄²⁻, the sulfate ion's overall charge is −2, but sulfur's oxidation number is +6 under usual oxygen assignments, and each oxygen is −2. The sum +6 + 4(−2) = −2 gives the ion charge. Calling sulfur −2 merely because sulfate is −2 would confuse an atom's oxidation number with a multi-atom species's total charge.

The same caution applies to neutral covalent molecules. Carbon is +4 in CO₂ by the usual formal rules, but CO₂ has no net ionic charge. The monatomic-ion rule is a special case because the entire charged species consists of exactly one atom. In a molecule or polyatomic ion, distribute the total among atoms according to the assignment conventions.

Ion charge is written with number before sign, such as Fe³⁺. Oxidation number is commonly written with sign before number, such as +3. This notation convention helps keep the two concepts distinct even when their numerical values agree for a monatomic ion. A charge of 3+ and oxidation number +3 convey different kinds of statements.

Step-by-step reasoning

1. Check that the species contains exactly one atom and is an ion. 2. Read its signed ion charge. 3. Assign the same signed value as oxidation number for that atom. 4. Compare with the same element in its starting or final species. 5. Classify a rise as oxidation or a fall as reduction.

Visual explanation

Draw three cards: Fe³⁺ with “ion charge 3+, oxidation number +3,” Cl⁻ with “charge 1−, number −1,” and SO₄²⁻ with “overall charge 2−; sulfur and oxygen numbers must be calculated separately.” The contrast shows the rule's boundary.

Real-world analogy

If one person alone owes three units, the group's balance and that person's balance are the same. If a group contains five people, the group's total debt does not identify each person's share. A monatomic ion has one atom, while a polyatomic ion requires distribution among several atoms.

Real-world example

An iron(II) solution may be converted to an iron(III) solution in a redox process. Fe²⁺ and Fe³⁺ have distinct charges and colors in many chemical environments. Their oxidation numbers, +2 and +3, show directly that iron oxidation has occurred when Fe²⁺ becomes Fe³⁺.

Why?

Why does oxidation number equal charge for a monatomic ion? There are no bonds to another atom within the species whose electron pair must be allocated. The formal electron count for the one atom accounts for the entire ion's net electrical charge.

Common misconception

“SO₄²⁻ means sulfur has oxidation number −2.” Sulfate contains five atoms, and −2 is the ion's overall charge. Sulfur is +6 when each of four oxygens is −2, making the sum −2.

Worked example

Determine redox changes in 2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺. Each monatomic ion's oxidation number equals its charge: Fe +3 → +2 is reduction by one unit for each of two ions; Sn +2 → +4 is oxidation by two units. The total decrease for two iron ions is two and the increase for tin is two. Charge checks: left 2(+3) + (+2) = +8; right 2(+2) + (+4) = +8.

Quick check

1. What oxidation number belongs to a single S²⁻ ion, and why? Answer: Sulfur is −2 because the monatomic ion's oxidation number equals its overall charge of two negative units.

Exam focus

Apply charge equals oxidation number only to monatomic ions. Include the sign and distinguish ionic-charge notation from oxidation-number notation. For polyatomic ions, use the sum rule rather than assigning the overall charge to one atom.

Advanced insight

Even in a monatomic ion, oxidation number is a formal assignment, though it equals an experimentally meaningful net charge. The equality becomes less direct in bonded species because electron density can be shared or delocalised. This is why one rule cannot replace all oxidation-state reasoning.

Summary

The oxidation number of a monatomic ion equals its charge. This makes changes such as Fe²⁺ → Fe³⁺ and Cl₂ → Cl⁻ easy to classify. A polyatomic ion's charge belongs to the entire species and must be distributed among its atoms by further rules.

Practice questions

1. What is the oxidation number of copper in Cu⁺? Answer: +1, equal to the charge of the monatomic copper ion. 2. What is oxygen's oxidation number in O²⁻? Answer: −2, equal to the charge of the monatomic oxide ion. 3. Is Fe²⁺ → Fe³⁺ oxidation or reduction? Answer: Oxidation, because iron's oxidation number rises from +2 to +3. 4. Why is sulfur not −2 in SO₄²⁻? Answer: The −2 value is the whole ion's charge; sulfur and four oxygen atoms must have numbers summing to it.