Rules for Assigning Oxidation Numbers

Word Cloud

What comes to mind when you hear the words OXIDATION and REDUCTION?

Multiple Choice

Which statement about oxidation numbers is true?

• They always match the actual charge of an atom

• They help determine electron transfer in redox reactions

• They are only used for covalent compounds

• They are the same as mass numbers

Rule 1
Free elements have an oxidation number of 0

• Any element in its pure form (not in a compound) has an oxidation number of 0.

• Examples: O₂, H₂, Na, Fe all have 0 oxidation numbers.

Multiple Choice

What is the oxidation number of nitrogen in N₂?

• +1
• -3
• +2
• 0

Rule 2
Monoatomic ions have oxidation numbers equal to their charge

• The oxidation number of a single-atom ion is the same as its charge.

• Examples: Na⁺ = +1, Cl⁻ = -1, Mg²⁺ = +2

Fill in the Blanks

What is the oxidation number of Fe³⁺?

Rule 3
Oxygen is usually -2, except in Peroxides (-1) or with Fluorine (+2)

• Oxygen is normally -2 (e.g., H₂O, CO₂).

• In peroxides (H₂O₂), oxygen is -1.

• With fluorine (OF₂), oxygen is +2.The oxidation number of a single-atom ion is the same as its charge.

Multiple Choice

What is the oxidation number of oxygen in H₂O₂?

• -1
• -2
• +2
• 0

Rule 4
Hydrogen is +1 with nonmetals and -1 with metals

• In compounds like HCl or H₂O, hydrogen is +1.

• In metal hydrides like NaH, hydrogen is -1.

Multiple Choice

What is the oxidation number of hydrogen in NaH?

• +1
• 0
• +2
• -1

Rule 5
The sum of oxidation numbers in a neutral compound is 0

• Example: H₂O → H (+1) x 2 + O (-2) = 0

Multiple Choice

What is the oxidation number of carbon in CO₂?

CO₂ → C (???) + O₂ (-2 x 2) = 0

• +1
• +4
• +2
• 0

• Example:

• SO₄²⁻ → S (+6) + O₄ (-2 x 4) = -2

• Example:

• NO₃⁻ → N (+5) + O₃ (-2 x 3) = -1

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​Rule 6
The sum of oxidation numbers in a polyatomic ion equals Its Charge

Multiple Choice

What is the oxidation number of sulfur in SO₄²⁻?

• +4
• +2
• 0
• +6

Use the rules for assigning oxidation numbers to answer the following questions.

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Let's Play!

Multiple Choice

What is the oxidation number of hydrogen in LiH?

• -1
• 0
• +1
• +2

Multiple Choice

What is the oxidation number of nitrogen in NH₃?

• -3
• 0
• +1
• +3

Multiple Choice

What is the oxidation number of sulfur in SO₃²⁻?

• 0
• +6
• +2
• +4

Multiple Choice

What is the oxidation number of phosphorus in P₄?

• -1
• +3
• +5
• 0

Multiple Choice

What is the oxidation number of bromine in Br⁻?

• 0
• +1
• -2
• -1

Multiple Choice

What is the most common oxidation number of oxygen in compounds?

• +2
• +1
• -2
• 0

Use the rules to answer the following questions AND a few surprise questions. :)

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​Challenge Round

Multiple Choice

What is the oxidation number of chromium in Cr₂O₇²⁻?

• +6
• +5
• +4
• +3

Multiple Choice

What is the oxidation number of chlorine in ClO₄⁻?

• +5
• +3
• 0
• +7

Multiple Choice

Identify the oxidation state of phosphorus in PO₄³⁻.

• +3
• +5
• -3
• +1

Multiple Choice

Which of the following represents an oxidation half-reaction?

• Fe²⁺ → Fe³⁺ + e⁻

• O₂ + 4e⁻ → 2O²⁻

• Cu²⁺ + 2e⁻ → Cu

• Ag⁺ + e⁻ → Ag

Multiple Choice

Which of the following represents a reduction half-reaction?

• Mg → Mg²⁺ + 2e⁻

• Zn → Zn²⁺ + 2e⁻

• Cl₂ + 2e⁻ → 2Cl⁻

• Fe²⁺ → Fe³⁺ + e⁻