Oxidation Number Rules Explained with Examples

📚 Quick Study Guide

• ⚛️ Oxidation number represents the hypothetical charge an atom would have if all bonds were completely ionic.
• ➕ The oxidation number of an atom in its elemental form is always 0 (e.g., $Na$, $O_2$).
• 🧪 The oxidation number of a monatomic ion is equal to its charge (e.g., $Na^+$ is +1, $Cl^-$ is -1).
• ⚗️ Oxygen usually has an oxidation number of -2, except in peroxides (like $H_2O_2$) where it's -1 or when bonded to fluorine (where it's positive).
• 💡 Hydrogen usually has an oxidation number of +1, except when bonded to metals in binary compounds, where it's -1 (e.g., $NaH$).
• ⚖️ The sum of the oxidation numbers in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion equals the charge of the ion.
• 🔎 Fluorine always has an oxidation number of -1. Other halogens usually have an oxidation number of -1, but can be positive when combined with oxygen or fluorine.

Practice Quiz

1. What is the oxidation number of carbon in $CO_2$?
   1. -4
   2. +4
   3. -2
   4. +2
2. What is the oxidation number of oxygen in $OF_2$?
   1. -2
   2. +2
   3. -1
   4. +1
3. What is the oxidation number of sulfur in $SO_4^{2-}$?
   1. +2
   2. +4
   3. +6
   4. +8
4. What is the oxidation number of hydrogen in $NaH$?
   1. +1
   2. -1
   3. 0
   4. +2
5. What is the oxidation number of chlorine in $HClO_3$?
   1. +3
   2. +5
   3. +1
   4. +7
6. What is the oxidation number of manganese in $KMnO_4$?
   1. +2
   2. +4
   3. +6
   4. +7
7. What is the oxidation number of chromium in $Cr_2O_7^{2-}$?
   1. +3
   2. +6
   3. +12
   4. +7