Oxidation Number — Revision Notes

⚡ 30-Second Revision

Elemental State: — ON = 0 ($O_2$, $Na$)
Monatomic Ions: — ON = charge ($Na^+ = +1$, $Cl^- = -1$)
Group 1 Metals: — ON = +1 (in compounds)
Group 2 Metals: — ON = +2 (in compounds)
Fluorine: — ON = -1 (always in compounds)
Hydrogen: — ON = +1 (usually); ON = -1 (in metal hydrides like $NaH$)
Oxygen: — ON = -2 (usually); ON = -1 (in peroxides like $H_2O_2$); ON = $-1/2$ (in superoxides like $KO_2$); ON = +2 (in $OF_2$)
Sum of ONs: — 0 for neutral compounds; equals ion's charge for polyatomic ions.
Oxidation: — Increase in ON (loss of electrons).
Reduction: — Decrease in ON (gain of electrons).

2-Minute Revision

Oxidation number (ON) is a hypothetical charge assigned to an atom, assuming complete electron transfer in bonds. It's a crucial tool for redox reactions. Remember the core rules: elements have ON=0; monatomic ions have ON equal to their charge.

Group 1 metals are always +1, Group 2 are +2. Fluorine is always -1. Hydrogen is usually +1, but -1 in metal hydrides. Oxygen is usually -2, but -1 in peroxides, $-1/2$ in superoxides, and +2 in $OF_2$.

The sum of ONs in a neutral compound is zero, and in an ion, it equals the ion's charge. An increase in ON signifies oxidation (electron loss), while a decrease signifies reduction (electron gain). This helps identify oxidizing agents (get reduced) and reducing agents (get oxidized).

Pay special attention to exceptions for oxygen and hydrogen, as they are common NEET traps.

5-Minute Revision

The oxidation number (ON) is a formal charge that helps us track electron movement in chemical reactions. It's based on the assumption that electrons in a bond are completely transferred to the more electronegative atom. Mastering the rules is essential:

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Elements: — ON is 0 (e.g., $Fe$, $Cl_2$).
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Monatomic Ions: — ON equals the ion's charge (e.g., $Na^+$ is +1, $S^{2-}$ is -2).
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Group 1 & 2 Metals: — Always +1 and +2 respectively in compounds.
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Fluorine: — Always -1 in compounds due to its high electronegativity.
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Hydrogen: — Generally +1, but -1 in metal hydrides (e.g., $CaH_2$).
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Oxygen: — Generally -2, but critical exceptions include -1 in peroxides ($H_2O_2$), $-1/2$ in superoxides ($KO_2$), and +2 in $OF_2$ (where F is more electronegative).
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Sum Rule: — The sum of ONs in a neutral compound is 0; in a polyatomic ion, it equals the ion's charge.

Calculation Example: Find ON of Mn in $MnO_4^-$. Let Mn be $x$. $x + 4(-2) = -1 Rightarrow x - 8 = -1 Rightarrow x = +7$.

Redox Connection: An increase in ON means oxidation (loss of electrons), and the species is a reducing agent. A decrease in ON means reduction (gain of electrons), and the species is an oxidizing agent. Fractional ONs (e.g., $Fe_3O_4$, $C_3O_2$) indicate an average of different integer ONs for the same element within the structure. Always be vigilant for exceptions, as they are frequently tested in NEET.

Prelims Revision Notes

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Definition: — Oxidation Number (ON) is the hypothetical charge on an atom if all bonds were ionic, with electrons assigned to the more electronegative atom.
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Rules for Assignment:

* Elemental State: ON = 0 ($O_2$, $Fe$, $P_4$). * Monatomic Ions: ON = charge ($Na^+ \rightarrow +1$, $Cl^- \rightarrow -1$). * Group 1 Metals (Li, Na, K, Rb, Cs): ON = +1 in compounds. * Group 2 Metals (Be, Mg, Ca, Sr, Ba): ON = +2 in compounds.

* Fluorine (F): ON = -1 in all compounds. * Hydrogen (H): Generally +1. Exception: -1 in metal hydrides (e.g., $NaH$, $CaH_2$). * Oxygen (O): Generally -2. Exceptions: * -1 in peroxides (e.

g., $H_2O_2$, $BaO_2$). * $-1/2$ in superoxides (e.g., $KO_2$). * +2 in $OF_2$ (Fluorine is more electronegative). * +1 in $O_2F_2$. * Halogens (Cl, Br, I): Usually -1, but can be positive when bonded to oxygen or a more electronegative halogen (e.

g., Cl in $HClO_4$ is +7). * Sum of ONs: For a neutral compound, sum = 0. For a polyatomic ion, sum = charge of the ion.

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Redox Reactions:

* Oxidation: Increase in ON (loss of electrons). The species is a reducing agent. * Reduction: Decrease in ON (gain of electrons). The species is an oxidizing agent.

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Fractional Oxidation Numbers: — Occur when an element exists in multiple oxidation states within the same compound (e.g., average ON of Fe in $Fe_3O_4$ is $+8/3$). Individual atoms always have integer ONs.
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Key Distinction: — Oxidation Number (hypothetical charge, signed, can be fractional) vs. Valency (combining capacity, positive integer, unsigned). Do not confuse them.
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Common Traps: — Peroxide linkages ($H_2SO_5$, $H_2S_2O_8$), metal hydrides, and compounds like $OF_2$ are frequently tested.

Vyyuha Quick Recall

Hydrogen is Positive, Oxygen Negative, Fluorine Always Minus One. Metals Give Electrons, Non-metals Take. Sum Zero for Neutral, Charge for Ions.