Rules for Assigning Oxidation Numbers — Revision Notes

⚡ 30-Second Revision

Elements in free state: — $ON = 0$ (e.g., $O_2$, $Na$).
Monatomic ions: — $ON = \text{ion charge}$ (e.g., $Na^+ = +1$, $Cl^- = -1$).
Group 1 metals: — $ON = +1$ in compounds.
Group 2 metals: — $ON = +2$ in compounds.
Fluorine: — $ON = -1$ in all compounds.
Hydrogen: — $ON = +1$ (most compounds); $ON = -1$ (metal hydrides, e.g., $NaH$).
Oxygen: — $ON = -2$ (most compounds);

- $ON = -1$ (peroxides, e.g., $H_2O_2$); - $ON = -1/2$ (superoxides, e.g., $KO_2$); - $ON = +2$ (with fluorine, e.g., $OF_2$).

Halogens (Cl, Br, I): — $ON = -1$ (most compounds); positive with O or more electronegative halogens.
Sum of ONs: — $0$ for neutral compounds; = ion charge for polyatomic ions.
Fractional ON: — Average of different integral states.

2-Minute Revision

Mastering oxidation number rules is crucial for NEET. Start with the basics: elements in their free state have an oxidation number of $0$. Monatomic ions have an oxidation number equal to their charge.

For compounds, remember the fixed states: Group 1 metals are always $+1$, Group 2 metals are always $+2$, and fluorine is always $-1$. Hydrogen is typically $+1$, but remember the critical exception: it's $-1$ in metal hydrides (like $NaH$).

Oxygen is usually $-2$, but has several important exceptions: it's $-1$ in peroxides ($H_2O_2$), $-1/2$ in superoxides ($KO_2$), and $+2$ when bonded to fluorine ($OF_2$). Halogens are generally $-1$, but can be positive when bonded to more electronegative elements like oxygen or fluorine.

Always remember that the sum of oxidation numbers in a neutral compound is zero, and in a polyatomic ion, it equals the ion's charge. If you encounter a fractional oxidation number, it simply means the element exists in multiple integral oxidation states within that compound, and the calculated value is an average.

Practice applying these rules, especially the exceptions, to complex ions and compounds to build speed and accuracy.

5-Minute Revision

A thorough understanding of oxidation number rules is non-negotiable for NEET. Let's consolidate the key points. The oxidation number is a hypothetical charge, not a real one, used for electron bookkeeping in redox reactions.

Core Rules (High Priority):

1
Free Elements: — $ON = 0$ (e.g., $Fe$, $Cl_2$, $P_4$).
2
Monatomic Ions: — $ON = \text{ion's charge}$ (e.g., $Mg^{2+}$ is $+2$).
3
Group 1 Metals (Li, Na, K, etc.): — Always $+1$ in compounds.
4
Group 2 Metals (Be, Mg, Ca, etc.): — Always $+2$ in compounds.
5
Fluorine (F): — Always $-1$ in compounds (most electronegative).

Variable Rules with Exceptions (Medium Priority):

1
Hydrogen (H): — Generally $+1$. Exception: $-1$ in metal hydrides (e.g., $NaH$, $CaH_2$).
2
Oxygen (O): — Generally $-2$. Exceptions:

* $-1$ in peroxides (e.g., $H_2O_2$, $Na_2O_2$). * $-1/2$ in superoxides (e.g., $KO_2$). * $+1$ in $O_2F_2$. * $+2$ in $OF_2$ (due to F's higher electronegativity).

1
Halogens (Cl, Br, I): — Generally $-1$. Exceptions: Positive when bonded to oxygen or a more electronegative halogen (e.g., $Cl$ is $+7$ in $HClO_4$, $Br$ is $+3$ in $BrF_3$).

Summation Rules:

1
Neutral Compounds: — Sum of ONs = $0$.
2
Polyatomic Ions: — Sum of ONs = ion's charge.

Fractional Oxidation Numbers: These occur when an element exists in multiple oxidation states within the same compound (e.g., $Fe_3O_4$ has Fe in $+2$ and $+3$ states, averaging to $+8/3$). Don't be alarmed by them; they are valid averages.

Worked Example: Find the oxidation number of sulfur in $Na_2S_2O_3$ (sodium thiosulfate).

$Na$ is Group 1, so $ON = +1$.
$O$ is usually $-2$.
Compound is neutral, so sum of ONs = $0$.
Let $ON(S) = x$.
$2 \times (+1) + 2 \times (x) + 3 \times (-2) = 0$
$2 + 2x - 6 = 0$
$2x - 4 = 0$
$2x = 4 implies x = +2$.

So, the average oxidation number of sulfur in $Na_2S_2O_3$ is $+2$. (Note: In reality, one S is $0$ and the other is $+4$ in this compound, averaging to $+2$).

Remember to apply these rules systematically, prioritizing the higher-priority rules and always being vigilant for exceptions. Practice is key to quick and accurate calculation, which is vital for NEET.

Prelims Revision Notes

For NEET, quick recall of oxidation number rules and their exceptions is critical. Here's a concise factual and formula-recall guide:

1
Elemental State: — Oxidation Number (ON) is $0$. Examples: $Na$, $Fe$, $O_2$, $N_2$, $S_8$, $P_4$.
2
Monatomic Ions: — ON equals the charge of the ion. Examples: $Na^+ (+1)$, $Mg^{2+} (+2)$, $Cl^- (-1)$, $O^{2-} (-2)$.
3
Group 1 Metals (Li, Na, K, Rb, Cs): — Always $ON = +1$ in compounds.
4
Group 2 Metals (Be, Mg, Ca, Sr, Ba): — Always $ON = +2$ in compounds.
5
Fluorine (F): — Always $ON = -1$ in compounds (highest electronegativity).
6
Hydrogen (H):

* Generally $ON = +1$ (e.g., $H_2O$, $HCl$, $NH_3$). * Exception: $ON = -1$ in metal hydrides (e.g., $NaH$, $CaH_2$).

1
Oxygen (O):

* Generally $ON = -2$ (most common). * Exceptions: * $ON = -1$ in peroxides ($O_2^{2-}$ ion, e.g., $H_2O_2$, $Na_2O_2$). * $ON = -1/2$ in superoxides ($O_2^-$ ion, e.g., $KO_2$, $RbO_2$). * $ON = -1/3$ in ozonides ($O_3^-$ ion, e.g., $KO_3$). * $ON = +1$ in $O_2F_2$. * $ON = +2$ in $OF_2$ (due to F's higher electronegativity).

1
Halogens (Cl, Br, I):

* Generally $ON = -1$ in compounds (especially with less electronegative elements). * Exceptions: Positive ON when bonded to oxygen or more electronegative halogens (e.g., $Cl$ in $HClO_4$ is $+7$; $Br$ in $BrF_3$ is $+3$).

1
Summation Rule for Neutral Compounds: — The algebraic sum of oxidation numbers of all atoms in a neutral compound is $0$.
2
Summation Rule for Polyatomic Ions: — The algebraic sum of oxidation numbers of all atoms in a polyatomic ion equals the net charge of the ion.
3
Fractional Oxidation Numbers: — Indicate an average of different integral oxidation states for the same element within a compound (e.g., $Fe_3O_4$, $Br_3O_8$).

Prioritization: Rules for Group 1/2 metals and Fluorine are generally highest priority. When in doubt, the more electronegative element dictates its oxidation state first.

Vyyuha Quick Recall

To remember the priority of common elements for oxidation numbers, think: 'For All Metals, Hydrogen, Oxygen, Halogens, Sum it up!'

Fluorine: Always -1 (highest priority)
Alkali Metals (Group 1): Always +1
Metals (Alkaline Earth, Group 2): Always +2
Hydrogen: +1 (usually), -1 (metal hydrides)
Oxygen: -2 (usually), -1 (peroxides), -1/2 (superoxides), +2 (with F)
Halogens (Cl, Br, I): -1 (usually), positive (with O or F)
Sum: 0 for neutral, charge for ion.