Oxidation number rules — assigning, change — Chemistry notes

What an oxidation number really is

The rules (and HOW each is derived)

$\sum_i n_i \cdot (\text{ox no})_i = Q_{\text{species}}$

where $n_i$ = number of atoms of element $i$ , $Q$ = net charge.

HOW to assign — worked examples

Oxidation change → redox

Figure — Oxidation number rules — assigning, change

Common mistake Steel-manned common errors

A. "O is always $-2$ ." Why it feels right: It's true ~95% of the time. The fix: Peroxides ( $-1$ ), superoxides ( $-\tfrac12$ ), and $OF_2$ ( $+2$ ) break it. Always check for O–O bonds or F.

B. "Oxidation number = real charge on the atom." Why it feels right: For monatomic ions ( $Na^+$ ) they are equal. The fix: In covalent molecules it's a pretend charge. S is $+6$ in $H_2SO_4$ but carries no real $+6$ .

C. Using net charge $=0$ for an ion. Why it feels right: Neutral compounds are more common in practice. The fix: For ions, sum $=$ the ion's charge (e.g. $-2$ for $Cr_2O_7^{2-}$ ).

D. "Oxidation numbers must be whole." Why it feels right: Charges are integers. The fix: Averages can be fractional ( $+2.5$ in $S_4O_6^{2-}$ , $+\tfrac{8}{3}$ in $Fe_3O_4$ ).

Recall Feynman: explain to a 12-year-old

Imagine kids sharing candy but the biggest kid always grabs the whole piece. Oxidation number is a scoreboard: we pretend the biggest kid took ALL the shared candy, then count how much extra ( $+$ ) or short ( $-$ ) each kid is. In a reaction, if a kid's score goes up, they gave candy away (got "oxidised"); if it goes down, they grabbed some. The scoreboard changes must cancel — candy is never destroyed, just moved.

What is an oxidation number?

The charge an atom would have if all shared bonding electrons were assigned to the more electronegative atom (pretend fully ionic).

Oxidation number of an atom in a free element (e.g. O₂, Na)?

Oxidation number of a monatomic ion?

Equal to its ionic charge.

Usual oxidation number of oxygen, and its exceptions?

−2 usually; −1 in peroxides, −½ in superoxides, +2 in OF₂.

Oxidation number of H in metal hydrides like NaH?

−1.

State the sum rule for oxidation numbers.

Sum of oxidation numbers of all atoms = net charge of the species.

Find S in H₂SO₄.

+6 (from 2(+1)+x+4(−2)=0).

Find Cr in Cr₂O₇²⁻.

+6 (from 2x+7(−2)=−2).

Define oxidation in terms of oxidation number.

An increase in oxidation number (loss of electrons).

Define reduction in terms of oxidation number.

A decrease in oxidation number (gain of electrons).

Why can oxidation numbers be fractional (e.g. +2.5)?

It is an average over non-equivalent atoms; it's bookkeeping, not a real per-atom charge.

In Zn + Cu²⁺ → Zn²⁺ + Cu, which is oxidised?

Zn (0 → +2), it is the reducing agent.

What does OIL RIG stand for?

Oxidation Is Loss, Reduction Is Gain (of electrons).

Connections

Redox reactions — balancing (half-reaction & ion-electron)
Electronegativity & periodic trends
Oxidising and reducing agents
Stoichiometry — mole conservation in reactions
Electrochemistry — cell potentials

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, oxidation number ek hisaab-kitaab ka trick hai. Molecule mein electrons share hote hain, par jo atom zyada electronegative (greedy) hota hai, wo shared electrons ko apni taraf kheench leta hai. Hum maan lete hain ki greedy atom ne poore electrons le hi liye — is pretend duniya mein har atom ka jo charge banta hai, wahi uska oxidation number hai. Yaad rakho: ye real charge nahi, sirf ek scoreboard hai.

Rules simple hain: free element ka $0$ , monatomic ion ka uska charge, F hamesha $-1$ , O usually $-2$ (par peroxide mein $-1$ ), H metals ke saath $-1$ . Sabse powerful rule hai sum rule: sabhi oxidation numbers ka total = species ka net charge. Isse ek unknown atom nikal jaata hai. Jaise $H_2SO_4$ mein S: $2(+1)+x+4(-2)=0 \Rightarrow x=+6$ . Ion ke liye zero mat lagana — $Cr_2O_7^{2-}$ mein total $-2$ hoga.

Redox samajhne ke liye change dekho: agar oxidation number badhta hai to oxidation (electron loss), agar ghatata hai to reduction (electron gain). Mnemonic — OIL RIG. Zn + Cu²⁺ waale example mein Zn $0 \to +2$ (oxidised), Cu $+2 \to 0$ (reduced), aur dono changes cancel ho jaate hain kyunki electrons na bante hain na khatam hote — bas transfer hote hain. Ye concept aage balancing, electrochemistry, sab mein kaam aayega, isliye rules ratne ke bajaye sum-rule se derive karna seekho.

Oxidation number rules — assigning, change

What an oxidation number really is

The rules (and HOW each is derived)

HOW to assign — worked examples

Oxidation change → redox

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Chemical Reactions & Stoichiometry

Connections

Atom	Before	After	Change
Zn	$0$	$+2$	$+2$ (↑ oxidised)
Cu	$+2$	$0$	$-2$ (↓ reduced)