Intuition The ONE core ideaAn oxidation number is a pretend charge : we imagine that every shared electron in a bond is grabbed entirely by the greedier atom, then count how many electrons each atom is "up" or "down" from neutral. Everything in this topic — the rules, the counting equation, oxidation vs reduction — is just careful counting of that one make-believe scoreboard.
This page assumes nothing . Before you can assign a single oxidation number, you need to genuinely understand a handful of small ideas: what an atom's charge is, what "sharing electrons" means, what "greedy" (electronegative) means, why a lone atom scores zero, and what the little symbols you'll meet (like + 6 before it is used.
Look at the figure. Two atoms are joined by a bond — a shared pair of electrons (the two dots in the middle). In reality that pair sits between them, tugged slightly toward the greedier atom. The whole trick of oxidation numbers is the thought experiment on the right : we pretend the greedy atom snatches BOTH electrons completely. Once we do that pretend snatch, each atom has a clean whole-number charge — and those charges are the oxidation numbers.
Hold this image. Every rule below is a shortcut for doing this pretend-snatch quickly.
A tiny particle carrying one unit of negative charge , written − 1 e − nucleus surrounded by these negative electrons.
Think of an electron as a single negatively-charged marble orbiting the atom. Oxidation numbers are, from start to finish, a way of asking: how many marbles does this atom "own" compared to when it was neutral and alone?
Owns fewer marbles than neutral ⇒ positive oxidation number.
Owns more marbles than neutral ⇒ negative oxidation number.
Owns exactly the same as neutral ⇒ oxidation number zero .
The symbol e − Z n 2 e −
Intuition Why a sign, not just a sizeThe sign tells you the direction of the imbalance, and the size tells you how much . When you read "S is + 6 H 2 S O 4 in the pretend fully-ionic world, sulfur is missing six electrons. The + −
Common mistake The sign is easy to dropWriting "6 + 6
Definition Free (elemental) state and the zero ruleAn atom is in its free or elemental state when it is bonded only to copies of itself (or to nothing at all): N a O 2 P 4 C l 2 0
Intuition WHY free elements score zero (from the marble picture)Go back to the pretend-snatch. A bond between two identical atoms — the two oxygens in O 2 right down the middle : each atom keeps exactly what it brought.
If each atom keeps its own electrons, none is short and none is extra ⇒ each still "owns" the same marbles it had when neutral ⇒ oxidation number 0 N a 0
This is the starting point of every reaction table: zinc metal begins at 0 0 change against.
Definition ElectronegativityA measure of how strongly an atom pulls shared bonding electrons toward itself . Higher electronegativity = greedier.
Intuition The picture (tug-of-war)Every bond between different atoms is a tug-of-war over the shared electron pair. The more electronegative atom wins the rope and drags the pair closer to itself. Fluorine is the greediest of all; oxygen is second; metals like sodium are among the least greedy (they let go easily).
For the trends behind who wins, see Electronegativity & periodic trends . You do NOT need to memorise values — you only need the ranking F > O > N ≈ C l > ⋯ > H > metals
The figure shows the tug-of-war: in H – O keeps both electrons, so O reads − 2 + 1
Notice each rule is just "who wins the rope": fluorine always wins, so it's always − 1 − 2 + 2 O F 2 the parent rules note ; here we only need to recognise them.
Definition Bond, molecule, ion
Bond = a shared pair of electrons holding two atoms together (the line in H – O – H Molecule = a neutral group of bonded atoms (e.g. H 2 O 0 Ion = a charged group of atoms — it has extra or missing electrons overall (e.g. C r 2 O 7 2 − − 2
Intuition Why the difference mattersThe oxidation numbers of all the atoms in a piece of matter must add up to that piece's overall charge . For a neutral molecule that target is 0
Common mistake The most common wrong answerStudents set the total to 0 even for an ion , because neutral compounds are more familiar. But C r 2 O 7 2 − − 2 − 2 0 that as your target.
Worked example Reading a formulaIn C r 2 O 7 2 −
C r 2 2 chromium atoms (the small 2 subscript = a count).O 7 7 oxygen atoms.the 2 − superscript ) is the net charge of the whole ion: − 2
Definition Oxidation number notationWritten as a signed number attached to an element: S + 6 O − 2 S + 6
Intuition What it looks like vs a real chargeA real ionic charge (N a + N a + oxidation number is a bookkeeping value we write separately or above the atom. They can coincide (for N a + + 1
Common mistake "Oxidation number = the atom's real charge"It feels true because for a monatomic ion like N a + + 6 H 2 S O 4 no real + 6 pretend -snatched. Keep the two ideas in separate boxes.
Definition The "sum" symbol∑ ∑ "add up all the terms of this kind." The little i
∑ i n i ⋅ ( ox no ) i = Q species
Intuition Why this equation exists at allElectrons are only moved around , never created or destroyed. So the pretend charges must reconcile to the real total charge Q one unknown atom (like x
The figure builds the H 2 S O 4 n i ⋅ ( ox no ) i 0
x x isolate x by undoing additions and multiplications.
Worked example The whole algebra, from zero, forH 2 S O 4 2 ( + 1 ) + x + 4 ( − 2 ) = 0
Multiply the known counts: 2 ( + 1 ) = + 2 4 ( − 2 ) = − 8 Collect: 2 + x − 8 = 0 ⇒ x − 6 = 0 Move the − 6 (add 6 x = + 6
That's it — no calculus, no fractions unless the atoms genuinely aren't equivalent. If they aren't (like the four sulfurs in S 4 O 6 2 − x average and can be fractional (+ 2.5
Definition Oxidation and reduction (as directions)
Oxidation = oxidation number goes up (atom loses electrons).Reduction = oxidation number goes down (atom gains electrons).
Intuition The number line picturePut oxidation numbers on a number line. Moving right (− 2 → + 2 left (+ 2 → 0 Redox reactions — balancing (half-reaction & ion-electron) and to Oxidising and reducing agents . Note the free-element zero from Section 3 is your fixed reference point — you measure every change from it.
O xidation I s L oss, R eduction I s G ain (of electrons). The atom that is oxidised gave electrons away, so it is the reducing agent .
Electron = one negative marble
Charge and plus minus signs
Oxidation number = pretend charge
Electronegativity = greedy atom wins
The oxidation number rules
Net charge Q of the species
Assign every oxidation number
Everything on this page funnels into the parent: the oxidation-number rules note .
Cover the right side and test yourself — if any answer is fuzzy, reread that section before the parent note.
What does an electron carry, and how is it written? One unit of negative charge, written − 1 e −
What does a + 3 The atom is short 3 electrons compared to neutral (too positive).
What is the oxidation number of an atom in its free (elemental) state, and why? 0
Give three examples of free elements. N a O 2 P 4 C l 2
What does electronegativity measure? How strongly an atom pulls shared bonding electrons toward itself — "greediness."
Rank F, O, and H by electronegativity. F > O > H
Why is fluorine always − 1 It is the greediest element, so it always wins the tug-of-war and keeps the shared electrons.
In C r 2 O 7 2 − 7 2 − Subscript 7 2 −
What is the target sum for a neutral molecule vs an ion? 0
What does Q The net charge of the whole species.
What does ∑ i n i ( ox no ) i For each element, multiply its atom count by its oxidation number, then add all those products.
Why does the sum rule equal the net charge? Electrons are only redistributed, never created or destroyed, so pretend charges must reconcile to the real total.
Solve 2 ( + 1 ) + x + 4 ( − 2 ) = 0 x = + 6
Why can an oxidation number be fractional like + 2.5 It is an average over atoms that are not all equivalent — bookkeeping, not a real per-atom charge.
On a number line, which direction is oxidation? Rightward (increase in oxidation number) = loss of electrons.
What does OIL RIG stand for? Oxidation Is Loss, Reduction Is Gain (of electrons).
