Suggest a list of the substances where carbon can exhibit oxidation states from –4 to +4 and nitrogen from –3

Exercises · 7.7

Q. Suggest a list of the substances where carbon can exhibit oxidation states from -4 to +4 and nitrogen from -3 to +5.

Rajasthan RbseTextbookSubjective· 2mImportance★★★★★est

22% · 17/78 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Oxidation Number Calculation

Oxidation Number Calculation: From Intuition to Precision

Imagine you're watching a tug-of-war between two atoms in a molecule. Each atom has a certain "pull" on the shared electrons — chemists call this electronegativity. The oxidation number is like a scorecard that tells us: If the more electronegative atom took all the shared electrons, what charge would each atom end up with?

This isn't a real charge — it's a bookkeeping tool. Real molecules don't have these exact charges. But this imaginary scorecard helps us track where electrons go during chemical reactions, especially in redox (reduction-oxidation) processes.

The Core Idea

Oxidation number (also called oxidation state) is the hypothetical charge an atom would have if all bonds to atoms of different elements were 100% ionic — meaning the more electronegative atom keeps all the shared electrons.

Note

For an atom bonded to another atom of the same element (like O₂ or N₂), the electrons are shared equally. So the oxidation number is zero — no one "wins" the tug-of-war.

The Rules (Your Toolkit)

These rules are applied in order — rule 1 overrides rule 2, and so on. Memorise them in this sequence:

Free elements (uncombined, like Fe, O₂, H₂, S₈) have oxidation number = 0.
Monatomic ions have oxidation number = their charge.

Example: Na⁺ = +1, Cl⁻ = −1, Mg²⁺ = +2.
Fluorine is always −1 in compounds (it's the most electronegative element — it always "wins").
Oxygen is usually −2, except:
- In peroxides (like H₂O₂) it's −1
- In OF₂ (with fluorine) it's +2 (fluorine wins)
Hydrogen is usually +1 when bonded to non-metals, −1 when bonded to metals (like NaH, CaH₂).
The sum of oxidation numbers in a neutral compound = 0.

In a polyatomic ion, the sum = the ion's charge.

Watch out

Never apply rule 6 before rules 1–5. The sum rule is your check, not your starting point.

How to Calculate: A Step-by-Step Example

Let's find the oxidation number of sulphur in H₂SO₄ (sulphuric acid).

Step 1: Write the known oxidation numbers.

Hydrogen: +1 (rule 5, bonded to non-metal oxygen)

Oxygen: −2 (rule 4, not a peroxide)

Step 2: Let the unknown be $x$ (for sulphur).

Step 3: Apply the sum rule (rule 6). The compound is neutral, so:

2(+1) + x + 4(-2) = 0

Step 4: Solve:

2 + x - 8 = 0

x - 6 = 0

x = +6

Important

Sulphur in H₂SO₄ has oxidation number +6.

Another Example: A Polyatomic Ion

Find the oxidation number of chromium in Cr₂O₇²⁻ (dichromate ion).

Oxygen: −2 (rule 4)

Let chromium = $x$

Sum of oxidation numbers = charge of ion (−2):

2x + 7(-2) = -2

2x - 14 = -2

2x = 12

x = +6

Tip

When you get a fractional oxidation number (like +2.5 in Fe₃O₄), it means the compound has two different oxidation states for the same element. Fe₃O₄ actually contains Fe²⁺ and Fe³⁺ in a 1:2 ratio.

Common Traps to Avoid

| Mistake | Why it's wrong |

|---------|----------------| …

Start your 14-day free trial to unlock the full solution →

Carbon's oxidation states span $-4$ (methane) to $+4$ (carbon dioxide), and nitrogen's

span $-3$ (ammonia) to $+5$ (nitric acid). A ladder of familiar compounds covers every

step: for carbon CH₄ (−4), C₂H₆ (−3), C₂H₄ (−2), C₂H₂ (−1), HCHO (0), OHC–CHO (+1),

HCOOH (+2), (COOH)₂ (+3), CO₂ (+4); for nitrogen NH₃ (−3), N₂H₄ (−2), NH₂OH (−1),

N₂ (0), N₂O (+1), NO (+2), HNO₂ (+3), NO₂ (+4), HNO₃ (+5).

Why the ranges are −4…+4 and −3…+5

An element's oxidation-state range is set by its valence electrons. Carbon has four:

sharing them with less electronegative partners (hydrogen) pushes carbon down to $-4$ ;

surrendering all four to more electronegative partners (oxygen, chlorine) pushes it up

to $+4$ . Nitrogen has five valence electrons but needs only three more for an octet, so

it runs from $-3$ (all bonds to hydrogen) up to $+5$ (all five electrons committed to

oxygen).

A reliable way to assign states in organic molecules: each C–H bond counts $-1$ to

carbon, each C–O (or C–Cl) bond counts $+1$ , and C–C bonds count $0$ (identical atoms

share equally).

Carbon: a compound for every state from −4 to +4

Oxidation state	Compound	Check
$-4$	Methane, $\ce{CH4}$	$x + 4(+1) = 0 \Rightarrow x = -4$
$-3$	Ethane, $\ce{C2H6}$	$2x + 6(+1) = 0 \Rightarrow x = -3$
$-2$	Ethylene, $\ce{C2H4}$ (also $\ce{CH3OH}$ )	$2x + 4(+1) = 0 \Rightarrow x = -2$
$-1$	Acetylene, $\ce{C2H2}$	$2x + 2(+1) = 0 \Rightarrow x = -1$
$0$	Formaldehyde, $\ce{HCHO}$ (also elemental C)	$x + 2(+1) + (-2) = 0 \Rightarrow x = 0$
$+1$	Glyoxal, $\ce{OHC-CHO}$	$2x + 2(+1) + 2(-2) = 0 \Rightarrow x = +1$
$+2$	Formic acid, $\ce{HCOOH}$ (also $\ce{CO}$ )	$x + 2(+1) + 2(-2) = 0 \Rightarrow x = +2$
$+3$	Oxalic acid, $\ce{(COOH)2}$	$2x + 2(+1) + 4(-2) = 0 \Rightarrow x = +3$
$+4$	Carbon dioxide, $\ce{CO2}$ (also $\ce{CCl4}$ )	$x + 2(-2) = 0 \Rightarrow x = +4$

Watch out

A common slip is assigning ethane's carbon $-2$ . Each carbon in $\ce{C2H6}$ carries

three C–H bonds ( $-3$ ) and one C–C bond ( $0$ ), so it sits at $-3$ , not $-2$ . Always

count bond by bond, or use the whole-molecule equation as in the table.

Nitrogen: a compound for every state from −3 to +5

| Oxidation state | Compound | Formula | …

Start your 14-day free trial to unlock the full solution →

←Previous question · exercises7.6 — Write formulas for the following compounds: (a) Mercury(II) chloride (…→Next question · exercises7.8 — While sulphur dioxide and hydrogen peroxide can act as oxidising as we…

Q.Suggest a list of the substances where carbon can exhibit oxidation states from –4 to +4 and nitrogen from –3 to +5.

Why the ranges are −4…+4 and −3…+5

Carbon: a compound for every state from −4 to +4

Nitrogen: a compound for every state from −3 to +5

🎓Unlock everything free for 14 days

Unlock everything free for 14 days