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NDA Chemistry · Chemical Bonding

Valency, Oxidation States and Molecular Formula

Valency is an element's combining capacity — the number of bonds it forms; the oxidation state is the charge it would carry if every bond were ionic; and crossing valencies turns the two into the formula of a compound.

All Chemical Bonding notesNDA Chemistry strategy

Why this matters

4 PYQs, all short recall or one-line work. The bank asks for the valency of an element from its group, the oxidation state of a metal in an oxide, and the molecular formula of a compound from the valencies of its ions. Nail the group→valency rule and the cross-over method for writing formulae, and these are one-step answers.

Concept 1 of 3

Valency — combining capacity from the outer shell

Intuition

Valency is how many bonds an atom forms — how many electrons it loses, gains or shares to reach a full outer shell. The number of electrons in the outermost shell fixes it, so elements in the same group share a valency. The bank gives you an element (or its group) and asks for its valency, or asks which anion carries a particular charge.

Definition

Valency rules the bank tests:

  • Valency = electrons lost, gained or shared to complete the octet. It is decided by the number of outer-shell electrons.
  • Same group → same valency. Group 1 (Na) → 1; Group 2 (Mg) → 2; Group 13 (Al) → 3; Group 14 (C, Si) → 4; Group 15 (N, P) → 3; Group 16 (O, S) → 2; Group 17 (Cl) → 1.
  • Nitrogen has 5 outer electrons and needs 3 more → valency 3; the nitride ion is N³⁻ (a −3 anion).
  • For non-metals near the right, valency = 8 − (group number's outer electrons) for the anion charge (O gains 2 → O²⁻; N gains 3 → N³⁻).

Common valencies

Na=1Mg=2Al=3C=4N=3O=2Cl=1\text{Na}=1 \quad \text{Mg}=2 \quad \text{Al}=3 \quad \text{C}=4 \quad \text{N}=3 \quad \text{O}=2 \quad \text{Cl}=1

Worked example

Which one of these has a valency of 3: sodium, magnesium, nitrogen or oxygen?
  1. Sodium (Group 1) has valency 1; magnesium (Group 2) has valency 2.
  2. Oxygen (Group 16) gains 2 electrons → valency 2.
  3. Nitrogen has 5 outer electrons and needs 3 to complete its octet → valency 3.
Answer:Nitrogen — it forms three bonds (valency 3), as in NH₃ and the nitride ion N³⁻.
Practice this conceptself-check · 5 quick reps

Try it yourself

Which anion has a valency (charge) of −3: chloride, oxide, nitride or sulphide?

Practice — Level 1 (5 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    What is the valency of nitrogen?
  2. 2.
    Which anion carries a −3 charge: chloride, oxide or nitride?
  3. 3.
    What is the valency of magnesium?
  4. 4.
    On what does the valency of an element depend?
  5. 5.
    What is the valency of aluminium?

From the bank · past-year question

Example 1Chemical BondingEASY
Which one among the following has valency of 3 ?

[Q115 · Sep · 2024]

Nitrogen's valency is 3, and nitride is N³⁻

Nitrogen has 5 outer electrons but a valency of 3 (it needs 3 to reach 8), and the nitride ion is N³⁻. Don't read 'valency 5' off the group number — count the electrons needed to complete the octet.
Drill 1 more on valency — combining capacity from the outer shell

Concept 2 of 3

Oxidation state — the charge if every bond were ionic

Intuition

The oxidation state is the charge an atom would carry if all its bonds were fully ionic. You find it by fixing the elements you know (oxygen is usually −2, hydrogen +1) and letting the molecule's total charge force the unknown. The bank gives a compound and asks for the oxidation state of one element in it.

Definition

Rules for assigning oxidation states:

  • The oxidation states in a neutral compound add up to 0; in an ion they add up to the ion's charge.
  • Oxygen is −2 (except in peroxides); hydrogen is +1 (except in metal hydrides).
  • A free element has oxidation state 0.
  • Example — in V₂O₅: let vanadium be x. Then 2x + 5(−2) = 0 → 2x = 10 → x = +5.

Sum rule for oxidation states

∑(oxidation states)=net charge\sum (\text{oxidation states}) = \text{net charge}
  • Ousually −2
  • Husually +1

Worked example

What is the oxidation state of vanadium in V₂O₅?
  1. Oxygen is −2, and there are 5 oxygens: total from oxygen = 5 × (−2) = −10.
  2. The compound is neutral, so the two vanadium atoms must total +10.
  3. Each vanadium = +10 / 2 = +5.
Answer:+5.
Practice this conceptself-check · 4 quick reps

Try it yourself

What is the oxidation state of sulphur in sulphuric acid, H₂SO₄?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Oxidation state of vanadium in V₂O₅?
  2. 2.
    Usual oxidation state of oxygen in a compound?
  3. 3.
    Oxidation state of a free element (e.g. O₂, Na metal)?
  4. 4.
    Oxidation state of manganese in KMnO₄?

From the bank · past-year question

Example 2Chemical BondingEASY
What is the oxidation state of Vanadium in V2O5V_2O_5 ?

[Q67 · Apr · 2024]

Let the known atoms force the unknown

Always fix oxygen at −2 and hydrogen at +1, then set the oxidation states to sum to the net charge and solve for the metal. For V₂O₅ the answer is +5, not +10 — the +10 is split between the two vanadium atoms.

Concept 3 of 3

Writing a molecular formula by crossing valencies

Intuition

To write a compound's formula, put the two ions side by side and 'cross over' their valencies — each ion's charge becomes the other's subscript. Polyatomic ions (like ammonium NH₄⁺ or carbonate CO₃²⁻) keep their bracket. The bank gives you two ions with their valencies and asks for the correct formula.

Definition

The cross-over method:

  • Write the cation (positive ion) first, then the anion. Write each ion's valency above it.
  • Cross the valencies to become subscripts: a +1 cation with a −2 anion gives the cation a subscript of 2.
  • A polyatomic ion taking a subscript > 1 is wrapped in brackets: e.g. ammonium NH₄⁺ becomes (NH₄)₂ when two are needed.
  • Example — ammonium ion NH₄⁺ (valency +1) with carbonate CO₃²⁻ (valency −2): cross over → (NH₄)₂CO₃, ammonium carbonate.

Cross-over rule

A+m B−n  ⟶  AnBmA^{+m}\,B^{-n} \;\longrightarrow\; A_{n}B_{m}
  • mvalency (charge) of the cation A
  • nvalency (charge) of the anion B

Worked example

Write the molecular formula of ammonium carbonate, given the ammonium ion is +1 and the carbonate ion is −2.
  1. Cation = NH₄⁺ (valency 1); anion = CO₃²⁻ (valency 2).
  2. Cross the valencies: the +1 cation takes subscript 2; the −2 anion takes subscript 1.
  3. Two ammonium ions need brackets: (NH₄)₂; carbonate stays CO₃.
Answer:(NH₄)₂CO₃.
Practice this conceptself-check · 4 quick reps

Try it yourself

Write the formula of aluminium oxide, given aluminium is +3 and oxide is −2.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Formula of ammonium carbonate (NH₄⁺ and CO₃²⁻)?
  2. 2.
    Formula of calcium chloride (Ca²⁺ and Cl⁻)?
  3. 3.
    Formula of aluminium oxide (Al³⁺ and O²⁻)?
  4. 4.
    When does a polyatomic ion need brackets in a formula?

From the bank · past-year question

Example 3Chemical BondingEASY
Which one of the following is the correct molecular formula of ammonium carbonate if the valency of ammonium ion is (+1)(+1) and carbonate anion is (−2)(-2)?

[Q60 · Apr · 2022]

Bracket a polyatomic ion before adding a subscript

Two ammonium ions are written (NH₄)₂, not NH₄₂ or N₂H₈. Wrap the whole polyatomic ion in brackets, then apply the subscript — otherwise the formula is misread.

Summary — formulas & gotchas at a glance

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Formulas (3)

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Mastery check — 1 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Chemical BondingEASY
Which of the following anions has a valency of -3 ?

[Q59 · Sep · 2024]

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