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SSC CGL Chemistry · Chemical Bonding

Valency, Oxidation Numbers and Atomicity

Writing a formula from valencies, variable valency and the old -ous and -ic names, working out an oxidation number, and how many atoms make up a molecule of an element.

All Chemical Bonding notes

Why this matters

Seven questions in the bank: four from the 2023 shifts and three from the 2024 shifts. The oxidation-number sum is the one with working; the rest are single facts.

Concept 1 of 2: Valency and oxidation numbers

A neutral compound's charges cancel. So the oxidation numbers of all its atoms add to zero, which lets you solve for the one you do not know.

Definition

Valency and formulas:

  • Cross the valencies: Mg (2) and OH (1) give Mg(OH)2.
  • Variable valency: iron is 2 (ferrous) or 3 (ferric).
  • Old names: -ous is the lower valency, -ic the higher. Argentous is silver(I); argentic is silver(II), valency +2.

Oxidation numbers: H is +1 and O is minus 2 in most compounds; the total for a neutral compound is 0.

Neutral compound

∑oxidation numbers=0\sum \text{oxidation numbers} = 0
  • 00The charge of a neutral compound

Worked example

Find the oxidation number of manganese in KMnO4.
Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2024 · Tier I, 12 Sep 2024, Shift 1 · Q43Easy

Example 1 · Chemical Bonding · Valency, Oxidation Numbers and Atomicity

Calculate the oxidation number of ‘S’ in H2S2O7\text{H}_2\text{S}_2\text{O}_7 .

Metal ions are never negative

A valency for argentic silver of minus 1 or minus 2 cannot be right: a metal ion is positive. Argentic is +2.
Drill 3 more on valency and oxidation numbers

Concept 2 of 2: Atomicity

Atomicity is how many atoms make up one molecule of an element. Noble gases stand alone; the common gases pair up.

Definition

The kinds:

  • Monoatomic: noble gases such as helium.
  • Diatomic: chlorine (Cl2), oxygen (O2), hydrogen, nitrogen. Carbon monoxide is a diatomic molecule too.
  • Triatomic: ozone (O3): 18 valence electrons, with one double and one single bond in its Lewis structure.
  • Polyatomic: phosphorus (P4), sulphur (S8).
ElementAtomicity
Helium1Q
Chlorine2Q
Ozone3
Phosphorus4
Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2023 · Tier I, 25 Jul 2023, Shift 2 · Q28Easy

Example 2 · Chemical Bonding · Valency, Oxidation Numbers and Atomicity

What is the atomicity of Chlorine?

Chlorine is diatomic

Chlorine gas is Cl2, so diatomic. Only noble gases are monoatomic among the gases SSC lists.
Drill 2 more on atomicity

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Reference tables (1)

Atomicity4 rows
ElementAtomicity
Helium1Q
Chlorine2Q
Ozone3
Phosphorus4

Watch out for (2)

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