(1) Metal ion has 2 valencies: primary (= oxidation state, ionizable) and secondary (= coordination number, non-ionizable). (2) Secondary valencies have fixed spatial arrangement → geometry of complex.
-- NCERT Class 12 Chemistry, Ch. 5, p. 119Werner Theory
8 MCQs9-step worked example
Source: NCERT Coordination CompoundsPYQ coverage: NEET 2026Official key: NTA-verifiedLast updated: 25 Sep 2026
Werner Theory, explained for NEET
The trap that costs marks here: confusing primary valency (oxidation state, ionizable) with secondary valency (coordination number, non-ionizable, directional). Werner distinguished these in 1893, and NEET questions exploit exactly this confusion — they give you a formula like CoCl₃·6NH₃ and ask how many ions it produces or what the coordination number is.
Werner's key postulates (NCERT Class 12 Chemistry Chapter 5, page 119):
- Metals in coordination compounds show two types of valency: primary (ionizable, satisfied by negative ions) and secondary (non-ionizable, satisfied by ligands in the coordination sphere).
- Secondary valency = coordination number. It is fixed for a given metal in a given oxidation state. For Co³⁺, this is almost always 6.
- Secondary valencies are directional — they determine the geometry (octahedral for CN 6, square planar or tetrahedral for CN 4).
How to read a formula: In [Co(NH₃)₆]Cl₃, everything inside the bracket is the coordination sphere (secondary valency = 6 NH₃ ligands). The 3 Cl⁻ outside are counter-ions satisfying primary valency. Conductivity measurements confirm 4 ions in solution: [Co(NH₃)₆]³⁺ + 3 Cl⁻.
Watch-out for NEET: When a species moves from outside to inside the bracket (compare CoCl₃·6NH₃ vs CoCl₃·5NH₃·H₂O vs CoCl₃·4NH₃), the number of ionizable Cl⁻ drops — and so does the number of ions produced. Questions test whether you can correctly partition species between the coordination sphere and the ionizable sphere.
Can you answer these Werner Theory MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
According to Werner's theory, the secondary valency of a metal ion in a coordination compound corresponds to:
Show answer and why every option is right or wrong
Answer: C. Werner defined secondary valency as the number of ligands directly bonded to the metal in the coordination sphere, which is the coordination number (NCERT Class 12 Chemistry Chapter 5, page 119).
Why A is wrong: A confuses primary valency (oxidation state, ionizable) with secondary valency (coordination number, non-ionizable).
Why B is wrong: B describes a measurable consequence of the theory (conductivity data), not the definition of secondary valency itself.
Why D is wrong: D is the net charge on the coordination entity, not the same as secondary valency — e.g. [Co(NH₃)₆]³⁺ has charge +3 but secondary valency (CN) = 6.
MCQ 2Easy RecallPractice
Werner's theory states that secondary valencies are:
Show answer and why every option is right or wrong
Answer: C. Secondary valencies are directional (they determine geometry) and non-ionizable (ligands are firmly bonded and do not dissociate as free ions), as stated in Werner's postulates (NCERT Class 12 Chemistry Chapter 5, page 119).
Why A is wrong: A assigns properties of primary valency (ionizable) to secondary valency, and incorrectly removes directionality which is a defining feature.
Why B is wrong: B correctly identifies directionality but wrongly adds ionizability — secondary valency bonds do not produce free ions in solution.
Why D is wrong: D correctly identifies non-ionizable character but removes directionality — the directional nature is what gives coordination compounds their definite geometry.
MCQ 3Direct ApplicationPractice
In the compound CoCl₃·5NH₃, the coordination number of cobalt and the number of ions produced in solution are, respectively:
Show answer and why every option is right or wrong
Answer: A. Werner's series: CoCl₃·5NH₃ is [CoCl(NH₃)₅]Cl₂. The coordination sphere has 5 NH₃ + 1 Cl⁻ = CN 6. Only 2 Cl⁻ are outside (ionizable), giving 3 ions: [CoCl(NH₃)₅]²⁺ + 2 Cl⁻ (NCERT Class 12 Chemistry Chapter 5, page 119).
Why B is wrong: B assumes CN = 5 by counting only NH₃ ligands, ignoring the one Cl⁻ inside the coordination sphere that also contributes to the coordination number.
Why C is wrong: C gives 4 ions, which corresponds to CoCl₃·6NH₃ where all 3 Cl⁻ are outside. Here one Cl⁻ has moved into the coordination sphere.
Why D is wrong: D gets CN wrong (5 instead of 6) by not counting the Cl⁻ inside the bracket, and gets ions wrong (4 instead of 3) by miscounting ionizable species.
MCQ 4Direct ApplicationPractice
The complex [Co(NH₃)₄Cl₂]Cl has how many ionizable chloride ions and what is the coordination number of Co?
Show answer and why every option is right or wrong
Answer: A. In [Co(NH₃)₄Cl₂]Cl, the bracket contains 4 NH₃ + 2 Cl⁻ = 6 ligand positions (CN = 6). Only the 1 Cl⁻ outside the bracket is ionizable — confirmed by conductivity giving 2 ions in solution.
Why B is wrong: B miscounts by treating the 2 Cl⁻ inside the coordination sphere as ionizable, and incorrectly gives CN = 4 by counting only NH₃ ligands.
Why C is wrong: C is wrong: three ionizable Cl⁻ would need all three chlorides outside the bracket, which is not the given formula; and with 4 NH₃ as the only ligands the coordination number would be 4, not 6.
Why D is wrong: D correctly identifies 1 ionizable Cl⁻ but gives CN = 4, counting only NH₃ and ignoring the 2 Cl⁻ ligands inside the coordination sphere.
MCQ 5Easy RecallPractice
Which of the following is NOT a postulate of Werner's theory?
Show answer and why every option is right or wrong
Answer: D. Werner stated that primary valency is satisfied by negative ions (anions), not neutral molecules. Neutral molecules (like NH₃, H₂O) satisfy secondary valency by acting as ligands (NCERT Class 12 Chemistry Chapter 5, page 119).
Why A is wrong: A is a correct postulate of Werner's theory — the two-valency distinction is the central claim.
Why B is wrong: B is a correct postulate — directionality of secondary valency explains fixed geometry.
Why C is wrong: C is a correct postulate — Werner equated secondary valency with coordination number.
MCQ 6Direct ApplicationPractice
Among CoCl₃·6NH₃, CoCl₃·5NH₃, and CoCl₃·4NH₃, which produces the fewest ions in aqueous solution?
Show answer and why every option is right or wrong
Answer: D. CoCl₃·4NH₃ is [CoCl₂(NH₃)₄]Cl — only 1 Cl⁻ ionizable, giving 2 ions total. CoCl₃·5NH₃ gives 3 ions; CoCl₃·6NH₃ gives 4 ions. As more Cl⁻ enter the coordination sphere, fewer are ionizable.
Why A is wrong: A (CoCl₃·6NH₃ = [Co(NH₃)₆]Cl₃) gives the MOST ions (4 total: the complex cation + 3 Cl⁻).
Why B is wrong: B (CoCl₃·5NH₃ = [CoCl(NH₃)₅]Cl₂) gives 3 ions — intermediate, not the fewest.
Why C is wrong: C ignores the central point of Werner's theory — the partitioning between coordination sphere and ionizable sphere differs across the series.
MCQ 7Concept TrapPractice
Werner proposed that secondary valencies are directional. This postulate directly explains which property of coordination compounds?
Show answer and why every option is right or wrong
Answer: B. Directionality of secondary valencies means ligands occupy fixed positions in space around the metal, giving coordination compounds their definite geometry (octahedral, tetrahedral, square planar, etc.).
Why A is wrong: A relates to primary valency (oxidation state) and is a general property of transition metals, not specifically explained by directional secondary valency.
Why C is wrong: C is explained by the ionizable vs non-ionizable distinction (primary vs secondary valency partition), not by the directionality of secondary bonds.
Why D is wrong: D is explained by crystal field theory (d-d transitions due to field splitting), not by Werner's geometric postulate itself.
MCQ 8CalculationPractice
A coordination compound of formula CrCl₃·6H₂O has a coordination number of 6. When treated with excess AgNO₃, one mole of this compound gives 3 moles of AgCl precipitate. What is the correct structural formula?
Show answer and why every option is right or wrong
Answer: B. 3 moles of AgCl means all 3 Cl⁻ are ionizable (outside the bracket). Since CN = 6 and no Cl is a ligand, all 6 coordination positions are occupied by H₂O. Structure: [Cr(H₂O)₆]Cl₃ — confirmed by having 4 ions in solution (complex cation + 3 Cl⁻).
Why A is wrong: A places all 3 Cl inside the sphere (0 ionizable), predicting 0 moles AgCl — contradicts the given data of 3 moles AgCl.
Why C is wrong: C places 1 Cl inside and 2 outside, predicting 2 moles AgCl — still contradicts the given 3 moles.
Why D is wrong: D places 2 Cl inside and 1 outside, predicting only 1 mole AgCl precipitate — contradicts the given 3 moles.
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How do you solve a Werner Theory question? A worked example
Pattern: Werner coordination number — identify primary and secondary valencies from formula (P.CHE.U12.WERNER_COORDINATION_NUMBER)
- 1
Given
A cobalt(III) compound has the empirical formula CoCl₃·4NH₃. Conductivity measurements show it produces 2 ions per formula unit in solution.
- 2
Required
Write the structural formula showing the coordination sphere and determine the coordination number.
- 3
Concept
Werner's theory: species inside the coordination sphere satisfy secondary valency (non-ionizable); species outside satisfy primary valency (ionizable). Number of ions = 1 (complex ion) + number of ionizable counter-ions.
- 4
Formula / Rule
Number of ions in solution = 1 + (number of ions outside the bracket)
Coordination number = total number of ligand donor atoms in the bracket. - 5
Substitution
2 ions total → 1 complex ion + 1 counter-ion outside.
Co(III) needs 3 Cl⁻ for primary valency. Only 1 Cl⁻ is outside → 2 Cl⁻ must be inside the coordination sphere as ligands. - 6
Calculation
Inside bracket: 4 NH₃ + 2 Cl⁻ = 6 donor positions.
Outside bracket: 1 Cl⁻ (ionizable).
Structural formula: [CoCl₂(NH₃)₄]Cl. - 7
Final answer
[CoCl₂(NH₃)₄]Cl, coordination number = 6.
Note: The integer counts (4, 2, 1, 6) are exact counting numbers and do not contribute to any significant-figure consideration. - 8
Common trap
Counting counter-ions as ligands: if you include the 1 Cl⁻ outside the bracket as a ligand, you'd wrongly get CN = 7. Only species inside the coordination sphere count toward CN.
- 9
Similar NEET-style question
"The compound PtCl₄·2NH₃ is non-electrolyte (zero ions in solution). Write its structural formula and determine the coordination number of Pt." (Answer: [PtCl₄(NH₃)₂], CN = 6, all Cl inside, zero ionizable ions.)
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What to remember before solving Werner Theory questions
More in Coordination Compounds: 2 exam traps and mistakes · 2 formulas · 2 question patterns from its other lessons.
Werner Theory questions from past NEET papers
1 question from NEET 2026. Answers verified against NTA official keys.
NEET 2026
A 1:3 electrolyte in an aqueous solution is
1[CoCl₂(NH₃)₄]Cl
2[CoCl(NH₃)₅]Cl₂
3[Co(NH₃)₆]Cl₃
4[Co(NH₃)₃(NO₂)₃]
NTA Answer: Option 3(final)
How does NEET ask about Werner Theory?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Identify primary and secondary valencies, coordination number from formula. Primary = ionizable ions; secondary = ligands.
InterpretationMedium
Common distractors
counts counter ions as ligands
Treats all attached species as ligands
Sources
NCERT refs: Class 12 Chemistry Chapter 5, p.119
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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