Group 1: +1; Group 2: +2; p-block shows variable states: e.g. group 13 +1/+3, group 14 -4/+2/+4, group 15 -3/+3/+5, group 16 -2/+4/+6, group 17 -1/+1/+3/+5/+7.
-- NCERT Class 11 Chemistry, Ch. 3, p. 22Valence Oxidation States
8 MCQs9-step worked example
Source: NCERT Classification of Elements and Periodicity in PropertiesPYQ coverage: NEET 2021, 2024Official key: NTA-verifiedLast reviewed: May 2026
Lesson
Valence is the combining capacity of an element — the number of bonds it can form. Oxidation state is the charge an atom would carry if all bonds were treated as fully ionic. These two concepts overlap but are not identical, and NEET exploits this gap.
Key distinction: Valence is always a positive whole number (or zero). Oxidation state can be positive, negative, or zero, and can be fractional in certain compounds (e.g., Fe₃O₄ gives Fe an average oxidation state of +8/3).
Periodic trends in valence:
- Across a period (left → right), valence with respect to hydrogen increases from 1 to 4 (Na to Si), then valence with respect to oxygen continues to increase while hydrogen valence decreases (P shows valence 3 with H but 5 with O).
- Group valence equals group number for s- and p-block elements (with respect to oxygen). For hydrogen compounds, valence = 8 − group number (for groups 15–17).
Variable oxidation states are characteristic of transition metals (d-block) because of the small energy gap between (n−1)d and ns electrons. Both sets participate in bonding. Example: Mn shows oxidation states from +2 to +7.
Watch-out for NEET: When a question asks "valence of nitrogen in NH₃," the answer is 3 (bonds formed). When it asks "oxidation state of nitrogen in NH₃," the answer is −3. Students who conflate the two lose marks on straightforward recall questions.
NCERT Class 11 Chemistry Chapter 3, page 22 lists representative oxidation states across periods and groups (NCERT Class 11 Physics Chapter 3).
Practice 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
The valence of phosphorus in PCl₅ is:
MCQ 2Direct ApplicationPractice
The oxidation state of sulphur in Na₂S₂O₃ is:
MCQ 3Easy RecallPractice
Which of the following statements about valence is correct?
MCQ 4Direct ApplicationPractice
The oxidation state of Mn in KMnO₄ is:
MCQ 5Direct ApplicationPractice
The oxidation state of Fe in Fe₃O₄ is:
MCQ 6Easy RecallPractice
Which of the following elements shows only one oxidation state in all its compounds?
MCQ 7Concept TrapPractice
The valence of carbon in CH₄ and CO₂ respectively are:
MCQ 8Concept TrapPractice
Transition metals exhibit variable oxidation states primarily because:
Worked Example
- 1
Given
Compound: K₂Cr₂O₇. Known oxidation states: K = +1, O = −2.
- 2
Required
Oxidation state of Cr.
- 3
Concept
In a neutral compound, the sum of all oxidation states equals zero. Assign known oxidation states to K and O, then solve for Cr algebraically.
- 4
Formula
Sum of oxidation states = 0 → 2(+1) + 2(x) + 7(−2) = 0
- 5
Substitution
2 + 2x − 14 = 0
- 6
Calculation
2x = 12 → x = +6 Note: The integers 2, 7, 14, and 12 appearing in this calculation are stoichiometric coefficients and arithmetic results — exact by definition. They do not limit significant figures.
- 7
Final answer
Oxidation state of Cr in K₂Cr₂O₇ = **+6**.
- 8
Common trap
Students sometimes divide the total positive charge requirement by the wrong number of Cr atoms. With 2 Cr atoms, the total charge on Cr must be +12 (to balance +2 from K and −14 from O), giving +6 per atom. Dividing by 7 (number of oxygens) instead gives a nonsensical answer.
- 9
Similar NEET-style question
"What is the oxidation state of S in Na₂S₄O₆ (sodium tetrathionate)?" Apply the same charge-balance method: 2(+1) + 4x + 6(−2) = 0 → x = +2.5. This tests comfort with fractional oxidation states — a common NEET twist. ---
Before solving, remember these
Formulas
Ionization energy of hydrogen-like atom
Energy required to ionize an electron from the n-th shell of hydrogen-like atom.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Z | nuclear charge | - |
| n | quantum number | - |
Valid when
- One-electron atom
- Non-relativistic
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student includes inert-gas radius in atomic-radius trends. But inert gases use van der Waals radius (much larger than covalent), making 'monotonic decrease across period' look broken.
When it triggers
Atomic radius comparison includes a noble gas or trends across period 2/3.
How to avoid
Compare like with like: covalent radii for non-noble gases. Noble gas radii are van der Waals (no covalent bond). Don't compare noble-gas radius directly to halogen.
Category: Inorganic Exception
Student expects monotonic increase in IE across period. Anomalies: Be(s²) > B(s²p¹); N(p³ half-filled) > O(p⁴).
When it triggers
Compare IE values across period 2 (Li, Be, B, C, N, O, F).
How to avoid
Be > B (s² stable; B's p¹ easier to remove). N > O (N has p³ half-filled stability; O loses one to attain p³). Memorise these two anomalies.
Root cause: concept gap
Correction
Be>B (s² stability); N>O (N's p³ half-filled stability). Memorise these two anomalies in period 2.
Root cause: concept gap
Correction
Don't compare different radius types. Noble gases use vdW radius (much larger); halogens use covalent radius. Compare like-with-like.
Past Year Questions
3 questions from NEET 2021, 2024. Answers verified against NTA official keys.
NEET 2024Revised key
1Li < Be < B < C < N
2Li < B < Be < C < N
3Li < Be < C < B < N
4Li < Be < N < B < C
NTA Answer: Option 2(revised_final)
NEET 2024Revised key
1Si < C < N < O < F
2Si < C < O < N < F
3O < F < N < C < Si
4F < O < N < C < Si
NTA Answer: Option 1(revised_final)
NEET 2021
1CO < SiO : Increasing 2 2 < SnO < PbO oxidizing power 2 2
2HF < HCl : Increasing acidic < HBr < HI strength
3H O < H S : Increasing pK 2 2 a < H Se < H Te values 2 2
4NH < PH : Increasing 3 3 < AsH < SbH acidic character 3 3
NTA Answer: Option 3(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Periodic trends comparison — atomic/ionic radii, ionization energy, electron gain enthalpy, electronegativity.
Direct ApplicationMedium
Common distractors
swapped classes
Tempts surface-level recall.
Sources
NCERT refs: Class 11 Chemistry Chapter 3, p.22
Test yourself on this topic with real past-paper questions:
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