Key Fact
Electronegativity
Tendency of an atom to attract bonding electrons. Increases across period; decreases down group. Pauling scale: F=4.0 (highest); Cs/Fr ~0.7 (lowest).
-- NCERT Class 11 Chemistry, Ch. 3, p. 91Chemical Reactivity
8 MCQs9-step worked example
Source: NCERT Classification of Elements and Periodicity in PropertiesPYQ coverage: NEET 2020, 2024Official key: NTA-verifiedLast updated: 26 Sep 2026
Chemical Reactivity, explained for NEET
Chemical reactivity of elements shows periodic trends that NEET exploits — particularly the contrast between metals and non-metals, and the exceptions students overlook.
The core principle: Metallic character (tendency to lose electrons) increases down a group and decreases across a period. Non-metallic character (tendency to gain electrons) follows the opposite trend. Chemical reactivity therefore depends on which side of the table you're examining.
For metals: Reactivity increases down a group. Alkali metals illustrate this clearly — lithium reacts mildly with water, sodium vigorously, potassium violently. The outermost electron is farther from the nucleus and easier to lose as atomic size grows (NCERT Class 11 Chemistry Chapter 3, page 88).
For non-metals: Reactivity decreases down a group. Fluorine is the most reactive halogen because of its small size and high effective nuclear charge — it gains an electron most readily. Moving to chlorine, bromine, and iodine, the incoming electron enters progressively larger shells, reducing the nuclear pull.
The trap NEET exploits: Students apply the same "increases down the group" rule uniformly without distinguishing metals from non-metals. A question asking "Which is most reactive: Na, K, Rb?" tests one trend, but "Arrange F, Cl, Br in decreasing reactivity" tests the opposite direction within non-metals. Conflating these is a common distractor strategy.
Watch-out: Reactivity of noble gases is essentially zero under normal conditions — they have stable octets. Don't confuse "least reactive element in a period" (noble gas) with "least metallic character" (rightmost non-metal before the noble gas).
Can you answer these Chemical Reactivity 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
Which of the following alkali metals reacts most vigorously with water?
Show answer and why every option is right or wrong
Answer: B. Rb (rubidium) is lowest in the group among the options. Metallic reactivity increases down the group for alkali metals because ionization energy decreases — the valence electron is most easily lost. NCERT Class 11 Chemistry Chapter 3, page 88.
Why A is wrong: A is wrong because Li is the smallest alkali metal with the highest IE in the group — it reacts least vigorously with water among these options.
Why C is wrong: C is wrong because K is reactive but Rb sits below it in Group 1 with lower IE and larger atomic size, making Rb more reactive.
Why D is wrong: D is wrong because Na is above K and Rb in the group — its reactivity with water is moderate, not the most vigorous.
MCQ 2Easy RecallPractice
Among halogens, the most reactive element is:
Show answer and why every option is right or wrong
Answer: C. Fluorine has the smallest atomic size and highest electronegativity among halogens — it gains an electron most readily, making it the most reactive non-metal. NCERT Class 11 Chemistry Chapter 3, page 91.
Why A is wrong: A is wrong because Cl, though highly reactive, has a larger atomic radius than F and lower electron-gaining tendency.
Why B is wrong: B is wrong because Br is in period 4 — its larger size means weaker attraction for an incoming electron compared to F.
Why D is wrong: D is wrong because I is the largest common halogen with the weakest electron-gaining tendency — it is the least reactive halogen.
MCQ 3Easy RecallPractice
Metallic character in the periodic table:
Show answer and why every option is right or wrong
Answer: B. Metallic character (tendency to lose electrons) increases down a group as atomic size increases and IE decreases. It decreases across a period as effective nuclear charge increases. NCERT Class 11 Chemistry Chapter 3, page 95.
Why A is wrong: A is wrong because increasing nuclear charge across a period makes electron loss harder — metallic character decreases left to right.
Why C is wrong: C is wrong because adding shells down a group reduces the hold on valence electrons, increasing metallic character.
Why D is wrong: D is wrong because metallic character clearly varies within a group — compare Li (moderate) with Cs (most metallic alkali metal).
MCQ 4Direct ApplicationPractice
Arrange the following in order of decreasing chemical reactivity: Na, Mg, Al.
Show answer and why every option is right or wrong
Answer: A. All three are metals in period 3. Moving left to right (Na → Mg → Al), ionization energy increases and metallic character decreases, so reactivity decreases in that order. NCERT Class 11 Chemistry Chapter 3, page 95.
Why B is wrong: B is wrong because Al has the highest IE among these three period-3 metals — it loses electrons least readily and is least reactive.
Why C is wrong: C is wrong because Na has the lowest IE in period 3 among these elements — it is more reactive than Mg.
Why D is wrong: D is wrong because Al is less reactive than Mg (Al has higher IE and requires more energy to lose electrons).
MCQ 5Direct ApplicationPractice
Which of the following correctly describes the reactivity trend of non-metals within a group?
Show answer and why every option is right or wrong
Answer: C. For non-metals, reactivity depends on electron-gaining tendency, which decreases down the group as atomic size increases and the incoming electron is farther from the nucleus. NCERT Class 11 Chemistry Chapter 3, page 90.
Why A is wrong: A is wrong because this is the metallic reactivity trend — non-metals show the opposite pattern (their electron-gaining ability weakens with size).
Why B is wrong: B is wrong because there is no inversion point — non-metal reactivity decreases monotonically down a group.
Why D is wrong: D is wrong because atomic size and shielding clearly change down a group, altering electron-gaining tendency.
MCQ 6Concept TrapPractice
A student claims that since reactivity increases down Group 1, it must also increase down Group 17. The error in this reasoning is:
Show answer and why every option is right or wrong
Answer: D. The student incorrectly applies the metallic trend (easier electron loss down group) to non-metals. For halogens, reactivity depends on electron-gaining tendency, which weakens as atomic radius increases down the group. NCERT Class 11 Chemistry Chapter 3, page 90.
Why A is wrong: A is wrong because Group 17 elements (halogens) are non-metals — the premise of this option is factually incorrect.
Why B is wrong: B is wrong because electronegativity decreases from F to I — it is not constant within Group 17.
Why C is wrong: C is wrong because halogens clearly show periodic trends (electronegativity, electron gain enthalpy, reactivity all vary systematically).
MCQ 7Direct ApplicationPractice
Among Li, Na, K, which reacts with water to produce hydrogen gas most slowly?
Show answer and why every option is right or wrong
Answer: D. Li has the smallest atomic radius and highest IE among alkali metals listed — it loses its valence electron least readily, so its reaction with water is the slowest (gentle fizzing vs violent reaction of K). NCERT Class 11 Chemistry Chapter 3, page 88.
Why A is wrong: A is wrong because K has the lowest IE among these three — it reacts vigorously (can ignite the hydrogen produced).
Why B is wrong: B is wrong because Na reacts more vigorously than Li (melts into a ball, rapid hydrogen evolution).
Why C is wrong: C is wrong because the rate clearly differs — Li fizzes gently while K can ignite, demonstrating the group trend.
MCQ 8Concept TrapPractice
Consider elements X (Group 1, Period 4) and Y (Group 17, Period 4). Which statement about their chemical reactivity is correct?
Show answer and why every option is right or wrong
Answer: A. X is potassium (Group 1, Period 4) — reactive because of low IE (easy electron loss). Y is bromine (Group 17, Period 4) — reactive because of high electron gain enthalpy (strong tendency to accept an electron). Different mechanisms drive their reactivity. NCERT Class 11 Chemistry Chapter 3, page 94.
Why B is wrong: B is wrong because although both are reactive, the mechanisms differ — X loses an electron (low IE) while Y gains an electron (high electron affinity). 'Same reason' is incorrect.
Why C is wrong: C is wrong because the statement 'metals are always less reactive than non-metals' is false — K is extremely reactive and more reactive than many non-metals like S or P.
Why D is wrong: D is wrong because K is among the most reactive metals and Br is a reactive halogen — both are highly reactive in their respective categories.
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How do you solve a Chemical Reactivity question? A worked example
- 1
Given
Elements: Be, Mg, Ca, Ba — all in Group 2 (alkaline earth metals).
- 2
Required
Arrange in order of increasing metallic reactivity (least reactive → most reactive).
- 3
Concept
For metals, reactivity increases down a group. As atomic number increases in a group, atomic size increases and ionization energy decreases — the valence electrons are more easily lost.
- 4
Formula
No quantitative formula needed. The ordering follows the group trend: reactivity ∝ position down the group (or equivalently, reactivity ∝ 1/IE for metals in the same group).
- 5
Substitution
Group 2 order (top to bottom): Be → Mg → Ca → Sr → Ba.
Therefore reactivity order (increasing): Be < Mg < Ca < Ba. - 6
Calculation
No arithmetic calculation. The ordering is determined by position in the periodic table.
- 7
Final answer
Increasing metallic reactivity: Be < Mg < Ca < Ba.
- 8
Common trap
A student might reverse the order, thinking smaller atoms are "more reactive" — this conflates the non-metal trend (where smaller size aids electron gain) with the metal trend (where larger size aids electron loss).
- 9
Similar NEET-style question
"Arrange Na, K, Rb, Cs in order of decreasing reactivity with water." Answer: Cs > Rb > K > Na (same principle — reactivity increases down Group 1).
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What to remember before solving Chemical Reactivity questions
More in Classification of Elements and Periodicity in Properties: 4 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
Chemical Reactivity questions from past NEET papers
2 questions from NEET 2020, 2024. Answers verified against NTA official keys.
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 2020
1(iv) (iii) (ii) (i)
2(i) (ii) (iii) (iv)
3(ii) (i) (iv) (iii)
4(iii) (iv) (i) (ii)
NTA Answer: Option 3(final)
All 10 past-paper questions from Classification of Elements and Periodicity in Properties →
Sources
NCERT refs: Class 11 Chemistry Chapter 3, p.88
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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