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. 20Chemical Reactivity
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
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 20).
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).
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
Which of the following alkali metals reacts most vigorously with water?
MCQ 2Easy RecallPractice
Among halogens, the most reactive element is:
MCQ 3Easy RecallPractice
Metallic character in the periodic table:
MCQ 4Direct ApplicationPractice
Arrange the following in order of decreasing chemical reactivity: Na, Mg, Al.
MCQ 5Direct ApplicationPractice
Which of the following correctly describes the reactivity trend of non-metals within a group?
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:
MCQ 7Direct ApplicationPractice
Among Li, Na, K, which reacts with water to produce hydrogen gas most slowly?
MCQ 8Concept TrapPractice
Consider elements X (Group 1, Period 4) and Y (Group 17, Period 4). Which statement about their chemical reactivity is correct?
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). ---
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.20
Test yourself on this topic with real past-paper questions:
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