JEE Main · Physics · Nuclei

Radioactive Decay: JEE Main previous year questions

Name: Radioactive Decay: JEE Main previous year questions
Creator: Lumi

40 questions in the 108 JEE Main shifts Lumi analysed, in 7 of 8 years. Pattern: Gone quiet. 2 came in 2024–26. Here is how it is asked and which ideas come back.

Questions: 40; in 35 of 108 shifts
Years asked: 7/8; last asked 2026
Since 2024: Quiet; 2 questions in 2024–26
Weighted estimate: –; history: 0.3 a shift on average, typically 0.1–0.6

Key findings

01
Asked in 7 of 8 years with analysed shifts and in 35 of 108 shifts: 40 questions, #82 of 196 topics. The years that asked it were 1.5 apart on average.
02
33% of its questions since 2020 tested an idea no earlier year had asked; 23 repeated an idea from the year before.
03
The most repeated idea is Half-Life Calculations, asked 10 times in 9 shifts (2020, 2021, 2022, 2023), most often as “time interval between fractional decays”.
04
JEE Main has asked 12 of the 51 ideas in this topic; 33 have never been asked, directly or inside another question.
05
Mostly asked as single correct (70%); 18% need a numerical answer, 57% are easy and 3% hard. It went down from 0.5 to 0.1 questions a shift since 2024.
06
The skill tested most is calculation (65%), then application (13%).

Questions a shift, year by year

Out of the 25 Physics questions in a shift. Years differ in how many shifts Lumi analysed, so each year is scaled to one shift.

Raw count: 2017: 1 in 1 shift · 2020: 2 in 12 shifts · 2021: 16 in 24 shifts · 2022: 14 in 24 shifts · 2023: 5 in 12 shifts · 2024: 0 in 18 shifts · 2025: 1 in 8 shifts · 2026: 1 in 9 shifts.

Up to 2023: 0.5; a shift, of 25
Since 2024: 0.1; -0.4
Usual gap: 1.5 yrs; between years asking it
Shifts with it: 35/108; 32% of analysed shifts

How it is asked

Difficulty, format and skill

Across all 40 questions on Radioactive Decay since 2017.

Difficulty

QUESTIONS

2316

Easy23 · 57%
Medium16 · 40%
Hard1 · 3%

Format

Single correct · 28 Q70%
Numerical value · 7 Q18%
Diagram based · 4 Q10%
Other · 1 Q3%

Skill tested

Calculation · 26 Q65%
Application · 5 Q13%
Data or graph · 4 Q10%
Multi-step · 3 Q8%
Conceptual · 2 Q5%

Repeats

New idea, or one asked before?

Each column is a year. Blue questions tested an idea no earlier year had asked; navy ones went back to an idea from an earlier year. 2017 is the first year, so everything in it counts as new. Heights are question counts, and Lumi analysed 1 to 24 shifts a year, so compare the split inside a column rather than heights across years.

’17’20’21’22’23’24’25’26

Idea asked for the first time
Idea asked in an earlier year

Coverage of the topic

JEE Main has asked 12 of the 51 ideas in this topic. 33 have never been asked yet.

12633

Asked directly12 · 24%
Touched inside another question6 · 12%
Never asked33 · 65%

Ideas come from Lumi’s JEE syllabus map. A never-asked idea is not a safe skip: across all of JEE Main, 22% of questions since 2024 tested an idea no earlier year had asked.

Drill down

Idea, year, question

Every idea JEE Main tested in this topic, with its eight years. Open one to read the questions.

ideaAskedShareYears’17’20’21’22’23’24’25’26

OpenRepeated10 · 25%25%4/8423
- 8 Apr ’23 S2 · Q44
  A radio active material is reduced to 1/8 of its original amount in 3 days. If $8\times10^{-3}$ kg of the material is left after 5 days…
  mediumSingle correctAsked the year before
- 10 Apr ’23 S2 · Q44
  The half life of a radioactive substance is T. The time taken, for disintegrating $\frac{7}{8}$ th part of its original mass will be:
  easySingle correctAsked the year before
- 15 Apr ’23 S1 · Q44
  The half-life of a radioactive nucleus is 5 years. The fraction of the original sample that would decay in 15 years is:
  easySingle correctAsked the year before
- 27 Jul ’22 S2 · Q48
  The activity of a radioactive material is $6.4\times10^{-4}$ curie. Its half life is 5 days. The activity will become $5\times10^{-6}$ …
  easySingle correctAsked the year before
- 29 Jun ’22 S1 · Q33
  The activity of a radioactive material is $2.56 \times 10^{-3}$ Ci. If the half life of the material is 5 days, after how many days the…
  easySingle correctAsked the year before
- 3 Aug ’21 S2 · Q18
  A radioactive isotope has a half life of T years. After how much time is its activity reduced to 6.25% of its original activity ?
  easySingle correctAsked the year before
- 3 Aug ’21 S2 · Q25
  A sample undergoing beta decay, reduced to $\frac{1}{16}^{\mathrm{th}}$ of its initial mass in 48 years. Its half life period is…
  easyNumerical valueAsked the year before
- 27 Aug ’21 S1 · Q19
  There are $10^{10}$ radioactive nuclei in a given radioactive element. Its half-life time is 1 minute. How many nuclei will remain after…
  easySingle correctAsked the year before
- 16 Mar ’21 S2 · Q10
  Read the question
  easySingle correctAsked the year before
- 5 Sep ’20 S1 · Q18
  Read the question
  mediumDiagram basedNew idea
OpenRepeated7 · 18%18%2/825
OpenRepeated5 · 13%13%4/82
OpenRepeated3 · 8%8%3/8
OpenRepeated3 · 8%8%2/82
OpenRepeated3 · 8%8%2/82
OpenRepeated2 · 5%5%2/8
OpenRepeated2 · 5%5%2/8
OpenRepeated2 · 5%5%2/8
Open1 · 3%3%1/8
Open1 · 3%3%1/8
Open1 · 3%3%1/8

Squares are the 8 years with analysed shifts, 2017 to 2026. A number is the questions that year when there is more than one; the shade is scaled to one shift, so a year with more analysed shifts (1 to 24 a year) does not look darker. Share is of the level above.

Ideas

Every idea inside this topic

12 different ideas were tested; 9 came more than once.

Half-Life Calculations10×
time interval between fractional decays
Single correct · 9 shifts · 1 numerical
Integer Half-Life Shortcuts7×
time for fractional decay
Single correct · 6 shifts · 3 numerical
Statistical Decay Law5×
time interval between fractional decays
Single correct · 5 shifts · 1 numerical
Successive Decay Chains3×
graph of successive radioactive decay masses
Diagram based · 3 shifts
Activity and Rate of Decay3×
activity of sample from half-life
Single correct · 3 shifts · 1 numerical
Counting Alpha and Beta Emissions3×
mass and atomic number after decay chain
Single correct · 3 shifts
Alpha Decay Kinematics and Q-Value2×
ratio of alpha energies from q value
Single correct · 2 shifts
Mean Life Calculation2×
relation of decay constants from half-life and mean life
Single correct · 2 shifts
Parallel Decay Branching2×
effective half-life from parallel decay processes
Single correct · 2 shifts
Fractional Remainder Shortcut1×
time to decay to given fraction
Numerical value · 1 shift · 1 numerical
Activity from Sample Mass1×
activity from mass and half-life
Single correct · 1 shift
Beta Decay Mechanisms1×
true statement selection on beta decay
Single correct · 1 shift

Idea	Times	Shifts	Format	Numerical
Half-Life Calculationstime interval between fractional decays	10	9	Single correctCalculation	1
Integer Half-Life Shortcutstime for fractional decay	7	6	Single correctCalculation	3
Statistical Decay Lawtime interval between fractional decays	5	5	Single correctCalculation	1
Successive Decay Chainsgraph of successive radioactive decay masses	3	3	Diagram basedData or graph	–
Activity and Rate of Decayactivity of sample from half-life	3	3	Single correctCalculation	1
Counting Alpha and Beta Emissionsmass and atomic number after decay chain	3	3	Single correctApplication	–
Alpha Decay Kinematics and Q-Valueratio of alpha energies from q value	2	2	Single correctCalculation	–
Mean Life Calculationrelation of decay constants from half-life and mean life	2	2	Single correctApplication	–
Parallel Decay Branchingeffective half-life from parallel decay processes	2	2	Single correctApplication	–
Fractional Remainder Shortcuttime to decay to given fraction	1	1	Numerical valueCalculation	1
Activity from Sample Massactivity from mass and half-life	1	1	Single correctCalculation	–
Beta Decay Mechanismstrue statement selection on beta decay	1	1	Single correctConceptual	–

Times and shifts count every analysed shift. Across JEE Main, 78% of questions since 2024 went back to an idea an earlier year had asked; the rest were new. All repeated ideas

Every question

Radioactive Decay questions, year by year

The opening words of each question as it appeared in the official paper, with the idea it tested. Open one for its options and the official answer.

Questions from the official JEE Main Paper 1 shifts published by NTA (CBSE for 2017). Questions NTA dropped are left out. Topic and idea tags, counts and notes are Lumi’s.

Key findings

Questions a shift, year by year

Difficulty, format and skill

Difficulty

Format

Skill tested

New idea, or one asked before?

Coverage of the topic

Idea, year, question

Every idea inside this topic

Radioactive Decay questions, year by year

JEE Main 20261 question in 1 shift

JEE Main 20251 question in 1 shift

JEE Main 20235 questions in 5 shifts

JEE Main 202214 questions in 12 shifts

JEE Main 202116 questions in 13 shifts

JEE Main 20202 questions in 2 shifts

JEE Main 20171 question in 1 shift

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