Nuclear reactions and conservation laws
Study the equations for nuclear reactions, including conservation of mass number, atomic number, and other physical quantities.
Why this shows up in the exam
NEET expects you to balance nuclear equations and apply conservation principles.
How NEET tests this
Learn the idea
In every nuclear reaction the total mass number (A) and total atomic number (Z) of the reactants equal those of the products. The single insight is to write the unknown product as X(A,Z) and solve the two simple equations A_reactants=A_products and Z_reactants=Z_products.
🧠 Memory hook: Think of A as the total "Army" of nucleons marching together, and Z as the "Zebra" stripe count – both armies must stay the same before and after the battle.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Mass number A = number of protons + neutrons (conserved)
- Atomic number Z = number of protons (conserved)
- Deuteron = 2H (A=2, Z=1)
- Alpha particle = 4He (A=4, Z=2)
- Neutron = 1n (A=1, Z=0)
- Write the reaction in the form: reactants → products + X(A,Z)
How to approach it
- 11. Write down the symbols of all known reactants and products, including their A and Z values.
- 22. Represent the unknown nucleus as X(A,Z).
- 33. Apply A‑conservation: sum of A on left = sum of A on right → solve for A of X.
- 44. Apply Z‑conservation: sum of Z on left = sum of Z on right → solve for Z of X. The pair (Z,A) gives the required nucleus.
Worked example — watch it click
A deuteron strikes ₈O¹⁶ nucleus with subsequent emission of an alpha particle. Identify the nucleus so produced:
- A)₃Li⁷
- B)₅B¹⁰
- C)₇N¹³
- ✅₇N¹⁴
The concept behind this problem
The example forces you to use both A‑ and Z‑conservation simultaneously to identify the unknown product, exactly the skill NEET tests in nuclear‑reaction questions.
Step by step
- 1Nuclear reaction: ₁H² + ₈O¹⁶ → ₂He⁴ + X.
- 2Conservation of mass number: 2 + 16 = 4 + A, so A = 14.
- 3Conservation of atomic number: 1 + 8 = 2 + Z, so Z = 7.
- 4The nucleus with Z=7 and A=14 is ₇N¹⁴ (nitrogen-14).
Watch out
Students often take the alpha as 2He (A=2) instead of the correct 4He, leading to a wrong A for the produced nucleus.
Common slip-ups that cost marks
- •Forgetting to count emitted neutrons or other light particles in the balance
- •Using the wrong mass number for an alpha particle (it is 4, not 2)
- •Mixing up A and Z – e.g., treating charge as mass number
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Practise it
These are real questions from past NEET papers that test this exact idea.
When a uranium isotope ²³⁵₉₂U is bombarded with a neutron, it generates ⁸⁹₃₆Kr, three neutrons and
Push further
More challenging4 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.
A ₁₁Na²³ nucleus is bombarded with a proton (₁H¹), leading to the formation of a ₁₂Mg²³ nucleus. What particle must have been emitted in this reaction?
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