In nuclear reactions, which quantities are conserved when writing and balancing reaction equations?

Introduction / Context:
Balancing nuclear reaction equations requires conservation principles analogous to balancing chemical equations. Instead of atoms and molecules, we track nucleons and charge to ensure physical consistency before and after a reaction event such as decay, fission, or fusion.

Given Data / Assumptions:

• Reaction written as target + projectile → products + particles (e.g., n, p, α, β).
• Relativistic mass–energy equivalence applies, but bookkeeping uses discrete mass number (A) and atomic number (Z) for balance.

Concept / Approach:
The mass number A (total nucleons = protons + neutrons) and atomic number Z (protons = net charge in units of e) must be conserved across the equation. Energy and momentum are also conserved, but the tabular balancing typically enforces A and Z conservation explicitly, accounting for emitted particles (e.g., α has A = 4, Z = 2; β− has A = 0, Z = −1; γ has A = 0, Z = 0).

Step-by-Step Solution:
Write all nuclides with A (superscript) and Z (subscript).Sum A on reactant side and match to product side.Sum Z on reactant side and match to product side including emitted/absorbed particles.Confirm energy/momentum consistency separately through Q-value if needed.

Verification / Alternative check:
Example: ²³⁵U (A=235, Z=92) + n (A=1, Z=0) → ¹⁴¹Ba (A=141, Z=56) + ⁹²Kr (A=92, Z=36) + 3 n (3*1, 0). A: 236 → 236; Z: 92 → 92, balanced.

Why Other Options Are Wrong:
A only or Z only: incomplete; both must balance.Neither: violates fundamental conservation.Neutron count only: not generally conserved; neutrons can be emitted/absorbed.

Common Pitfalls:
Forgetting to include the charge of β particles in Z balance.Confusing conservation of A with conservation of rest mass (mass defect converted to energy is allowed).

Final Answer:
Both mass number (A) and atomic number (Z)