Question: Isotones are the nuclides having :
Step 1 (Definition & Governing Principle): [0.5 Mark]
According to the NCERT Physics textbook (Chapter 13: Nuclei), nuclides are classified based on their composition of protons ($Z$) and neutrons ($N$). Isotones are defined as nuclides that possess the same number of neutrons ($N$) but different atomic numbers ($Z$).
The relationship for a nucleus is given by:
$$A = Z + N$$Where:
- $A$ = Mass number
- $Z$ = Atomic number (number of protons)
- $N$ = Neutron number
Step 2 (Step-by-Step Logical Deduction): [0.25 Mark]
To satisfy the condition of being isotones, two different nuclides ($_{Z_1}X^{A_1}$ and $_{Z_2}Y^{A_2}$) must satisfy:
$$N_1 = A_1 - Z_1$$ $$N_2 = A_2 - Z_2$$For isotones, $N_1 = N_2 = N$. Since the nuclides are different, $Z_1 \neq Z_2$, which consequently implies $A_1 \neq A_2$.
Step 3 (Final Concluded Answer): [0.25 Mark]
Comparing the derived condition with the given options, the correct choice is (C).
Correct Option: (C) same neutron number, but different atomic number
💡 EL10 Concept Insight: Think of isotones like two different teams that have a different number of players (atomic number), but both teams happen to have the exact same number of substitute players (neutrons) sitting on the bench.