Question

{"given":"Mass number $A$ = number of protons ($Z$) + number of neutrons ($N$), i.e., $A = Z + N$. Atomic number = $Z$.","key_observation":"Since $N \\geq 0$, we have $A = Z + N \\geq Z$. Therefore, mass number is never less than atomic number. It equals atomic number only when $N = 0$ (e.g., $^1_1\\mathrm{H}$), and exceeds it when $N \\geq 1$.","option_analysis":[{"label":"(A)","text":"always less than its atomic number","verdict":"incorrect","explanation":"Since $A = Z + N$ and $N \\geq 0$, the mass number is always greater than or equal to atomic number, never less."},{"label":"(B)","text":"always more than its atomic number","verdict":"incorrect","explanation":"For $^1_1\\mathrm{H}$ (protium), $A = 1$ and $Z = 1$, so $A = Z$; thus mass number is not always more."},{"label":"(C)","text":"sometimes equal to its atomic number","verdict":"correct","explanation":"For $^1_1\\mathrm{H}$, $A = Z = 1$, so mass number equals atomic number. This is true for any nuclide with zero neutrons."},{"label":"(D)","text":"sometimes more than and sometimes less than its atomic number","verdict":"incorrect","explanation":"Mass number is never less than atomic number since $A = Z + N \\geq Z$ for all nuclei; thus 'sometimes less' is factually wrong."}],"answer":"(C)","formula_steps":[]}