Concepts Of Physics MCQ Edition [Volume 2]PhysicsThe Nucleus
The mass number of an atomic nucleus is
Options
- Aalways equal to its atomic number
- Balways larger than its atomic number
- Calways smaller than its atomic number
- Dsometimes larger than and sometimes equal to its atomic number
Correct answer
D. sometimes larger than and sometimes equal to its atomic number
Step-by-step solution
The mass number A of an atomic nucleus is the sum of the number of protons Z (atomic number) and the number of neutrons N . This gives the relation A = Z + N . For an ordinary hydrogen nucleus ( ¹H ), there are no neutrons ( N = 0 ), so its mass number is equal to its atomic number ( A = Z = 1 ). For all other atomic nuclei, there is at least one neutron ( N > 0 ), making the mass number strictly greater than the atomic number ( A > Z ). Therefore, the mass number is sometimes larger than and sometimes equal to its