Q. An atom has 7 electrons in its M - shell and contains 18 neutrons in its nucleus. What is its mass number
A) 27
B) 25
C) 43
D) 35
✔ Correct Answer: 35
Exam Insight
The mass number of the atom is 35, calculated as the sum of protons (17, inferred from the electron configuration) and neutrons (18).