If the mass number of an atom is A = 40 and its electron configuration is $1 s^2, 2 s^2, 2 p^6, 3 s^2, 3 p^6$, the number of neutrons and protons in its nucleus will be
Answer & explanation
Correct answer: option 1
Number of protons = 2 + 2 + 6 + 2 + 6 = 18
Number of neutrons = 40 – 18 = 22.