No, this set of values is impossible.
Work Step by Step
1. Is the atomic number equal to the number of protons? - Yes, they are equal. (18) 2. Is the sum of the number of protons and neutrons equal to the mass number? Sum: $18 + 40 = 58$ Mass number: 40 - No, they aren't equal ($58 \ne 40$). - Therefore, this set of values is impossible.