Chemistry

Question Analysis
This question tests the student's ability to calculate the mass number of an isotope given the number of protons and neutrons.

Key Concept Explanation
The mass number (A) is the sum of the number of protons and neutrons in the nucleus.

Step-by-step Solution
1. Identify the number of protons and neutrons in the isotope: 19 protons and 20 neutrons.
2. Calculate the mass number by adding the number of protons an...