If a nucleus gains a neutron and then undergoes beta emission, what happens to its atomic number and mass number?

• A. Both the atomic number and mass number increase by 1.
• B. The atomic number increases by 1, and the mass number remains unchanged.
• C. The atomic number remains unchanged, and the mass number decreases by 1.
• D. The atomic number decreases by 1, and the mass number remains unchanged.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand what happens when a nucleus gains a neutron. A neutron is a neutral particle, so when it is added to the nucleus, the mass number (total number of protons and neutrons) increases by 1, but the atomic number (number of protons) remains the same because no protons are added or removed.
2. Step 2: Recognize what beta emission (beta decay) involves. In beta emission, a neutron in the nucleus is converted into a proton and an electron (beta particle). The electron is emitted, and the proton remains in the nucleus.
3. Step 3: Analyze the effect of beta emission on atomic and mass numbers. Since a neutron changes into a proton, the atomic number increases by 1 (because there is one more proton), but the mass number remains unchanged (one neutron is lost, one proton is gained, total nucleons stay the same).
4. Step 4: Combine the two processes. First, the nucleus gains a neutron, increasing the mass number by 1 with no change in atomic number. Then, beta emission converts that neutron into a proton, increasing the atomic number by 1 but keeping the mass number the same as after the neutron gain.
5. Step 5: Conclude the net effect. After both steps, the atomic number increases by 1, and the mass number remains unchanged compared to the original nucleus before gaining the neutron.