Question

what happens to an atom when it emits a positron?
the atomic number decreases by one and the atomic mass remains constant.
the atomic number increases by one while the atomic mass remains constant.
the atomic number and the atomic mass remain constant.
the atomic number decreases by four and the atomic mass decreases by two.
the atomic number decreases by two and the atomic mass decreases by four.

Explanation:

Positron emission is a type of beta - plus decay. In positron emission, a proton in the nucleus is converted into a neutron, a positron, and a neutrino ($p
ightarrow n + e^{+}+
u_{e}$). The atomic number (which is equal to the number of protons) decreases by 1 because a proton is converted to a neutron. The atomic mass (sum of protons and neutrons) remains constant because the number of nucleons (protons + neutrons) doesn't change (a proton is just converted to a neutron). Now let's analyze each option:

• Option 1: "The atomic number decreases by one and the atomic mass remains constant." This matches our understanding of positron emission.
• Option 2: Atomic number should decrease, not increase, so this is wrong.
• Option 3: Atomic number changes (decreases by 1), so this is wrong.
• Option 4: This is the case for alpha decay (emission of a helium nucleus, $_{2}^{4}He$), not positron emission, so wrong.
• Option 5: This is also related to alpha decay, not positron emission, so wrong.

Answer:

A. The atomic number decreases by one and the atomic mass remains constant.