Final answer:
The atom's mass decreases when a neutron leaves a nucleus, as seen in decay processes such as beta decay and positron emission that alter the atomic number but not the mass number.
Step-by-step explanation:
When a neutron leaves a nucleus, the atom's mass decreases. This is because the neutron has mass, and its removal from the nucleus results in a lower total mass for the atom. In radioactive decay processes, such as beta decay and positron emission, although the mass number stays the same, there is a change in the atomic number. Beta decay increases the atomic number while positron emission decreases it. In beta decay, a neutron is transformed into a proton and a beta particle is emitted, increasing the atomic number by one but not changing the mass number. With positron emission, a proton is transformed into a neutron and a positron is emitted, decreasing the atomic number by one while keeping the mass number constant.