American Chemical Society (ACS) Chemistry Test 2025 – 400 Free Practice Questions to Pass the Exam

When a neutron is lost from the nucleus of an atom, it does not change the atomic number of that atom. The atomic number is defined as the number of protons in the nucleus, which determines the element and its properties. Since neutrons have no charge and do not alter the number of protons when they are removed, the atomic number remains the same.

In contrast, losing a proton would decrease the atomic number, potentially changing the element to a different one. Electrons, being charged particles outside the nucleus, also do not affect the atomic number but rather influence the ionization state of the atom. A positron, which is the antimatter counterpart of the electron, would be involved in processes like beta decay but would still not affect the atomic number when discussing stability and structure beyond standard decay processes.

Thus, the loss of a neutron is the only action in this context that maintains the atomic number, making it the correct choice.