When you add a neutron to an atom, the atomic mass of the atom increases. This is because the atomic mass is determined by the combined mass of the protons and neutrons in the atom's nucleus. Protons and neutrons both have a mass of approximately 1 atomic mass unit (amu).
If you add a neutron to the atom, you are increasing the number of neutrons in the nucleus while keeping the number of protons constant. Since neutrons have a mass of approximately 1 amu, adding a neutron increases the total mass of the nucleus without changing the atom's identity.
For example, let's consider the hydrogen atom (H). The most common isotope of hydrogen, known as protium, has one proton and no neutrons. Its atomic mass is approximately 1 amu. If you add a neutron to hydrogen, you form deuterium (D), an isotope of hydrogen with one proton and one neutron. The atomic mass of deuterium is approximately 2 amu.
So, by adding a neutron to an atom, you increase the atomic mass of the atom without changing its elemental identity.