This statement is false. While it is true that the emission of an alpha particle does decrease the atomic mass of the atom, it also affects the atomic number, which ultimately changes the identity of the element itself.
An alpha particle is made up of two protons and two neutrons, essentially a helium nucleus. When an atom emits an alpha particle, it loses these particles from its nucleus. This results in a decrease in atomic mass because the total number of nucleons (protons and neutrons) in the nucleus is reduced.
However, since the emission of an alpha particle also removes two protons from the nucleus, the atomic number of the atom decreases by two. This means the atom transforms into a different element on the periodic table. For instance, if Uranium-238 emits an alpha particle, it transforms into Thorium-234, both differing in atomic number and identity.
In summary, while the emission of an alpha particle does decrease the atomic mass, it also changes the atomic number and thus the element itself; hence the statement is false.