The emission of an alpha particle from an atom will decrease the atomic number of the atom by 2.
An alpha particle is composed of two protons and two neutrons, which is equivalent to a helium nucleus. When an alpha particle is emitted from an atom, it means that two protons and two neutrons are ejected from the nucleus. Since the atomic number of an element is determined by the number of protons in its nucleus, removing two protons will decrease the atomic number by 2.
For example, if an atom with an atomic number of 88 (such as radium) emits an alpha particle, the resulting atom will have an atomic number of 86 (since 88 - 2 = 86), which corresponds to the element radon.