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How does alpha decay change an element's atomic number and mass number?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transuranium element Modern version of the periodic table of the elements (printable).

In alpha decay, an unstable nucleus spontaneously ejects an alpha particle, which consists of two protons and two neutrons. This process results in the following changes to the parent nucleus:12

Atomic Number: The atomic number (the number of protons) decreases by two units.342
Mass Number: The atomic mass number (the total of protons and neutrons) decreases by four units.342

Because the atomic number changes, the daughter nucleus represents a different chemical element than the parent. This mode of decay is particularly common among heavy elements, including all transuranium elements, which possess isotopes that are alpha-unstable.15