When an atom changes by alpha decay, its atomic number decreases because of the losing of proton and neutron.
An alpha particles is a helium nucleus that contains 2 protons and 2 neutrons. When the alpha decay occur, the atom loses an alpha particle which causes the loss of proton and neutron thus decreases the mass number as well as atomic number occur.
So we can conclude that When an atom changes by alpha decay, its atomic number decreases because of the losing of proton and neutron.
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