what is the atomic number of 216/84 after it goes under alpha decay

The atomic number of an element is equal to the number of protons in its nucleus. In alpha decay, a nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. This means that the atomic number of the new nucleus is reduced by 2, since it has lost two protons.

Therefore, after undergoing alpha decay, the atomic number of the nucleus with 216/84 becomes 214/82.