The transformation of an unstable atomic nucleus into a stable nucleus spontaneously is known as radioactive decay. This results in the release of energy or matter from the nucleus of an atom. The released radiation is in the form of beta particle, alpha particle, gamma rays, etc.
During the radioactive decay, the sum of atomic number and the sum of mass number will be same on both the sides (reactant and product side) of the nuclear decay equation.
Atomic number of
is 82 and the atomic number of
is 80.
The given decay can be written as:

Let X be the other product formed where z is the atomic number of X and a is the mass number.
in order to determine the identity of X:
The value of a:


The value of z:


The element with atomic number 2 is Helium, 
The radioactive decay of
to
will be written as:

Hence, In the radioactive decay of 204pb to 200hg the other product is
.