An atom has 81 electrons, 84 neutrons, and 82 protons. What element is this atom?

Lead

Explanation:

The subatomic particles within an atom can be used to know the atom or element given.

Of particular interest is the number of protons within the atom.

The periodic table is based on the atomic number of atoms. This atomic number is the number of protons within an atomic space.

So; If we know the number of protons within an atom, we can know the element.

The number of protons given is 82, the element is  therefore lead.

The atomic number of polonium is 84. The atomic number lead is 82.

Explanation:

if you mean hydrogen and oxygen then water is the product