What is the isotope symbol for an atom that contain 82 protons and 125 neutrons?

May 18, 2018

""_82^207Pb or just ""^207Pb.

Explanation:

The isotope of an element $X$ is given by ""_Z^AX, where $Z$ is the proton number of the element and $A$ is the mass number of the element.

The mass number of this isotope will be $82 + 125 = 207$ units, while it has $82$ protons.

Looking on the periodic table, element number $82$ is lead, and its symbol is $P b$.

Therefore, the isotope in this case will be written as:

""_82^207Pb

Alternatively, we can write it even shorter like this,

""^207Pb

since we know that lead can only have an atomic number of $82$.