206 82 Pb is an isotope. What is its atomic number? What is its mass number? What is its neutron number?
This exercise is about understanding the nomenclature used to write down an atomic isotope. According to convention, the atomic number, that is the number of protons which is what defines the identity of the element, is written as a subscript. So in this case, the atomic number of this element, lead, is 82.
Next, we look for the mass number. This is the value written as a superscript. The mass number equals the number of protons plus the number of neutrons in an isotope. For this isotope of lead, it is 206.
Finally, we want to know the number of neutrons in this isotope. That number isn’t written down to identify the isotope, but we can solve for it based on what is. Recall that the mass number is equal to the number of protons plus the number of neutrons, and that the atomic number is the number of protons in the sample.
So to find the number of neutrons, we subtract the atomic number from the mass number. In our case, 206 minus 82 which equals 124. That’s the number of neutrons in this lead isotope.