Question

The atomic number of lead (Pb) is 82. Its mass number is 207. How many protons, neutrons, and electrons does this neutral atom of lead (Pb) contain?

Answer 1

An element’s atomic number is equal to the number of protons in that element’s nucleus. The mass number is the total number of an atom’s protons and neutrons. Protons have a positive charge; electrons have a negative charge; and neutrons are electrically neutral.

Putting it all together, given that the atomic number of lead is 82, the number of protons a lead atom contains is 82. The number of neutrons would be the difference between 207 and 82, or 125 neutrons. Finally, since you have a neutral atom, there must be an equal number of electrons as the number of protons—that is, 82 electrons.

Thus, you’ve got 82 protons, 125 neutrons, and 82 electrons.