An atom has $82$ protons, $86$ electrons, and $125$ neutrons. Which element is it?

• A. Radon (Rn)
• B. Mercury (Hg)
• C. Lead (Pb)
• D. Polonium (Po)

Answer: (C)

Verified step by step guidance

1. Identify the atomic number of the element, which is equal to the number of protons. Here, the atom has 82 protons, so the atomic number is 82.
2. Use the atomic number to determine the element on the periodic table. The element with atomic number 82 is Lead (Pb).
3. Note that the number of electrons (86) is different from the number of protons (82), indicating the atom is an ion with a charge. Since there are more electrons than protons, it is a negatively charged ion.
4. Calculate the mass number by adding the number of protons and neutrons: mass number = protons + neutrons = 82 + 125.
5. Confirm the element identity by matching the atomic number to the element symbol and name, which in this case corresponds to Lead (Pb).