Atomic Mass of Lead (Pb)
The relative atomic mass of Lead is 207.2 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
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Lead by the numbers
One Lead atom carries 82 protons in its nucleus; add the neutrons of its commonest isotope (about 125) and you get its mass number, roughly 207.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 82 |
| Relative atomic mass | 207.2 u |
| Protons | 82 |
| Neutrons (commonest isotope) | 125 |
| Electrons | 82 |
Atomic mass vs mass number
Mass number (A) counts particles — protons + neutrons in one specific atom, always a whole number. Relative atomic mass is the experimentally measured average over all natural isotopes, so it comes out fractional (207.2). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Lead a whole number?
No. 207.2 u is the average of Lead's natural isotopes, which is why it lands between whole numbers. The mass number of a single isotope, though, is always whole.
How many neutrons does Lead have?
Neutrons = mass number − atomic number. For Lead's commonest isotope that is about 207 − 82 = 125, though the exact count differs between isotopes.