Atomic Mass of Iodine (I)
The relative atomic mass of Iodine is 126.90 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
Teachers call it "one fact, fully handled" — exactly what this page does for Iodine. The answer sits at the top in bold; scroll for the working, the shell-by-shell breakdown and the two questions students ask most. Need the whole picture instead — history, characteristics, uses, exam questions? The full Iodine page is one click away.
Iodine by the numbers
Z = 53 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (126.90 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 53 |
| Relative atomic mass | 126.90 u |
| Protons | 53 |
| Neutrons (commonest isotope) | 74 |
| Electrons | 53 |
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 (126.90). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Iodine a whole number?
No. 126.90 u is the average of Iodine'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 Iodine have?
Neutrons = mass number − atomic number. For Iodine's commonest isotope that is about 127 − 53 = 74, though the exact count differs between isotopes.