Atomic Mass of Chlorine (Cl)
The relative atomic mass of Chlorine is 35.45 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 Chlorine. 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 Chlorine page is one click away.
Chlorine by the numbers
Z = 17 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (35.45 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 17 |
| Relative atomic mass | 35.45 u |
| Protons | 17 |
| Neutrons (commonest isotope) | 18 |
| Electrons | 17 |
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 (35.45). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Chlorine a whole number?
No. 35.45 u is the average of Chlorine'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 Chlorine have?
Neutrons = mass number − atomic number. For Chlorine's commonest isotope that is about 35 − 17 = 18, though the exact count differs between isotopes.