Atomic Mass of Arsenic (As)
The relative atomic mass of Arsenic is 74.922 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
Looking for Arsenic's atomic mass without wading through a whole chapter? Here is the direct answer first, then the tables and derivations examiners want to see. Every number on this page comes from the same engine that powers the interactive table. Need the whole picture instead — history, characteristics, uses, exam questions? The full Arsenic page is one click away.
Arsenic by the numbers
Z = 33 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (74.922 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 33 |
| Relative atomic mass | 74.922 u |
| Protons | 33 |
| Neutrons (commonest isotope) | 42 |
| Electrons | 33 |
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 (74.922). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Arsenic a whole number?
No. 74.922 u is the average of Arsenic'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 Arsenic have?
Neutrons = mass number − atomic number. For Arsenic's commonest isotope that is about 75 − 33 = 42, though the exact count differs between isotopes.