Atomic Mass of Selenium (Se)
The relative atomic mass of Selenium is 78.971 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
This is the fast lane: one page, one fact, zero digressions. Read the quick answer, check the table, and if your exam needs the full derivation, the sections below walk through it step by step — in Bangla and English, like every page here. Need the whole picture instead — history, characteristics, uses, exam questions? The full Selenium page is one click away.
Selenium by the numbers
One Selenium atom carries 34 protons in its nucleus; add the neutrons of its commonest isotope (about 45) and you get its mass number, roughly 79.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 34 |
| Relative atomic mass | 78.971 u |
| Protons | 34 |
| Neutrons (commonest isotope) | 45 |
| Electrons | 34 |
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 (78.971). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Selenium a whole number?
No. 78.971 u is the average of Selenium'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 Selenium have?
Neutrons = mass number − atomic number. For Selenium's commonest isotope that is about 79 − 34 = 45, though the exact count differs between isotopes.