Atoms & structure
Atomic mass — পারমাণবিক ভর
Quick answer
Atomic mass is the average mass of an element's atoms, measured in unified atomic mass units (u).
Definition
The atomic mass of an element is the weighted average mass of its naturally occurring isotopes, measured in unified atomic mass units. One unit, 1 u, equals one twelfth of a carbon-12 atom. Because it is an average, atomic mass is usually not a whole number: chlorine reads 35.45 u since about three quarters of its atoms are Cl-35 and one quarter Cl-37. A mass number belongs to one isotope; atomic mass belongs to the element as a whole.
For example
Chlorine's average works out as (35 × 75 + 37 × 25) ÷ 100 = 35.5 u — the classic exam sum, straight from the Class 9–10 book.