Atoms & structure
Mass number — ভর সংখ্যা
Quick answer
The mass number is the total of protons plus neutrons in one nucleus, written as symbol A.
Definition
The mass number, symbol A, is the sum of protons and neutrons in a particular nucleus: A = Z + N. It is always a whole number, because you cannot have half a particle, and it is written at the top left of a symbol, as in ¹²C or ²³⁸U. Two atoms of the same element can have different mass numbers — those are isotopes. Do not confuse it with atomic mass, which is the weighted average for the element as found in nature.
For example
A sodium-23 nucleus packs 11 protons and 12 neutrons: 11 + 12 = 23. Your table salt is built from atoms like this.