Atoms & structure
Atomic number — পারমাণবিক সংখ্যা
Quick answer
The atomic number is the number of protons in an atom's nucleus — the element's identity.
Definition
The atomic number, symbol Z, counts the protons in a nucleus, and it alone fixes the element's identity: every atom with 6 protons is carbon, every atom with 26 is iron. In a neutral atom the electron count equals Z, which is why Z also predicts the electron arrangement and hence the chemistry. The modern periodic table is ordered by atomic number, not atomic mass — Henry Moseley's insight, which finally put every element in its right place.
For example
On the Soren table, iron is element 26 because every iron atom carries 26 protons; add one more proton and it is no longer iron but cobalt.