Atoms & structure
Electron configuration — ইলেকট্রন বিন্যাস
Quick answer
Electron configuration is the neat listing of how an atom's electrons occupy its shells and subshells.
Definition
The electron configuration writes out where every electron lives, filling subshells from lowest energy upward (the aufbau principle): 1s, 2s, 2p, 3s, 3p, 4s, then 3d, and so on. Sodium reads 1s² 2s² 2p⁶ 3s¹, or more briefly [Ne] 3s¹, borrowing the previous noble gas as shorthand. Iron reads [Ar] 3d⁶ 4s². The configuration decides everything important: which block an element sits in, how many valence electrons it has, and how it bonds.
For example
Compare sodium's [Ne] 3s¹ with iron's [Ar] 3d⁶ 4s²: one outer electron to give away easily, versus a roomy 3d subshell — a reactive metal versus the backbone of steel.