Atoms & structure
Electron shell — ইলেকট্রন শেল
Quick answer
An electron shell is an energy level around the nucleus where electrons stay, named K, L, M, N and so on.
Definition
Electrons do not orbit anywhere they like; they occupy shells — fixed energy levels spreading out from the nucleus, named K, L, M, N in order. Each shell holds at most 2n² electrons, so K takes 2, L takes 8, M takes 18. Shells fill from the inside out, and the outermost one holds the valence electrons that do all the bonding. An element's shell arrangement, like sodium's 2, 8, 1, is the first thing to write in almost every chemistry answer.
For example
Sodium's 11 electrons sit as 2, 8, 1 — two filling K, eight filling L, and one lonely electron in M that it readily donates to chlorine.