Question

We know M - shell contain 18 electrons but in K atom the electronic configuration is 2,8,8,1 - why?

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Answer 1

" There are certain rules for arranging atoms in shells 1) Maximum number of electrons in a shell is given by the formula 2n^2. 2) The outermost ( valence) shell can accomodate only 8 electrons. Let's take an example of calcium whose number of electrons = 20 K shell - 2 L shell - 8 M shell -8 N shell - 2 If we put these 2 electrons also in the M shell then it would violate the second rule because valence electrons would we equal to 10 but it should be only 8. Hope you got it.

All shells (except the 1st shell) have enough space for a maximum of 8 electrons. The 1st shell has space for only 2 electrons because it is so close to the nucleus. Electrons will fill each shell completely before moving on to the next one. So an atom with 18 electrons, would fill the shells like: 2 (in the 1st shell) , 8 (in the second shell) , and 8 (in the 3rd shell). 2+8+8=18 "

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