The electronic configuration of potassium(K) is 2,8,8,1 instead of 2,8,9 thogh the M shell can accommodate up to 18 electrons. Explain
if a shell attains 8 electrons( except for 1st shell) then the extra can move into the next shell. this is the reason. but for example: zinc configuration is 2,8,18,2, the maximum which the 3rd shell can hold is 18 and and because the first two shells are complete....the third shell is 18 and not in configuration 2,8,8,8,4.
The last filled up shell of an atom can have only 8 electrons.in this case although M can have 18 electrons, Since its the last, it can hold only 8.this is also why atoms react . they are constantly trying to attain an octet or 8 electrons in the last shell by giving or taking electrons from other atoms.how many atoms they give or take is the valency.
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