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it is said that no. of electrons in a shell is given by the formula 2n square. but it is also said that outer most shell should contain 8 electrons only .why is it so?

The molecules or ions tend to be most stable when the outermost electron shells of their constituent atoms contain eight electrons. The rule is commonly used in drawing Lewis dot structures (see the Related Questions links to the left of this answer). It is known that the formula used to find how much electrons (e-1) can fit a shell or orbit is 2n2 eg: Shell 1 = 2 x (1)2 = 2 e-1
Shell 2 = 2 x (2)2 = 8 e-1 which is the maximum number a shell/orbit/orbital can hold 


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