By firstly knowing the capacity of all shells and then keeping the electron like wise and the precaution not to keep more than 8 in one except in noble gas.
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The electron configuration of an atom is the representation of the arrangement of electrons distributed among the orbital shells and subshells. Commonly, the electron configuration is used to describe the orbitals of an atom in its ground state, but it can also be used to represent an atom that has ionized into a cation or anion by compensating with the loss of or gain of electrons in their subsequent orbitals.