Question

How does an atom attain stability

Answer 1

a atom can attain stability by loosing or gaining electrons.
by forming ionic or covalent bond between them

Answer 2

The stability of atoms depends on whether or not their outer-most shell is filled with electrons.  If the outer shell is filled, the atom is stable.  Atoms with unfilled outer shells are unstable, and will usually form chemical bonds with other atoms to achieve stability.

they attain stability when there are 8 electrons in the outermost shell of an atom