The outer electrons are the electrons of the last occupied shell
Example:
The selenium atom has 34 electrons:
34Se $1s^22s^22p^63s^23p^6$$4s^2$$3d^{10}$$4p^4$
Last shell: n=4 !
Those are the electrons drawn in the Lewis structure of selenium !
- red dashes:
electron pairs = $2e^-$ in one orbital
- red points:
unpaired electrons = $1e^-$ in one orbital