To achieve an octet, 3 more electrons must be gained to the atom.
Work Step by Step
The electron configuration of the atom $$1s^22s^22p^3$$ To achieve an octet, besides having 8 valence electrons, the atom must also have fully occupied $s$ and $p$ subshells. Here we see that for subshells $1s$ and $2s$, each is occupied by 2 electrons. And $s$ subshell may be occupied by, at most, 2 electrons. Therefore, both subshells $1s$ and $2s$ are fully occupied. Subshell $2p$ is occupied by 3 electrons. A $p$ subshell can have 6 electrons in maximum, which means subshell $2p$ still has 3 spots left unoccupied. Therefore, to achieve an octet, 3 more electrons must be gained to fill the unoccupied spots in subshell $2p$ so that it can be fully occupied.