For the following species,determine whether the octet rule is obeyed or not obeyed. Provide the reason for each:
$(1) BeF_4^{2-}$
$(2) BF_3$
$(3) CO_2$
$(4) NO$
$(5) NH_3$

(N/A) The octet rule states that atoms tend to gain,lose,or share electrons to achieve a stable configuration of $8$ electrons in their valence shell.
$(1) BeF_4^{2-}$: Obeyed. The central $Be$ atom shares $4$ pairs of electrons,resulting in $8$ electrons in its valence shell.
$(2) BF_3$: Not obeyed. The central $B$ atom has only $6$ electrons in its valence shell (electron-deficient molecule).
$(3) CO_2$: Obeyed. Both $C$ and $O$ atoms complete their octets through double bonds.
$(4) NO$: Not obeyed. This is an odd-electron molecule with $11$ valence electrons,leaving the $N$ atom with $7$ electrons.
$(5) NH_3$: Obeyed. The central $N$ atom has $8$ electrons in its valence shell ($3$ shared pairs and $1$ lone pair).