Concept: Covalent bonds involve the sharing of electrons between two atoms. Coordinate bonds (or dative bonds) are a special type of covalent bond where both shared electrons are contributed by only one of the participating atoms (the donor atom) to another atom (the acceptor atom).

Why (B) is correct:

The tetrafluoroborate ion, ${\text{BF}}_4^{-}$, is formed when a fluoride ion (${\text{F}}^{-}$) donates a lone pair of electrons to a boron trifluoride (${\text{BF}}_3$) molecule. In ${\text{BF}}_3$, boron has only six valence electrons (an incomplete octet). When ${\text{F}}^{-}$ donates a lone pair to boron, a coordinate bond is formed. The other three B-F bonds in ${\text{BF}}_4^{-}$ are regular covalent bonds.

Option Analysis:

• A) ${\text{H}}_2\text{O}$: Water contains only covalent bonds between oxygen and hydrogen atoms.
• B) ${\text{BF}}_4^{-}$: As explained above, it contains three covalent B-F bonds and one coordinate B-F bond.
• C) ${\text{NH}}_3$: Ammonia contains only covalent bonds between nitrogen and hydrogen atoms.
• D) ${\text{SF}}_2$: Sulfur difluoride contains only covalent bonds between sulfur and fluorine atoms.

Correct Answer: (B)