Covalent bonding and coordinate (dative covalent) bonding — common mistakes

When drawing a dot-and-cross diagram for a species containing a coordinate bond, like $[NH_4]^+$, ensure you show both electrons for that specific bond coming from the donor atom (e.g., both as crosses if nitrogen is the 'cross' atom). Crucially, always enclose the final ion in square brackets and write the overall charge in the top right corner. Forgetting the brackets and charge is a common way to lose marks.

No. Once formed, a coordinate bond is indistinguishable from any other covalent bond in the molecule or ion. For example, in the ammonium ion ($NH_4^+$), all four N-H bonds are identical in length and strength. The distinction is only in the 'history' of where the electrons came from.

Yes, absolutely. A common example is the adduct formed between ammonia and boron trifluoride, $F_3B-NH_3$. The nitrogen donates its lone pair to the electron-deficient boron atom. Another example is carbon monoxide, $CO$, which is best represented with a triple bond, one of which is a coordinate bond.

In boron trifluoride, the central boron atom forms three single covalent bonds with three fluorine atoms. This gives the boron atom only six electrons in its outer shell (3 bonds × 2 electrons/bond = 6 electrons), not a full octet of eight. This makes it 'electron deficient' and able to accept a lone pair from a donor atom.