How does Lewis structure works

Actually, the Lewis structure doesn't "work" for anything, it is just a way of expression of how many electrons are on a molecule or atom, and how they locate. This is of course not the real way of alignment of electrons as you might think and wish they are.

But basically, we represent an ionic bond we express on compartment receiving another's electrons by drawing 1 or 2 or 3 more on it.

Take NaCl as an example, we usually do not draw inner shell electrons, so we just leave Na+ as Na+ in the Lewis dot structure. So where has the one electron gone? To the Cl atom to form a Cl- ion. Represented by eight dots around the Cl atom (best group them into pairs). You can use brackets to include the ions inside to represent they are seperate ions, out side the brackets, you should also give the sign, like [Na]+

But for covalent bonds, we do not seperate matters. We just like up what we've got. Like the dichlorine molecule. For a colvalent bond, we think of them each giving one electron for bonding between them, the chlorine atoms. And for a bond, we can put the electrons in between the atoms to represent it. In this bond, each electron get chance of share of two electrons, so in total gaining one.

So each chlorine atom have 6 electrons that haven't been drawn out, so we draw them out surround each chlorine atom.


And for dative bond, it is totally equivalent to a covalent bond. Some chemist would like to seperate them because sometimes an atom seem to donate a pair of electrons for bonding with each other, instead of 1 from each atom. But i have to clarify that dative bond is absolutely the same as any covalent bond. The only think to note is that a charge is usually encountered in a dative bond.

Consider BF3NH3, BF3 has three covalent bonds, but as boron still need two electrons to reach octet, so it readily accept the electron from ammonia to form 4 bonds, with a negative charge developed on itself, so of course, a positive charge is developed on ammonia. This is what some chemists call a dative bond.

Consider the ammonium ion, NH4+. Consider pure gaseous chemistry, it is originally ammonia and H+, but the H+, essentially a proton, have extremely high charge density that it would like to approach any electron it could reach. So it readily accept the lone pair electron from ammonia and become the ammonium ion. But chemists usually don't call this dative bond due to some cultural difference. But you can see that there are absolutely no difference.

Talking back to Cl2, we can consider Cl2 as originally Cl+ and Cl-, (of course again gaseous chemistry, and you will see that Cl+ is not that unstable). So why is not Cl-Cl bond a dative bond? This is again, culturally, we consider Cl2 as originally two Cl atoms.

So whether a bond is dative or not, say it yourself. Tell of dative bond in the exams, but keep in mind that it's all your perceptions.