Formaldehyde contains 12 valence electrons 2 from 2 H, 4 from C, and 6 from O. C neighbors the other atoms, so draw one bond from C to each of the remaining atoms:. The drawing contains only 6 electrons 3 bonds , so 6 electrons are leftover. O is the only terminal atom that lacks an octet, so we add all 6 electrons to O as lone pairs:.
Although the rules don't call for it, I've added formal charges to this intermediate drawing. C is the only atom lacking an octet, and O is the only C neighbor that possesses a lone pair.
Therefore, we convert an O lone pair into an additional CO bond. Formal charge calculation. There are now four bonds in the skeleton structure, which leaves only four nonbonding electrons. This is enough, however, to satisfy the octets of the carbon and oxygen atoms. Every once in a while, we encounter a molecule for which it is impossible to write a satisfactory Lewis structure. There are three covalent bonds in the most reasonable skeleton structure for the molecule.
Because it takes six electrons to form the skeleton structure, there are 18 nonbonding valence electrons. Each fluorine atom needs six nonbonding electrons to satisfy its octet. Thus, all of the nonbonding electrons are consumed by the three fluorine atoms. As a result, we run out of electrons while the boron atom has only six valence electrons. Because neither boron nor fluorine falls in this category, we have to stop with what appears to be an unsatisfactory Lewis structure.
It is also possible to encounter a molecule that seems to have too many valence electrons. When that happens, we expand the valence shell of the central atom. Example: Consider the Lewis structure for sulfur tetrafluoride SF 4 which contains 34 valence electrons. There are four covalent bonds in the skeleton structure for SF 4. Because this requires using eight valence electrons to form the covalent bonds that hold the molecule together, there are 26 nonbonding valence electrons.
Because there are four of these atoms, so we need 24 nonbonding electrons for this purpose. But there are 26 nonbonding electrons in this molecule.
We have already satisfied the octets for all five atoms, and we still have one more pair of valence electrons. We therefore expand the valence shell of the sulfur atom to hold more than eight electrons. This raises an interesting question: How does the sulfur atom in SF 4 hold 10 electrons in its valence shell?
The electron configuration for a neutral sulfur atom seems to suggest that it takes eight electrons to fill the 3 s and 3 p orbitals in the valence shell of this atom. But let's look, once again, at the selection rules for atomic orbitals. Because the 3 d orbitals on a neutral sulfur atom are all empty, one of these orbitals can be used to hold the extra pair of electrons on the sulfur atom in SF 4. This oxygen already has these two electrons that it's sharing hanging around.
So in order to get to the octet rule, it needs six more. So let's give it six electrons. So, one, two, three, four, five, six. Well I've just used up the remaining six valence electrons. So I don't really have any more to play with, but let's see how the other atoms are feeling.
So hydrogen here, it's able to share these two electrons that are in this covalent bond, so it's feeling good. It can kind of pretend that it has a full outer shell, 'cause its outer shell is just that one, that first shell, that's filled with two electrons. Same thing for this other hydrogen. So at least the terminal atoms, the oxygen and the two hydrogens, are feeling like they have a full outer shell. But then in the fourth step, we're going to look at our central atom. So, let's focus on the central atom, central atom, and do we need more bonds, or do we need to do something interesting here?
And what we see is that carbon, it's able to have two, four, six electrons hanging around it, but it would love to have eight. Carbon would love to have a full outer shell, so how could we do that?
Well, we could add more bonds. Where could the bonds come from? Well it would come from some lone pair of electrons. Well the only lone pairs of electrons are hanging around this oxygen. Radius of the earth in meters. Room temperature in Kelvin. How many Jupiters can fit in the sun. If your mass is 60 kg on earth, what would your mass be on the moon? Orion is visible on winter evenings in the northern hemisphere but not summer evenings because of.
0コメント