These answers should be obvious if you've tried to write several hundred Lewis structures.
An odd electron comes from an odd number of electrons. Right?
An expanded octet comes from having too many electrons. That is, after assigning electrons and there are eight around each atom, if electrons are left over, those are added to the central atom in the extra (usually d) orbitals.
Incomplete octet is when you just don't have enough electrons to go around and provide eight for each element (two in the case of H).
In the case of BH3, you have
B = 1B x 3e each = 3 electrons
H = 3H x 1e each = 3 elecrons
Add them and we have six electrons. So we have
So you have satisfied each of the H atoms with two electrons an you've used all six of the electrons you have. There aren't anymore even though B might like to have two more. BUT, that makes compounds like BH3, BCl3, BF3, etc, VERY susceptible to adding almost anything that has a free pair of unshared electrons. For example, NH3 has a pair, Cl^-, F^- etc have a pair so it's easy to form NH3BH3 or BF4^- and BCl4^-. The incomplete octet of B compounds makes it a very strong Lewis Acid (it wants to accept electrons.)
• Count the total number of valence electrons.
• Link the central atom to the outer atoms with shared electron pairs. Count the shared electrons.
• Complete the octets of the outer atoms.
• Subtract (Total number - Shared number ) = ___? That is the number of unshared electrons on the central atom.
The total number of valence electrons in BCl3 is 24.
The number of shared electrons in BCl3 is 6 electrons (3 shared pairs between B and Cl).
The number of electrons needed to completre the octets of the three Cl's is 18.
The number of nonbonding electrons on the boron, B, is 24 - (6 + 18) = 0
Conclusion: the central atom, boron, is surrounded by 6 shared electrons only. That is an incomplete octet.
Expanded octets on the central atom are determined in a similar way: 1) Count the total number of valence electrons. 2) Assume the outer atoms have complete octets. 3) Place 10 or 12 electrons around the central atom to get the correct total number.
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