how can i tell if a molecule has one of the following features

-incomplete octet
-expanded shell
-odd electron molecule

for example BCl3 is incomplete octet but i don't know why

also for B when speaking about lewis structures and the octet rules, does it want to have 6 electrons rather than 8??

thanks

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
H
..
B:H
..
H
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.
Example 1:
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.

To determine if a molecule has an incomplete octet, expanded shell, or contains odd electrons, you need to consider the valence electrons and the Lewis structure of the molecule. Here are the steps to determine the features you mentioned:

1. Determine the total number of valence electrons for the molecule. You can find this by adding up the valence electrons of each atom in the molecule. Be sure to consider any charges on the ions, if applicable.

2. Build the Lewis structure by placing the atoms together and connecting them with single bonds. Remember, hydrogen always makes a single bond, and other atoms typically form as many bonds as possible to achieve an octet (except for hydrogen and helium, which have a duet instead of an octet).

3. Distribute the remaining valence electrons around the atoms to satisfy the octet rule. Each atom (except hydrogen and helium) should strive to have eight electrons around it.

- Incomplete octet: A molecule has an incomplete octet when an atom in the molecule has fewer than eight electrons in its valence shell. This typically occurs with elements from Group 2 (such as Be or Mg) or Group 13 (such as B or Al). For example, in BCl3, boron has only six electrons around it in the Lewis structure, making it an incomplete octet.

- Expanded shell: A molecule has an expanded shell when an atom has more than eight electrons around it in the Lewis structure. This usually occurs with elements from Period 3 or beyond in the periodic table, where d-orbitals become available for bonding. For example, sulfur (S) can accommodate more than eight electrons in its valence shell. A molecule like SF6 has sulfur bonded to six fluorine atoms, resulting in an expanded shell for sulfur.

- Odd electron molecule: A molecule with an odd number of valence electrons is called an odd electron molecule. This often happens when there is an odd number of electrons contributed by an odd-electron atom or an odd number of lone pairs. For example, the nitrogen dioxide molecule (NO2) has an odd number of valence electrons due to the presence of a lone pair on nitrogen.

Regarding boron (B) in Lewis structures and the octet rule, it is true that B prefers to have only six electrons instead of eight. This is due to its position in Group 13, which has three valence electrons. By forming three covalent bonds, boron can achieve a stable configuration with six electrons around it. Examples of this include BCl3, where boron forms three bonds with chlorine, resulting in six valence electrons around boron.

I hope this explanation helps you understand how to identify incomplete octets, expanded shells, and odd electron molecules, as well as the octet rule for boron. Let me know if you have any further questions!