1. Atoms in second row can have less than an octet. Ex.
QUESTION:
In which of the following atoms must have an octet when sharing electrons with other atoms?
A. N, B. B, C. P, D. none of these
The octet rule is not a very strict rule. It’s a good rule to follow to get to a good starting point for drawing the structure for a molecule.
Atoms in second row can have less than an octet. Ex.
Be in BeH2 is surrounded by four valence electrons.
B in BF3 is surrounded by six valence electrons.
N in NO is surrounded by seven valence electrons.
Atoms in third row can have an “expanded octet”. Ex. P in PCl5 is surrounded by 10 valence electrons
In which of the following atoms must have an octet when sharing electrons with other atoms?
A. N, B. B, C. P, D. none of these
PAUSE
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The octet rule is not a very strict rule. It’s a good rule to follow to get to a good starting structure for a molecule. But there are a lot of exceptions.
Atoms in second row can have less than an octet. For example,
Be in BeH2 is surrounded by four valence electrons.
B in BF3 is surrounded by six valence electrons.
N in NO is surrounded by seven valence electrons.
CLICK Atoms in third row can have an “expanded octet”. For example, P in PCl5 is surrounded by 10 valence electrons
The correct answer is D. None of these.
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2. Here are some examples of structures with atoms that do not follow the octet rule.
Structures A, B, and C are resonance structures of boron trifluoride.
Note that the Boron atom in structure A only six valence electrons around it.
Experimental evidence and formal charges suggest that this resonance structure is more significant than
structures B and C.
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Here are two possible resonance structures for the nitric oxide molecule. There is now way to satisfy
the octet rule for both atoms in this case because the total number of valence electrons is an odd number.
Recall that nitrogen belongs to group 5A and oxygen belongs to group 6A. Therefore the total number
of valence electrons is 11. The presence of an unpaired electron can be verified by subjecting the
molecules to a magnetic field. The presence of an unpaired electron makes a molecule paramagnetic.
Here’s the structure for phosphorus pentachloride. Note that the phosphorus here is surrounded by
10 valence electrons.
HIGHLIGHT single bonds as you count.
Phosphorus is a third-row element. Atoms of elements in the third row have a 3d subshell in their outermost shell. This
allows them to accommodate more than 8 electrons.

3. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08