Textbook Question
b. Which of these compounds or ions is an exception to the octet rule: nitrogen dioxide, borohydride (BH4−), borazine (B3N3H6 which is analogous to benzene with alternating B and N in the ring), or boron trichloride?
Ch.8 - Basic Concepts of Chemical Bonding
Chapter 8, Problem 66
Draw the Lewis structures for each of the following molecules or ions. Identify instances where the octet rule is not obeyed; state which atom in each compound does not follow the octet rule; and state how many electrons surround these atoms: a. NO, b. BF3, c. ICl2−, d. OPBr3 (the P is the central atom), e. XeF4.
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Draw the Lewis structure for NO: Start by counting the total number of valence electrons. Nitrogen has 5 valence electrons and oxygen has 6, totaling 11 electrons. Arrange the atoms with nitrogen and oxygen bonded together. Distribute the electrons to satisfy the octet rule as much as possible. Note that NO is a radical and does not fully obey the octet rule.
Draw the Lewis structure for BF_3: Boron has 3 valence electrons and each fluorine has 7, totaling 24 electrons. Arrange boron in the center with three fluorine atoms around it. Distribute the electrons to satisfy the octet rule for fluorine. Boron will have only 6 electrons, not obeying the octet rule.
Draw the Lewis structure for ICl_2^-: Iodine has 7 valence electrons, each chlorine has 7, and the extra negative charge adds 1 more electron, totaling 22 electrons. Place iodine in the center with two chlorine atoms and one lone pair around it. Distribute the electrons to satisfy the octet rule for chlorine. Iodine will have more than 8 electrons, exceeding the octet rule.
Draw the Lewis structure for OPBr_3: Oxygen has 6 valence electrons, phosphorus has 5, and each bromine has 7, totaling 40 electrons. Place phosphorus in the center with oxygen and three bromine atoms around it. Distribute the electrons to satisfy the octet rule for oxygen and bromine. Phosphorus will have more than 8 electrons, exceeding the octet rule.
Draw the Lewis structure for XeF_4: Xenon has 8 valence electrons and each fluorine has 7, totaling 36 electrons. Place xenon in the center with four fluorine atoms around it. Distribute the electrons to satisfy the octet rule for fluorine. Xenon will have more than 8 electrons, exceeding the octet rule.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Lewis Structures
Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They use dots to represent valence electrons and lines to indicate bonds between atoms. Understanding how to draw Lewis structures is essential for visualizing molecular geometry and predicting the behavior of molecules in chemical reactions.
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Lewis Dot Structures: Ions
Octet Rule
The octet rule is a chemical guideline stating that atoms tend to bond in such a way that they each have eight electrons in their valence shell, achieving a stable electron configuration similar to that of noble gases. While this rule applies to many main-group elements, there are exceptions, particularly for elements in the third period and beyond, which can accommodate more than eight electrons.
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Exceptions to the Octet Rule
Certain molecules and ions do not follow the octet rule, either because they have fewer than eight electrons (as in the case of some electron-deficient compounds) or because they can have expanded octets (as seen in elements like phosphorus and sulfur). Identifying these exceptions is crucial for accurately drawing Lewis structures and understanding the electronic structure of various compounds.
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Related Practice
Textbook Question
In the vapor phase, BeCl2 exists as a discrete molecule. (a) Draw the Lewis structure of this molecule, using only single bonds. Does this Lewis structure satisfy the octet rule?
Textbook Question
In the vapor phase, BeCl2 exists as a discrete molecule. (c) On the basis of the formal charges, which Lewis structure is expected to be dominant for BeCl2?
Textbook Question
(a) Describe the molecule xenon trioxide, XeO3, using four possible Lewis structures, one each with zero, one, two, or three Xe¬O double bonds. (b) Do any of these resonance structures satisfy the octet rule for every atom in the molecule? (c) Do any of the four Lewis structures have multiple resonance structures? If so, how many resonance structures do you find? (d) Which of the Lewis structures in part (a) yields the most favorable formal charges for the molecule?