Exceptions to the Octet Rule

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Basic Concepts of Chemical Bonding > Exceptions to the Octet Rule
Exceptions to the Octet Rule
• The Incomplete Octet
• Odd-Electron Molecules
• The Expanded Octet
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Basic Concepts of Chemical Bonding > Exceptions to the Octet Rule
The Incomplete Octet
• The octet rule states that atoms with an atomic number below 20 tend to combine
in such a way that they each have eight electrons in their valence shells, giving
them the same electronic configuration as a noble gas.
• The two elements that most commonly fail to complete an octet are boron and
aluminum, both of which readily form compounds in which they have six valence
electrons, rather than the usual eight predicted by the octet rule.
• While molecules exist that contain atoms with fewer than eight electrons in their
valence shell, these compounds are often reactive and can react to form species
with eight valence electrons.For example, BF3 will readily bind a fluoride anion to
form the
BF4-
anion, in which boron follows the octet rule.
Carbon Dioxide
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Basic Concepts of Chemical Bonding > Exceptions to the Octet Rule
Odd-Electron Molecules
• While the majority of compounds formed from atoms below atomic number 20
follow the octet rule, there are many examples of compounds that do not follow
the octet rule.
• Having an odd number of electrons in a molecule guarantees that it does not
follow the octet rule, because the rule requires eight electrons (or two for
hydrogen) around each atom.
• The most commonly encountered stable species that exist with an odd number of
electrons are nitrogen oxides, such as nitric oxide, NO, and nitrogen dioxide, NO2,
both of which are free radicals and disobey the octet rule.
Nitric Oxide
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Basic Concepts of Chemical Bonding > Exceptions to the Octet Rule
The Expanded Octet
• Main group elements that form more bonds than would be predicted by the octet
rule are called hypervalent compounds, and have what is known as an expanded
octet (more than eight electrons around one atom).
• The octet rule can be 'expanded' by some elements by utilizing the d-orbitals
found in the third principal energy level and beyond.Sulfur, phosphorus, silicon,
and chlorine are common examples of elements forming an expanded octet.
• Phosphorus pentachloride, PCl5, and sulfur hexafluoride, SF6, are examples of
molecules that deviate from the octet rule by having more than 8 electrons around
the central atom (10 for P in PCl5 and 12 for S in SF6).
Phosphorus Pentachloride
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Appendix
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Basic Concepts of Chemical Bonding
Key terms
• atomic number The number, equal to the number of protons in an atom that determines its chemical properties.Symbol: Z
• expanded octet A case where an atom shares more than eight electrons with its bonding partners.
• free radical Any molecule, ion, or atom that has one or more unpaired electrons.They vary in their reactivity and stability: some
are highly reactive and often only occur as transient (i.e., short-lived) species, while others are metastable.
• hypervalent molecule A molecule containing an atom from a main group element which deviates from the octet rule by sharing
more than eight electrons.
• main group element Elements that are not part of the transition metal block in the periodic table.
• metastable Of or pertaining to a physical or chemical state that is relatively long-lived, but may decay to a lower energy state
when perturbed.
• octet rule Atoms lose, gain, or share electrons in order to have a full valence shell of eight electrons.Hydrogen is an exception
because it can hold a maximum of two electrons in its valence level.
• octet rule Atoms gain, lose, or share electrons with other atoms in order to fill their valence level with eight electrons.
• valence electrons The electrons in the outermost (valence) principal energy level of an atom that can participate in the
formation of chemical bonds with other atoms.
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Basic Concepts of Chemical Bonding
Nitrogen Dioxide
Nitrogen dioxide is another stable molecule that disobeys the octet rule.Note the seven electrons around nitrogen.Formal charges and the molecule's
resonance structures are indicated.
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Wikibooks. "File:Stickstoffdioxid.svg - Wikibooks, open books for an open world." CC BY-SA 3.0
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Basic Concepts of Chemical Bonding
Energies of the Highest Occupied Orbitals of the Elements
This diagram illustrates the Aufbau rules as they are applied to all the elements.Note especially how the energies of the n th d orbitals fall between the
(n+1) s and (n+1) p orbitals.For example, the 3d orbitals begin to fill after the 4s orbital is filled, but before electrons populate the 4p orbitals.A similar
relation exists with d- and f-orbitals.
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Basic Concepts of Chemical Bonding
Boron Trifluoride Lewis Structure
Each pair of dots represents a pair of electrons.When placed between two atoms, the electrons are in a bond.A bond can also be drawn as a line
between two atoms, also indicating two electrons.Notice that the central boron atom has only 6 electrons in the final Lewis diagram/structure of this
molecule.
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Basic Concepts of Chemical Bonding
Boron Trifluoride-Ammonia Complex
This covalent compound (NH3BF3) shows that boron can have an octet of electrons in its valence level.
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Basic Concepts of Chemical Bonding
Sulfur Hexafluoride
In the SF6 molecule, the central sulfur atom is bonded to six fluorine atoms, so sulfur has 12 bonding electrons around it.The overall geometry of the
molecule is depicted (tetragonal bipyramidal, or octahedral), and bond angles and lengths are highlighted.
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Basic Concepts of Chemical Bonding
Carbon Dioxide
A Lewis dot diagram for carbon dioxide.
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Basic Concepts of Chemical Bonding
Phosphorus Pentachloride
In the PCl5 molecule, the central phosphorus atom is bonded to five Cl atoms, thus having 10 bonding electrons and violating the octet rule.The overall
geometry of the molecule is depicted (trigonal bipyramidal), and bond angles and lengths are highlighted.
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Basic Concepts of Chemical Bonding
Nitric Oxide
Nitric oxide, NO, is an example of a stable free radical.It does not obey the octet rule on the nitrogen atom.Each line around the atoms represents a pair
of electrons.
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Basic Concepts of Chemical Bonding
Which of the following compounds illustrates a deviation from the
octet rule?
A) BF3
B) H2
C) All of these answers
D) AlCl3
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Basic Concepts of Chemical Bonding
Which of the following compounds illustrates a deviation from the
octet rule?
A) BF3
B) H2
C) All of these answers
D) AlCl3
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Basic Concepts of Chemical Bonding
Nitric oxide violates the octet rule because __________.
A) it has more than 8 electrons around the nitrogen atom
B) it forms a double bond instead of a single bond
C) both its atoms have negative formal charges
D) it has an unpaired electron
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Basic Concepts of Chemical Bonding
Nitric oxide violates the octet rule because __________.
A) it has more than 8 electrons around the nitrogen atom
B) it forms a double bond instead of a single bond
C) both its atoms have negative formal charges
D) it has an unpaired electron
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Basic Concepts of Chemical Bonding
C cannot have an expanded octet because ________.
A) it is a second period element
B) it has no d orbitals
C) Neither of these answers
D) Both of these answers
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Basic Concepts of Chemical Bonding
C cannot have an expanded octet because ________.
A) it is a second period element
B) it has no d orbitals
C) Neither of these answers
D) Both of these answers
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Basic Concepts of Chemical Bonding
Which of the following elements is most likely to form compounds
involving an expanded valence shell of electrons?
A) P
B) O
C) Na
D) N
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Basic Concepts of Chemical Bonding
Which of the following elements is most likely to form compounds
involving an expanded valence shell of electrons?
A) P
B) O
C) Na
D) N
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Basic Concepts of Chemical Bonding
Attribution
• Wikibooks. "General Chemistry/Octet Rule and Exceptions." CC BY-SA 3.0
http://en.wikibooks.org/wiki/General_Chemistry/Octet_Rule_and_Exceptions
• Wikipedia. "Octet rule." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Octet_rule
• Connexions. "Covalent Bonding and Electron Pair Sharing." CC BY 3.0
http://cnx.org/content/m12584/latest/?collection=col10264/latest
• Wikipedia. "valence electrons." CC BY-SA 3.0 http://en.wikipedia.org/wiki/valence+electrons
• Wiktionary. "atomic number." CC BY-SA 3.0 http://en.wiktionary.org/wiki/atomic+number
• Wikipedia. "Nitrogen dioxide." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Nitrogen_dioxide
• Wikipedia. "Nitric oxide." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Nitric_oxide
• Wikibooks. "General Chemistry/Octet Rule and Exceptions." CC BY-SA 3.0
http://en.wikibooks.org/wiki/General_Chemistry/Octet_Rule_and_Exceptions
• Wiktionary. "free radical." CC BY-SA 3.0 http://en.wiktionary.org/wiki/free+radical
• Wiktionary. "octet rule." CC BY-SA 3.0 http://en.wiktionary.org/wiki/octet+rule
• Wiktionary. "metastable." CC BY-SA 3.0 http://en.wiktionary.org/wiki/metastable
• Wikibooks. "General Chemistry/Octet Rule and Exceptions." CC BY-SA 3.0
http://en.wikibooks.org/wiki/General_Chemistry/Octet_Rule_and_Exceptions
• Wikipedia. "Octet rule." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Octet_rule
• Wikipedia. "Hypervalent molecule." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Hypervalent_molecule
• Steve Lower's Website. "Covalent bonding." CC BY-SA http://www.chem1.com/acad/webtext/chembond/cb03.html#COMP
• Wiktionary. "main group element." CC BY-SA 3.0 http://en.wiktionary.org/wiki/main+group+element
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