(a) Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride?

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Understand the octet rule: Atoms tend to form compounds in ways that give them eight valence electrons, resembling the electron configuration of a noble gas.

Identify the compounds: Carbon dioxide (CO₂), water (H₂O), ammonia (NH₃), phosphorus trifluoride (PF₃), and arsenic pentafluoride (AsF₅).

Analyze each compound: Determine the central atom and count the number of valence electrons around it after forming bonds.

Check for exceptions: The octet rule exceptions include molecules with an odd number of electrons, molecules in which one or more atoms possess more or fewer than eight electrons, and molecules with central atoms from the third period or beyond that can have expanded octets.

Identify the exception: Compare the electron configurations of each compound's central atom to see which one does not follow the octet rule, focusing on the possibility of expanded octets in elements like phosphorus or arsenic.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Octet Rule

The octet rule is a chemical guideline that states 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. This rule primarily applies to main group elements and helps predict the bonding behavior of atoms in molecules.

Exceptions to the Octet Rule

Certain elements, particularly those in the third period and beyond, can expand their valence shell to accommodate more than eight electrons. This phenomenon occurs due to the availability of d-orbitals, allowing elements like phosphorus and arsenic to form compounds with more than four bonds, thus violating the octet rule.

Molecular Geometry and Bonding

The molecular geometry of a compound influences its bonding and the number of electrons around the central atom. Understanding the shapes and angles formed by the bonds helps in identifying whether a compound adheres to the octet rule or if it exhibits exceptions, as seen in compounds like phosphorus trifluoride and arsenic pentafluoride.

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Related Practice

Textbook Question

Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:(a) Draw all of the resonance structures of naphthalene. How many are there?

Textbook Question

Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:

(b) Do you expect the C—C bond lengths in the molecule to be similar to those of C—C single bonds, C ═C double bonds, or intermediate between C—C single and C ═C double bonds?

Textbook Question

Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:

(c) Not all of the C—C bond lengths in naphthalene are equivalent. Based on your resonance structures, how many C—C bonds in the molecule do you expect to be shorter than the others?

Textbook Question

(b) Which of these compounds or ions is an exception to the octet rule: borohydride (BH4-), borazine (B3N3H6, which is analogous to benzene with alternating B and N in the ring), or boron trichloride?

Textbook Question

For Group 13–17 elements in the third row of the periodic tableand beyond, the octet rule is often not obeyed. A friendof yours says this is because these heavier elements are morelikely to make double or triple bonds. Another friend ofyours says that this is because the heavier elements are largerand can make bonds to more than four atoms at a time.Which friend is more correct?

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