Chemistry Chapter 3.8: Octet Rule Exceptions

Questions and Answers

What are the 3 exceptions to the octet rule?

The species has an odd number of electrons 2. Incomplete octets- when there are too few valence electrons 3. Expanded octets- when the central atoms can have 10, or even 12 electrons (if it is in the 3rd row or lower)

For exception 1, (odd number of electrons), what must there end up being in the Lewis structure?

A lone electron, which is an unpaired electron

Where is a lone electron usually placed in a Lewis structure?

It is placed such that the structure will have the lowest/most appropriate set of formal charges

What is formal charge?

The charge an element would have in a molecular compound if all electrons were shared equally Signup and view all the answers

Is it common to have molecules with an odd number of electrons?

False Signup and view all the answers

What is a free radical?

A highly reactive species with an odd number of electrons Signup and view all the answers

How are free radicals usually shown?

By placing a dot in front of the chemical symbol Signup and view all the answers

Give an example of when exception 2 (incomplete octet due to too few valence electrons) occurs?

BF3 Signup and view all the answers

Explain when exception 3 occurs.

Exception 3: expanded octets generally occur when very electronegative terminal atoms are involved and a non-metal central atom found in period 3 or below. Signup and view all the answers

What is the octet rule based on?

The space available in ns and np orbitals Signup and view all the answers

What allows an expanded octet to occur at the 3rd period?

Beginning with principal quantum number n=3, an empty d subshell becomes available to accommodate extra electrons. Signup and view all the answers

What is the relationship between the size of the central atom and expanded octets?

The larger the central atom is, the larger the number of electrons which can surround it is (more room) Signup and view all the answers

When do expanded octets generally occur?

When a central atom is bonded to small, very electronegative elements (F, Cl, O) Signup and view all the answers

In cases where an octet is possible, is it plausible that expanded octets are actually more favorable?

True Signup and view all the answers

Study Notes

Exceptions to the Octet Rule

Exceptions to the octet rule include species with an odd number of electrons, incomplete octets, and expanded octets.
Odd-electron species result in a lone unpaired electron in Lewis structures.
Incomplete octets occur when there are too few valence electrons, exemplified by BF3.
Expanded octets can have central atoms with 10 or even 12 electrons, usually involving electronegative terminal atoms or non-metals in periods 3 or below.

Lone Electrons and Formal Charge

A lone electron is strategically placed in a Lewis structure to minimize formal charges.
Formal charge reflects the hypothetical charge of an element if electrons are equally shared in a compound.
Molecules with odd numbers of electrons, or free radicals, are typically reactive, seeking to pair their unpaired electrons.

Free Radicals

Free radicals are characterized as reactive species with an odd number of electrons.
They are generally depicted by placing a dot next to the chemical symbol in structural representations.

Octet Rule Principles

The octet rule is grounded in the spatial arrangement available in ns and np orbitals.
For period 3 elements and beyond, an empty d subshell becomes accessible, accommodating expanded octets.

Central Atom Size and Electron Capacity

A larger central atom allows for a greater number of surrounding electrons, promoting expanded octets.
Expanded octets are common when central atoms are connected to smaller, highly electronegative atoms, such as F, Cl, and O.

Favorability of Expanded Octets

In scenarios where octets are feasible, expanded octets might be favored to achieve optimal formal charges, highlighting the importance of considering formal charges during structure evaluation.

Description

Explore the exceptions to the octet rule in this flashcard quiz. Understand the concepts of odd electron species, incomplete octets, and expanded octets. Perfect for mastering key chemistry principles related to molecular structure.