Chemistry Chapter on Resonance Structures

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Questions and Answers

How many resonance structures can be drawn for the nitrate ion, NO2-?

2 (correct)

What is the bond order of the C-O bonds in the carbonate ion, CO32-?

1.67

1.33 (correct)

1.5

Which statement accurately describes resonance structures?

Structures with the same arrangement of atoms and electrons.

Structures with the same arrangement of atoms, but different arrangements of electrons. (correct)

Structures with different arrangements of atoms, but the same arrangement of electrons.

Structures with different arrangements of atoms and electrons.

What implication does resonance have on the stability of a molecule?

More equivalent structures lead to increased stability. Signup and view all the answers

What is a limitation of the Lewis model when describing resonance?

It considers individual structures as fully accurate. Signup and view all the answers

Which bond order corresponds with the structure of benzene due to resonance?

1.5 Signup and view all the answers

In carbon dioxide (CO2), which resonance structure is most representative of the actual molecule?

A structure with two double bonds. Signup and view all the answers

What does the concept of resonance help explain?

The delocalization of electrons in molecules. Signup and view all the answers

Which formula correctly calculates the formal charge of an atom?

<h1>valence e– - 1/2 (bond e–) - (lone pair e–)</h1> Signup and view all the answers

What does a nonzero formal charge on an atom indicate?

The atom has a deviation from its typical number of bonds. Signup and view all the answers

When determining formal charge, which electron count is compared with the number of valence electrons assigned in a molecule?

The number of electrons in the free neutral atom. Signup and view all the answers

What is the significance of putting a negative charge on the most electronegative atom in a Lewis structure?

It minimizes the formal charges within the structure. Signup and view all the answers

Which resonance form is considered the most important for the cyanate ion, OCN–?

The form with the least formal charges. Signup and view all the answers

What is a free radical?

An odd-electron molecule with an unpaired electron. Signup and view all the answers

Which of the following structures represents NNO in a Lewis diagram?

N=N=O Signup and view all the answers

Which aspect is NOT essential when predicting stability among resonance structures?

The overall charge of the molecule. Signup and view all the answers

Which compound shows an atom with an expanded octet?

XeF4 Signup and view all the answers

In Lewis structures, which process is used to complete octets around terminal atoms?

Use lone pairs around terminal atoms Signup and view all the answers

What do noble gases typically exhibit in terms of chemical reactivity?

They usually do not gain or lose electrons Signup and view all the answers

Which of the following statements is true about the Lewis model?

It works well with atoms like P and S Signup and view all the answers

What is generally observed about the ionization energy of noble gases as their atomic radius increases?

Ionization energy decreases Signup and view all the answers

Which of the following compounds is known to be stable at low temperatures?

HArF Signup and view all the answers

Why might the Lewis model of bonding be seen as limited?

It is not useful for describing ionic bonds Signup and view all the answers

Which compound has an atom that can participate in expanded octet bonding?

XeF4 Signup and view all the answers

Which atom can typically have less than an octet in its Lewis structure?

Boron Signup and view all the answers

What is a characteristic of atoms that can form expanded octets?

They can accommodate more than 8 electrons. Signup and view all the answers

In the Lewis structure for SF4, how would you recognize the need for an expanded octet?

There are fewer shared electrons than expected. Signup and view all the answers

What is the first step in the expanded octet Lewis structure recipe?

Count the valence electrons present. Signup and view all the answers

Why do atoms with a higher atomic number tend to form expanded octets?

They can hold more electrons due to larger atomic size. Signup and view all the answers

When calculating the difference between theoretical and present electrons, what is the next step after finding this difference?

Divide by two. Signup and view all the answers

Which of the following elements is most likely to exhibit an expanded octet?

Sulfur Signup and view all the answers

What effect does expanding the valence shell typically have on the central atom's formal charge?

It decreases the formal charge. Signup and view all the answers

Which statement best describes a limitation of Lewis structures?

They do not always provide a correct description of bonding. Signup and view all the answers

What does the steric number (SN) represent in the context of molecular geometry?

The number of electron groups around a central atom. Signup and view all the answers

Which factor is most important in determining the arrangement of electrons according to VSEPR theory?

Minimizing the repulsion between electron groups. Signup and view all the answers

Why might the Lewis structure not represent the bond angles correctly?

It shows a two-dimensional representation. Signup and view all the answers

What is the primary purpose of utilizing condensed and line structures in organic chemistry?

To simplify complex molecular representations. Signup and view all the answers

When determining molecular geometry, which step involves ignoring lone pairs?

Naming the final shape of the molecule. Signup and view all the answers

Which statement is an exception to the octet rule?

Some elements can accommodate more than eight electrons. Signup and view all the answers

What is the relationship between molecular shape and chemical/physical properties?

Molecular shape significantly influences properties. Signup and view all the answers

Which of the following is true about the arrangement of atoms in molecules?

Molecules can have multiple valid Lewis structures. Signup and view all the answers

What role does VSEPR theory play in molecular geometry?

It predicts the molecular shape based on electron repulsion. Signup and view all the answers

Study Notes

Resonance Structures

Resonance structures show different possible arrangements of electrons in a molecule.
They are not real, but rather ways of describing the delocalization of electrons.
The molecule is actually an average of all the resonance structures.
More equivalent resonance structures mean a more stable molecule (e.g. benzene).

Formal Charge

Formal Charge: The number of valence electrons for an atom in a Lewis structure minus the number of electrons assigned to it in the molecule.
- Formula: # valence e⁻ - 1/2 (bond e⁻) - (lone pair e⁻).
A nonzero formal charge indicates deviation from the atom's typical number of bonds.
Best Lewis structure: Fewest formal charges, negative charge on the most electronegative atom (when necessary).

Lewis Structures and Stability

Predicting Stability: Draw resonance structures for different arrangements of atoms to assess stability based on formal charge.
Less than Full Octets: Can occur for atoms like Be and B, which are small and not very electronegative.
Expanded Octets: Possible for atoms beyond the 2nd period (e.g. phosphorus, sulfur) as they have larger size and a more positive charge. Occurs with central atom having Z>12 bonded to strongly electronegative elements (F, O, Cl).
Recognizing Expanded Octets: If the number of bonding pairs is less than the number of connections needed through the Lewis structure recipe, expand the octet on the central atom.

Noble Gases

Group 18: Group of non-metals with very low boiling points and completely filled s and p subshells.
Traditionally considered unreactive due to complete outer shells.
Exceptions:
- Neil Bartlett discovered Xe compounds with halogens in 1962 (e.g. XeF2, XeF4, XeF6).
- KrF2 is the only stable compound of krypton.
- Ar forms HArF, stable only at very low temperatures.

Octet Rule Exceptions

The Octet Rule (atoms tend to gain, lose or share electrons to achieve eight valence electrons) has many exceptions.
Lewis structures are most useful for period 2 p-block elements (C, N, O, H) as they tend to follow the octet rule.

Molecular Shapes

Lewis structures do not show 3D geometry of molecules.
VSEPR (Valence Shell Electron Pair Repulsion) Theory: Predicts the arrangement of electrons around a central atom to minimize repulsions.
Steps for Determining Molecular Geometry:
1. Draw the Lewis structure.
2. Determine the steric number (SN) of the central atom (SN = # of electrons groups around an atom).
3. Arrange electron groups to minimize repulsions (electron group geometry).
4. Ignore lone pairs when naming the final shape of the molecule (molecular geometry).

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Description

Explore the concepts of resonance structures, formal charge, and Lewis structures in this informative quiz. Understand how electron arrangements affect molecular stability and learn to predict the best Lewis structures based on formal charges. Test your knowledge on these essential chemistry topics!