Chapter 9 - Molecular Geometry and Bonding Theories

Describe pi bonds

- A type of covalent bond - Overlap regions lie above and below the internuclear axis - There is no probability of electrons being found on the internuclear axis - Since the overlap is sideways, these bonds are slightly weaker

Describe how energy is related to hybrid orbital formation

- As the distance between two atoms decreases, the overlap between their orbitals decrease - Due to the resulting increase in electron density between the nuclei, the potential energy of the system decreases - Because potential energy of the system decreases, the strength of the bond increases, and it also becomes more stable However, at the same time: - As atoms come close together, the electrostatic repulsion between the nuclei, and therefore the energy, increases rapidly - Therefore, the observed bond length is the distance at which the attractive forces between unlike charges (electrons and nuclei) are balanced by repulsive forces between like charges (electronx2 and nucleus x2)

Describe hybridization involving d orbitals

- Atoms in the third period and beyond can use d orbitals to form hybrid orbitals - One s orbital, three p orbitals, and one d orbital leads to five sp3d hybrid orbitals (for trigonal bipyramidal geometry) - One s orbital, three p orbitals, and two d orbitals leads to six sp3d2 hybrid orbitals (for octahedral geometry)

Describe electron-domain geometry for 5 electron domains

- Creates a trigonal bipyramidal electron-domain geometry shape - There are two types of positions for the electron domains to point towards: axial and equatorial - 2 electron domains point towards axial positions (top and bottom) - 3 electron domains point towards a equatorial position (basically flat/planar) - Axial and equatorial domains have a 90 degree angle - Equatorial and equatorial domains have a 120 degree angle

Why do nonbonding pairs (of a trigonal bipyramidal; 5 electron domain geometry molecule) always occupy equatorial positions?

- Equatorial domains are 90 degrees away from 2 other domains (the two axial domains) - Axial domains are 90 degrees away from 3 other domains (three equatorial domains) - Therefore, equatorial domains experience less repulsion than axial domains - Since domains from nonbonding pairs exert larger repulsions than those from bonding pairs, they always occupy equatorial positions

Why orbitals hybridize (even though it requires energy)

- Hybrid orbitals have one large lobe: therefore, they can be directed at atoms more easily than unhybridized atomic orbitals - They can overlap more strongly with orbitals of other atoms (as compared to atomic orbitals), which creates stronger bonds - Although energy is required to promote electrons and hybridize orbitals, the energy released by the formation of bonds offsets this

How to determine if molecules/ions have delocalized bonding

- Molecules with two or more resonance structures - Molecules with pi bonding

Describe electron-domain geometry for 6 electron domains

- Octahedral (electron domain geometry) - Has 2 axial positions and 4 equatorial positions - All bond angles are 90 degrees - The 6 vertices are equivalent - Therefore, nonbonding domains can be placed at any of the vertices - 1 non-bonding domain: square pyramidal; the nonbonding domain is placed at any vertice - 2 non-bonding domains: square planar; the nonbonding domains are placed at opposite sides to reduce repulsions

Valence-bond theory

- The buildup of electron density between two nuclei is visualized as occurring when: a valence atomic orbital of one atom merges with that of another atom - The overlap then allows the orbitals to share a region of space - Orbital overlap allows two electrons of opposite spin to share the common space between the nuclei, forming a covalent bond

What different "things" can produce an electron domain?

1. A nonbonding pair of electrons 2. A single bond 3. Multiple bonds

Steps to predict hybrid orbitals of an atom in bonding

1. Draw Lewis structure 2. Determine electron domain geometry 3. Specify the hybrid orbitals needed to accomodate the electron pairs, based on their geometric arrangement

Steps of the VSEPR model to predict the shapes of molecules/ions

1. Draw the Lewis structure, and count the total number of electron domains around the central atom (each nonbonding domain, single bond, double bond, and triple bond) 2. Determine the electron domain geometry based on the number of electron domains 3. Use the arrangement of the bonded atoms to determine the molecular geometry

Describe how to determine pi bonds

1. Draw the lewis structure of a molecule, while determining the total number of electrons 2. Determine the hybridization of the central atoms, and draw out the hybrid orbitals (along with regular orbitals) 3. Add up the number of electrons accounted for. Then, the remaining electrons are added to unhybridized p orbitals, which then form pi bonds

In VSEPR, how do electron domain geometry and molecular geometry relate?

1. Electron domain geometry can be predicted: from knowing how many electron domains there are 2. Molecular geometry can be predicted: from knowing how many of these domains are due to nonbonding pairs

Summary of hybrid orbitals

1. Every bonded pair of atoms shares electrons. In each bond, at least one pair of electrons is localized in the space between atoms in a sigma bond. 2. The electrons in sigma bonds are localized between two bonded atoms 3. When atoms share more than one electron pair, one pair is used to form a sigma bond and the additional pairs form pi bonds 4. Molecules with multiple resonance structures can have pi bonds that extend over more than two bonded atoms, which are delocalized electrons

Describe pi and sigma bonds in ethylene, C2H4

1. Four C-H sigma bonds are formed by the overlap of sp2 hybrid orbitals and 1s hybrid orbitals 2. The C-C sigma bond is formed by the overlap of two sp2 hybrid orbitals 3. The remaining 2 electrons reside in the unhybridized 2p orbitals, with one electron on each carbon. These p orbitals overlap sideways above and below the axis, creating a pi bond.

How to determine if a molecule is dipole or nonpolar

1. Look at the individual bond dipoles 2. Look at the overall dipole moment of a molecule: by considering bond dipoles as vector quantities with direction and magnitude. The overall dipole moment is the vector sum of its bond dipoles. 3. The entire molecule must have a distinct positive and negative region, to be dipole aka polar

Describe sp hybridization

1. One s orbital and one p orbitals can be mixed to create two sp orbitals 2. the sp orbitals will have 2 lobes, with one lobe being larger than the other 3. The two new orbitals are identical in shape, but face opposite directions - Sp hybridization is created with linear electron domain arrangements

For a molecule consisting of 2+ atoms, what does the dipole moment depend on?

1. The polarities of the individual bonds 2. The geometry, or shape, of the molecule

Give the electron-domain geometry and bond angle for the following electron domains: 2, 3, 4, 5, 6

2: Linear; 180 3: Trigonal Planar; 120 4: Tetrahedral; 109.5 5: Trigonal bipyramidal; 120 and 90 6: Octahedral; 90

Pi bond

A bond that is created when two p orbitals overlap in a sideways fashion, and are oriented perpendicularly to the internuclear axis

Bond polarity

A measure of how equally the electrons in a bond are shared between two atoms of the bond. As electronegativity difference increases, so does bond polarity.

Which atoms on the periodic table may have more than an octet of electrons?

Atoms on the third period and beyond, as they have empty d orbitals that can be filled

How does the bond angle change as the number of nonbonding electron pairs increases? Why?

Bond angles decreases. This is because a bonding pair of electrons is attracted by both nuclei of bonded atoms. Nonbonding electrons are only attracted by one nucleus, meaning there is less attraction and its electron domain is spread out more into space.

Localized bonding

Bonding where sigma and pi electrons are completely associated with the two atoms that form the bond

Sigma bond

Bonds formed by head-on overlapping of atomic orbitals; the line joining the two nuclei passes through the middle of the overlap region and is along the internuclear axis

Lewis theory

Covalent bonding occurs when atoms share electrons, which causes electron density to concentrate between two nuclei

Given 2 electron domains, determine: electron-domain geometry, molecular geometry, and hybridization

Electron-domain geometry: Linear Molecular Geometry (2 bonding domains, 0 nonbonding domains): Linear Hybridization: sp

Given 6 electron domains, determine: electron-domain geometry, molecular geometry, and hybridization

Electron-domain geometry: Octahedral Molecular Geometry (6 bonding domains, 0 nonbonding domains): octahedral Molecular geometry (5 bonding domains, 1 nonbonding domain): Square pyramidal Molecular Geometry (4 bonding domains, 2 nonbonding domains): Square planar Hybridization: sp3d2

Given 4 electron domains, determine: electron-domain geometry, molecular geometry, and hybridization

Electron-domain geometry: Tetrahedral Molecular Geometry (4 bonding domains, 0 nonbonding domains): Tetrahedral Molecular Geometry (3 bonding domains, 0 nonbonding domains): Trigonal pyramidal Molecular Geometry (2 bonding domains, 2 nonbonding domains): Bent Hybridization: sp3

Given 3 electron domains, determine: electron-domain geometry, molecular geometry, and hybridization

Electron-domain geometry: Trigonal Planar Molecular Geometry (3 bonding domains, 0 nonbonding domains): Trigonal planar Molecular Geometry (2 bonding domains, 1 nonbonding domain): Bent Hybridization: sp2

Given 5 electron domains, determine: electron-domain geometry, molecular geometry, and hybridization

Electron-domain geometry: Trigonal bipyramidal Molecular Geometry (5 bonding domains, 0 nonbonding domains): trigonal bipyramidal Molecular geometry (4 bonding domains, 1 nonbonding domain): Seesaw Molecular Geometry (3 bonding domains, 2 nonbonding domains): T-shaped Molecular Geometry (2 bonding domains, 3 nonbonding domains): Linear Hybridization: sp3d

Electron domains for multiple bonds exert a ____ repulsive force on adjacent electron domains than do electron domains for single bonds.

Greater

Electron domains for nonbonding electron pairs exert ____ repulsive forces on adjacent electron domains, and tend to ___ bond angles.

Greater; compress

In what kinds of molecules is bonding sometimes not localized?

In molecules with 2 or more resonance structures, involving pi bonds

Describe how the VSEPR model can be extended to the shapes of larger molecules

Individual central atoms of a larger molecule can be looked at, their electron domain #'s determined, and therefore the electron domain geometry predicted as well

What does the number of hybrid orbitals created equal to?

It equals the number of mixed atomic orbitals

What are the two shape possibilities for AB2 molecules?

Linear (bond angle 180) or bent (bond angle not 180)

Five basic shapes of ABn molecules, from which most molecular shapes can be derived from

Linear, Trigonal Planar, Tetrahedral, Trigonal Bipyramidal, Octahedral

What is the size of multiple bond electron domains?

Multiple bonds have larger electron domains, as they contain more electrons

Describe sp3 hybridization

One s orbital and three p orbitals can be mixed to create four sp3 orbitals - Each sp3 orbital has a large lobe that points towards a vertex of a tetrahedron

Describe sp2 hybridization

One s orbital and two p orbitals can be mixed to create three sp2 orbitals - The sp2 orbitrals lie in the same plane, 120 degrees apart - Three sp2 orbitals are used to make three equivalent bonds - Used in trigonal planar electron-domain geometry

Hybrid orbitals

Orbitals created by combining two atomic orbitals, creating a shape unique from the original 2 orbitals

What is one positive and one negative of Lewis structures?

Positive: they help us understand the compositions of molecules, the number/types of bonds, etc. Negative: don't show the shape, size, bond angles, etc. of the molecule

Single bonds, double bonds, and triple bonds: sigma/pi bonds

Single bonds: one sigma bond Double bonds: one sigma, one pi bond Triple bonds: one sigma, two pi bonds

Bond angle

The angles made by the lines joining the nuclei of the atoms in a molecule

Electron domain geometry

The arrangement of electron domains around the central atom of a molecule/ion

Molecular geometry

The arrangement of only the atoms in a molecule or ion around the central atom (doesn't include nonbonding pairs)

What determines the shape and size of a molecule?

The bond angles and the bond lengths

How do bond angles change when surrounding atoms and electron domains aren't identical?

The bond angles deviate from the predicted bond angles

Delocalized bonding

The delocalization of electrons in pi bonds is when pi bonds are not individual electron pair bonds between neighboring atoms, and the electrons are seemingly shared between multiple atoms

When all electron domains in a molecule arise from bonds, what is the molecular geometry?

The molecular geometry equals the electron domain geometry

What is the best arrangement of electron domains?

The one that minimizes repulsions among them

Hybridization

The process of mixing atomic orbitals as atoms approach each other to form bonds

When some electron domains in a molecule arise from nonbonding pairs, how can molecular geometry be predicted?

These nonbonding pair electron domains must be ignored when predicting molecular shape/geometry

When are double and triple bonds (and hence pi bonds) more common?

They are more common in molecules made up of small atoms, especially C, N, and O

What do the shapes of ABn molecules/ions depend on?

They depend on the number of electron domains surrounding the central atom

Why are hybrid orbitals useful?

They help describe covalent bonding in molecules, and also relate to the electron domain geometries of the VSEPR model

What are the two electron domain geometries for molecules with 5 and 6 electron domains?

Trigonal bipyramidal (5 electron domains) and octahedral (6 electron domains)

What are three shape possibilities for AB3 molecules?

Trigonal planar, trigonal pyramidal, T-shaped

VSEPR

Valence Shell Electron Pair Repulsion; explains the different structures of molecules

When do orbitals that make up pi bonds achieve good overlaps?

When all of the atoms/molecules lay in the same plane (when the atoms/molecule are planar)

When do pi bonds form?

When unhybridized p orbitals are present on the bonded atoms (aka, atoms with sp or sp2 hybridization)

Bonding pair

a pair of electrons shared between two atoms

Electron domain

in the VSEPR model, a region about a central atom in which an electron pair can be found

Nonbonding pair

unshared pair of valence electrons in a molecule