Quiz 4 Study Guide - lectures and book notes for quiz 4 chem 1410 PDF

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lectures and book notes for quiz 4 chem 1410...

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Quiz 4 VSEPR: Valence Shell Electron Pair Repulsion Model      

Geometry is dependent on the accommodations of regions of electron density (or electron domains) about the central atom within a molecule The best geometry is that which minimizes electron to electron repulsions Any type of bond counts as a single electron domain Nonbonding pairs of electrons count as one electron domain The number of electron domains around an atom is that atom’s steric number Molecular geometry depends on o Number of electron groups around the central atom o How many of those electron groups are bonding or lone pairs

Steric Numbers 2 and 3  

#2- 2 electron domains, linear geometry, 180 degrees o Separation is maximized with a 180 degree bond angle, they repel each other #3- 3 electron domains, trigonal planar geometry, 120 degrees o Maximize separation is maximized with a 120 degree bond angles o BUT different types of electron groups exert slightly different repulsions resulting in small differences in bond angles

Steric Number 4: a 3-D Structure 

4 electron domains, tetrahedral geometry, 109.5 degrees

Steric Number 5: expanded octet    

5 electron domains, trigonal bipyramidal Angles are not all the same The angles between the equatorial positions ( the three bonds in the trigonal plane) are 120 degrees The angle between the axial positions (the two bonds on either side of the trigonal plane) and the trigonal plane is 90 degrees

Steric Number 6: expanded octet    

Six domains, octahedral geometry Four of the groups lie in a single plane There is one group above the plane, one below Angles are all 90 degrees

The effect of lone pairs 

Nonbonding electrons count as electron domains too

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Electron geometry and molecular geometry are different o Electron geometry- geometrical arrangement of the electron groups o Molecular geometry- geometrical arrangement of the atoms Lone pairs generally exert slightly greater repulsions than bonding electrons The lone pair occupies more of the angular space around a nucleus

Steric Number 3 

One lone pair, 2 bonding domains

Steric Number 4:  

One lone pair o 3 bonding domains, trigonal pyramid Two lone pairs o 2 bonding domains, bent

Steric Number 5:   

One lone pair, see saw Two lone pairs, T-shaped Three lone pairs, linear

Steric Number 6:  

One lone pair, square pyramidal Two lone pairs o Location matters here o First and second lone pairs should be as far apart as possible o

Square planar

Rules; 1. 2. 3. 4.

Draw Lewis Dot Structure Count electron domains Determine root geometry If molecule has lone pairs, choose location that minimizes lone-pair repulsions

Larger Molecules  

Draw Lewis Dot structure Determine local geometry about each molecule

Molecular Dipole Moments: molecular shape and polarity

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Bond polarity: if two bonded atoms have differing electronegativities, the bond between them has a dipole moment Molecular polarity: the sum of the dipole moments of all the bonds within a molecule gives the net dipole moment of the molecule Bond dipole moments may cancel each other out- nonpolar molecule The sum is a sum of vector quantities: quantities that have both magnitude and direction If the molecular geometry is such that the dipole moments of individual polar bonds sum together to a net dipole moment, then the molecule is polar

Generalities about molecular shape and polarity   

When there are no lone pairs on the central atom and all of the atoms bound to the central atom are identical, the sum of bond dipole moments cancel out: molecules are nonpolar If there are lone pairs on the central atom, the molecule will usually be polar If the atoms bound to the central atom differ from each other, the molecule will usually be polar

Ch. 10 Part 2 Molecular Orbital Theory   

In atoms, electrons occupy atomic orbitals. In molecules they occupy molecular orbitals. How do we construct these? Eventually, we will use linear combinations of atomic orbitals located on separate atoms to produce molecular orbitals that bind the molecule Find atomic orbitals of appropriate geometry

Valence Bond Theory  

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We construct molecular orbitals by considering the interaction between atomic orbitals existing on the two atoms If the interaction results in constructive interference between the two atomic orbitals, the molecule orbital formed will have enhanced electron density between the two atoms o This results in a stable electronic configuration we call a chemical bond The most effective bonds will form when the atomic orbitals are of appropriate geometry and energy

VBT: orbital overlap as a chemical bond    

When two atoms approach each other, the electrons and nucleus of one atom interact with those of the other atom In VBT, we calculate the effect of these interactions on the energies of the electrons in the atomic orbitals If the energy in the system is lowered because of the interactions, a chemical bond forms If the energy is raised a chemical bond does not form

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Interaction energy is usually calculated as a function of the internuclear distance between the two bonding atoms When the atoms are far apart, the interaction energy is nearly zero because the two atoms do not interact to any significant extent As the atoms get closer the interaction energy becomes negative If the atoms get too close, the interaction energy is nearly zero because the two atoms do not interact to any significant extent As the atoms get closer, the interaction energy becomes negative If the atoms get too close, the interaction energy begins to rise because of the mutual repulsion of the two positively charged nuclei Equilibrium point at a certain bond length General observation: the interaction energy is usually negative/ stabilizing when the interacting atomic orbitals contain a total of two electrons that can spin pair o When two atoms with half-filled orbitals approach each other, the half-filled orbitals overlap and the electrons occupying them align with opposite signs

VBT: Hybrid Atomic Orbitals- Atomic orbitals of appropriate geometry for VSEPR structure     

VSEPR does a good job of predicting the appropriate geometries of many molecules Often though, our known set of atomic orbitals (s, p, d) are NOT of appropriate geometry to create bonds that would give molecules their VSEPR structures Hybrid orbitals minimize the energy of the molecule by maximizing the orbital overlap in a bond Use hybrid orbitals: created by mixing standard atomic orbitals General rules: o Number of hybrid orbitals formed= number of orbitals mixed o The particular combinations of standard atomic orbitals added together determines the shapes and energies of the hybrid orbitals formed o The type of hybridization that occurs is the one that yields the lowest overall energy for the molecule

Linear Atomic orbitals  

We need 2 atomic orbitals on the central atom of appropriate geometry to create two molecular orbitals oriented at 180 degrees Sp

Atomic Orbitals for Trigonal Planar geometry  

Need three atomic orbitals to create three molecular orbitals at 120 degrees If we take linear combinations of three of our existing atomic orbitals: one s, two p’s, we can created 3 hybrid orbitals of appropriate symmetry, sp2

Atomic Orbitals for Tetrahedral Geometry

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Take linear combination of four of our existing atomic orbitals- one s and three p’s

Hybrid atomic orbitals for expanded octects  

Trigonal bipyramidal structures: sp3d hybrid orbitals Octahedral- sp3d2 hybrid orbitals

VBT and Multiple Bonds      

Our concept of bonding when atomic orbitals oriented along a bonding axis have a significant degree of overlap A new type of bond: overlap of orbitals whose orientation is orthogonal (perpendicular) to the bonding axis is also possible Multiple bonds between two atoms are always made up of one sigma bond and one or more pi bonds The geometry about the atoms dictates the type of hybrid orbitals used in forming the sigma bond The pi bond is formed from overlap of unhybridized p orbital A consequence of pi bonds: rotation about the bond is hindered o Free rotation about a single bond (sigma) o Rotation restricted by double bond (sigma and pi), rotation would disrupt degree of overlap

Determining VBT Bonding scheme for a molecule 1. Draw the Lewis Dot Structure and determine geometry with VSEPR 2. Use geometry to determine the hybrid orbitals used in the sigma bond framework Molecular Orbital Theory 

VBT describes boding MO’s that are formed from the overlap of AO’s located on the bonded atoms

Linear combination of Atomic Orbitals        

Weighted linear sum of valence atomic orbitals of the atoms in a molecule The bonding orbital is lower in energy than either of the two atomic orbitals that formed it A molecule has more than one molecular orbital. The next molecular orbital is approximated by making one of the atomic orbitals negative. Electrons in antibonding orbitals have higher energies than they did in their respective atomic orbitals 1s+1s= constructive interference= bonding 1s-1s= destructive interference=antibonding The bonding orbital has an increased electron density in the internuclear region Anti-bonding orbital has a node in the internuclear region

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Bond order= ½[#e- in bonding MO’s-#e- in antibonding MO’s] The higher the bond order, the stronger the bond A negative or zero bond order indicates that a bond WILL NOT form between the atoms

The Homonuclear Diatomics      

H2, He2, Li2, Be2, B2, C2, N2, O2, F2, Ne2 We will use energy diagrams to schematically represent the energies of the MOs relative to the AOs used in their creation MOs from compos of p orbitals that are aligned with the bonding axis leads to sigma type bonding and antibonding orbitals Combinations of p orbitals that are perpendicular to the bonding axis leads to pi type bonding and antibonding Paramagnetic- atoms with unpaired electrons Diamagnetic- atoms with paired electrons

MOs for heteronuclear diatomics 

The energies of the more electronegative atom is slightly lower

Delocalized Orbitals in MO Theory MO Theory and Resonance MO Theory and Spectroscopy   

Atomic spectroscopy- absorption of light by atoms Molecular spectroscopy- absorption of light by molecules Conjugated polyenes- organic molecules with alternating double and single bonds...