# Chemical Bonding II Molecular Geometry and Valence Bond

Chemical Bonding II Molecular Geometry and Valence Bond / Molecular Orbital Theory Chapter 10 Molecular Shapes Sugars and sweeteners taste sweet because

they fit into spots on the tongue that trigger that response Artificial or no calorie sweeteners fit in the same place but do not get metabolized as quickly or at all meaning they pass through fast and do not absorb

VSEPR Theory The Valence Shell Electron Pair Repulsion (VSEPR) Theory States that electron pairs (defined as lone pairs or single, double, triple bonds, and even single e-s) repel one another and want to push as far away as possible

Affects the shape (molecular geometry) Shapes The preferred shape of a molecule is the one that allows maximum distance between electrons (bonding and lone pairs)

Depends on 1. How many come off the central atom(s) 2. How many lone pairs / bonding pairs TWO electron Groups LINEAR: Maximizes separations by 180 Draw out BeCl2

CO2 THREE electron Groups Trigonal planar geometry: maximum separation is ~120 Draw BF3

Draw CH2O Notice the bond angles (ok to round to 120 FOUR electron Groups Tetrahedron geometry maximum bond

angle is 109.5 Draw CH4 FIVE electron Groups Trigonal Bipyramidal Geometry 2 bond angles for maximum separation: 90 & 120

Draw out PCl5 Axial chlorines (top and bottom) vs the three equatorial Chlorines SIX electron Groups Six electron groups around a central atom

assume an octahedral geometry Draw (think about) SF6 Maximum bond angle is ALL 90 Practice Determine the molecular geometry of NO3 Draw it out first

Resonance ? How many bonding sites?? Three what does this tell about its shape? Bonded vs Un-bonded electrons The previous shapes were all bonded electron groups Think about what happens with un-bonded electron

pairs (lone electron pairs) VSEPR theory says these are also greatly (-) and will push away and repel from one another in a similar fashion Only they create different shapes because there is not an atom to see on the lone pairs Nonbonding electrons

still push (very, very negative) They still have repulsions and according to the VSEPR theory they will migrate as far away as possible to alleviate this

stress REPULSIONS Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair Most Repulsive Least Repulsive

Think about how this can affect bond angles The more repulsive (the greater they push away) the greater the bond angle FOUR electron groups with ONE LONE PAIR

Think about ammonia There are 4 electron groups (3 bonded and 1 lone) So, push them apart just like a tetrahedron but there is nobody bonded on one side This push creates a pyramidal shape (trigonal pyramidal)

Trigonal Pyramidal FOUR electron groups with TWO LONE PAIRS Think about Water There are 4 electron groups (2 bonded and 2 lone)

So, push them apart just like a tetrahedron but there is nobody bonded on two sides This push creates a BENT shape Bent Molecular Geometry

FIVE electron groups with ONE LONE PAIR Draw out / Consider SF4 In order to get these as far away as possible: SEESAW

Geometry FIVE electron groups with TWO LONE PAIRS Draw out / Consider BrF3 In order to get these as far away as possible:

T-SHAPED Geometry FIVE electron groups with THREE LONE PAIRS Draw out / Consider XeF2 In order to get these

as far away as possible: LINEAR Geometry SIX electron groups with ONE LONE PAIR

Think about BrF5 There are 6 electron groups (5 bonded and 1 lone) So, push them apart just like a octahedron but there is nobody bonded to one of the sides This push creates a SQUARE PYRAMIDAL MOLECULAR GEOMETRY

SIX electron groups with TWO LONE PAIRS Think about XeF4 There are 6 electron groups (4 bonded and 2 lone) So, push them apart just like a octahedron (AGAIN)

but there is nobody bonded to TWO of the sides This push creates a SQUARE PLANAR MOLECULAR GEOMETRY Summary of VSEPR Theory The geometry of a molecule is determined by the number of electron groups on the central atom(s)

The number of e- groups can be determined from the Lewis structure of the molecule. If the molecule has resonance structures, use one of them doesnt matter which one Each of the following counts: lone pair, single bond, double bond, triple bond, or a single electron

Summary (contd) The geometry of the e- groups is determined by minimizing their repulsions: Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair Bond angles will vary, the presence of lone pairs will usually make bond angles smaller

than the ideal angle of the particular geometry Page 414-415 Copy this table On a piece of Computer paper (provided)

Review sheet / Study guide / Summary for Molecular geometry Quick Practice

Which of the following statements is always true according to VSEPR theory? a) The shape of a molecule is determined by repulsions among bonding electron groups. b) The shape of a molecule is determined by repulsions among nonbonding electron groups c) The shape of a molecule is determined by the polarity

of its bonds d) The shape of a molecule is determined by repulsions among all electron groups on the central atom VSEPR Theory: Predicting Shapes 1. Draw the Lewis structure 2. Determine the total number of electron

groups around the central atom 3. Determine the number of bonding groups and the number of lone pairs around the central atom 4. Use Table 10.1 to determine the electron geometry and molecular geometry

Polarity As we discussed in chapter 9, bonds can be polar (unequal sharing of electrons, EN) Entire molecules can be POLAR!! Polarity Bonds are easy just look at the differences in

electronegativity For molecules, there must be a pull in different directions, but not equal and opposite WE STILL NEED TO USE OUT TABLE FROM CHAPTER 9!

Nonpolar Molecules Think about two kids playing tug-o-war If they are equal in strength, there will be no net dipole moment They cancel out Go nowhere

Polar Molecules Consider the same two kids pulling if now instead, one kid pulls up and one kid pulls to the left there WILL be a net

movement (they will not cancel out In summary, to determine whether a molecule is polar: Draw a Lewis structure, determine the molecular geometry

Determine whether the molecule contains polar bonds Determine whether the polar bonds add together to form a net dipole moment (add up vectors) what is a vector??? Vector Addition

Refer to page 420 for VECTOR ADDITION One dimension = two kids pulling left and right (left / left OR right/ right) Two dimensions = pulling up and to the right (in different directions, off of the same plane) Page 421:

Practice Determine whether NH3 is polar 1. Draw the Lewis Structure 2. Determine where the molecule has polar bonds 3. Determine whether the bonds add together to form a net dipole moment

The three dipole moments sum to a net dipole moment. The molecule is POLAR! PRACTICE #2 Determine whether CF4 is polar.

Interactions Nonpolar and Polar molecules DO NOT MIX Think about water (polar) and oil (nonpolar) They will separate and keep their distance Polar molecules interact STRONGLY with other polar molecules

So when you mix P and NP together, the P will group and bunch up while the NP stay away and will not penetrate the group Page 422 Chemistry in your day How Soap Works

Read through this excerpt and answer the question in your notes Consider the detergent molecule below (in your book). Which end do you think is polar? Which end is nonpolar?

Hybridization Hybridization is a mathematical preocedure in which the standard atomic orbitals are combined to form new atomic orbitals called Hybrid Orbitals It is essentially adding two orbitals together SP3 hybridization ___ ___ ___ ___

Pi and Sigma bonds A pi () bond) bond is the overlap between the halffilled p orbitals overlap side-by-side A sigma () bond) bond is the overlap end-to-end A pi () bond) bond is formed after the sigma bond

A sigma () bond) bond is always the FIRST BOND formed https://www.youtube.com/watch? v=YcSPPKESpwc Short unit

Molecular shapes through the pi / sigma bonds

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