Chapter 7 Covalent Bonds and Molecular Architecture

Octet rule: Main group elements tend to undergo reactions that leaves them with either 2 or 8 electrons in their outer or valence shell achieved by sharing electrons. Exceptions to the rule includes all elements that have d levels close in energy. However, much of their chemistry can still be explained by the octet rule. It is important to realize that this is a model that can also help us understand molecular structure.

Main Group Elements

Covalent bonding and Lewis structures If H would form a covalent bond, how many bonds would it form? Octet rule: Elements tend to undergo reactions to form covalent bonds that leaves them with either 2 or 8 electrons in their outer or valence shell. Valence electrons are the electrons with the highest principle quantum number.

molecular orbitals atomic orbitals H2 has an electronic environment similar to He One way chemists have of indicating a sharing of two electrons is to use a line to connect the two atoms, H2 is drawn H-H or H:H. These are called Lewis structures.

If Cl would form a covalent bond with H, how many bonds would it form? chlorine is 1 electron short of a filled 3p level hydrogen is 1 electron short of a filled 1s level How many valence electrons does each atom posses? (valence electrons are the electrons with the highest principle quantum number) How many electrons does H need to share to have an outer shell that resembles an inert gas? Which inert gas? How many does Cl need to share to have an outer shell that resembles an inert gas? Which inert gas? H Cl

How many valence electrons does C posses? If C forms covalent bonds, how many bonds would it form to satisfy the octet rule? If H would form a covalent bond with C, how many bonds would it form? 4 4 H H-C-H Lewis structure

How many valence electrons does N posses? If N forms covalent bonds with H, how many bonds would it form to satisfy the octet rule? If H would form a covalent bond with N, how many bonds would it form? H-N-H H 5 3 1

6 How many valence electrons does O possess? If O forms covalent bonds with H, how many bonds would it form to satisfy the octet rule? If H would form a covalent bond with O, how many bonds would it form? 2 1 H ׀ :O:

If B would form a covalent bond, how many bonds would it form? If H would form a covalent bond with B, how many bonds would it form? How many valence electrons does each atom posses? How many does B need to share to have an outer shell that resembles an inert gas? Which inert gas? Boron is a hopeless mess; much of its chemistry is very different from other elements in that the only way it can obtain an octet of electrons is by forming charged complexes; similarly with aluminum H-B-H ׀ H H -1 ׀ Na+ H-B-H H

Geometry of molecules Suppose two groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion ( how would they arrange themselves to minimize repulsion)?

Suppose two groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion ( how would they arrange themselves to minimize repulsion)?

Suppose three groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion?

Suppose three groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion? .

Suppose four groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion?

Suppose four groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion? .

Suppose four groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion? .

Suppose five groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion?

Suppose five groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion?

Suppose five groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion?

Suppose six groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion?

Suppose six groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion?

Suppose six groups were attached at a single point in space, and suppose these groups repelled each other, what geometric arrangement would these group chose to minimize their repulsion? octahedral geometry http://www.jcrystal.com/steffenweber/POLYHEDRA/p_10.html

Let’s define a group as either an atom or a pair of valence electrons not involved in bonding, ignore bonding electrons and electrons in inner shells, draw the Lewis structures, and predict the geometry of the following molecules H2O What is the central atom? How many valence electrons around O? How many groups around the central atom? O . 6 :O: 4 What is the geometry of the molecule?

The geometry of a molecule is determined only by location of the nuclei. The electrons can not be located because of the uncertainty principle

CCl4; CH4 What is the central atom? How many valence electrons around C? What is the Lewis structure? How many groups around the central atom? What is the geometry of the molecule? C 4 4 tetrahedral

NH3 What is the central atom? How many valence electrons around N? What is the Lewis structure? How many groups around the central atom? What is the geometry of the molecule? N 5 4 pyramidal

Draw Lewis structures and predict the shape of the following compounds: 1. SiCl4 2. CH5N 3. CH2O 4. C2H2Cl2 5. C3H4

PCl5 What is the central atom? SF6 Remember that we can only locate the position of a heavy atom; the position of electrons is not determined. P S

We have partially explained the geometry observed when atoms combine to form molecules. The geometry of the molecule is determined locally by the central atom. How do we identify central atoms? Central atoms are determined by the number of bonds needed to complete the octet. H, Halogens are seldom central atoms B, C, N O in the first row Al, Si, P, S in the second row …

Shapes of molecules linear trigonal planar bent tetrahedral trigonal pyramidal trigonal bipyramidal seesaw T shaped octahedral square pyramidal square planar http://intro.chem.okstate.edu/1314F00/Lecture/Chapter10/VSEPR.html

What experimental evidense is there that CH4 is tetrahedral and not square planar? Consider CH2Cl2: H Cl H C Cl H C H Cl Cl No Are there two compounds with the formula CH2Cl2?

Consider CHBrClF Are these the same?

Consider CHBrClF Are these the same?

Consider CHBrClF Are these the same?

You recall that we were able to explain atomic structure using s, p , d, f orbitals. Can we explain the structure of molecules using these same orbitals? Remember the shape of these orbitals: s: spherically symmetric p: 3 orbitals each with two lobes touching each other at the nucleus and oriented 90 ° to each other. d: 5 orbitals with a more complicated structure.

d s p

Chemists like to think that the electrons are the glue that hold atoms together. Therefore, can the structure of the molecules we just described, for example, CH4, PCl5, be explained using the hydrogen atom atomic orbitals just shown?

Schroedinger Equation is a differential equation. : Properties of a differential equation: 1. the equation may have more than one solution. 2. any combination of solutions (sum or difference) is also a solution 2s = 1/4(1/2a3).5(2-r/a)(2.718)r/2a 2p = 1/4(1/2a3).5(r/a)(2.718)r/2acos  … Linus Pauling: hybridization of atomic orbitals

What were to happen if we combined ½ of a 2s orbital with one of the ½ 2p orbitals mathematically? These hybrid orbitals are directional, pointing 180° away from each other and are called sp hybrid orbitals 2 s 2 p a 2s + b 2p - + + - + a 2s - b 2p - + + - +

Combining a 2s orbital with 2 2p orbital can result in 3 sp2 hybrid orbital that point at 120 ° to each other

Combining a 2s orbital with 2 2p orbital results in 3 sp2 hybrid orbital that point at 120 ° to each other; note that one p orbital remains unchanged by these mathematics.

A summary of the types of hybridization necessary to product maximum electron density in the necessary direction as dictated by experimental geometries sp hybridization: 2 orbitals pointing 180 ° to each other; 2 atomic p orbitals remain unchanged sp2 hybridization: 3 orbitals pointing 120 ° to each other; 1 atomic p orbitals remains unchanged sp3 hybridization: 4 orbitals pointing to the corners of a regular tetrahedron; all atomic p orbitals used dsp2 hybridization: 4 orbitals pointing to the corners of a square; 4 d orbitals, 1 p orbital unchanged dsp3 hybridization: 5 orbitals pointing to the corners of a trigonal bipramid; 4 d orbitals unchanged d2sp3 hybridization: 6 orbitals pointing to the corners of a octahedron; 3d orbitals unchanged

sp2 d2sp3 Shapes of the hybrid orbitals sp3

Draw the Lewis structure of C2H4 so that every carbon has a filled octet and each hydrogen has a He configuration How many groups around each carbon? 3 What is the geometry at each carbon? trigonal

Atomic p orbitals on each C sp2 hybrid orbitals

Draw the Lewis structure of C2H2 so that every carbon has a filled octet and each hydrogen has a He configuration How many groups around each carbon? 2 What is the geometry about each carbon? digonal: 180 °

H-CC-H

Bond lengths C-C 1.54 *10-10 m C=C 1.34*10-10 m CC 1.2* 10-10 m

How do we explain the formation of CH4 using the electronic configuration of C Valence electrons of C _____ _____ _____2p ____ 2s promote 1 electron: requires investment of energy hybridize add electrons from H: allows the formation of 4 bonds instead of 2 and satisfies the octet rule

How do we explain the structure of molecules such as SF4? promote an electron 3d ____ _____ _____ ______ ____ 3p ___ ____ ____ 3s ____ hybridize 3d _____ _____ ______ ____ ____ ___ ____ ____ _____ dsp3 add electron from F

How do we explain the structure of molecules such as SF6? promote 2 electrons 3d ____ _____ _____ ______ ____ 3p ___ ____ ____ 3s ____ hybridize add electron from F 3d _____ _____ ______ ____ ___ ____ ____ _____ _____

How good are Lewis structures at explaining molecular properties? For compounds of carbon: excellent very few exceptions For other elements: very good, some exceptions Consider the Lewis structures of the diatomic molecules of the elements H2, O2 N2 F2 Any exceptions?

How good is this model at explaining molecular properties? For compounds of carbon: excellent very few exceptions For other elements: very good, some exceptions Consider the Lewis structures of the diatomic molecules of the elements H2, O2 N2 F2 Any exceptions? O2 is paramagnetic

Molecular orbital Theory Basic tenets: molecules are formed by combining atomic orbitals on each atom that have the proper orientation. Whenever two atomic orbitals combine to form a molecular orbital, one combination is obtained by mathemetically adding the two together; this orbital goes down in energy relative to the atomic level. The other, obtained by the mathematical difference between the two orbitals, goes up in energy. If a level goes down in energy relative to the atomic level, it is referred to as a bonding molecular orbital If a level goes up in energy relative to the atomic level, it is called an antibonding level If a level is not affected relative to the atomic level it is called a non-bonding orbital

H2

This model predicts that H2- should have some stability; the total energy of He2 is the same as two isolated He atoms so nothing keeps the molecule together and it falls apart in two He atoms.

O2

F2

N2

Draw the Lewis structure of ozone: O3

Draw the Lewis structure of ozone - : :O: : :O: :O: : +

Resonance structures: two structures that are identical except for the location of the electrons Symbol chemists use to denote resonance structures

Draw the structure of sulfuric acid; H2SO4 Draw the structure of the sulfate ion; SO4-2

Why do covalent bonds form? Covalent bonds: the sharing of electrons

Electronegativity: generally meant to identify the unevenness in sharing electrons in a covalent bond