experiment 6

BONDING AND MOLECULAR SHAPES

Experiment 6

THEORIES OF COVALENT BONDValence Bond Theory (VBT) • the build up of electron density between two nuclei is

visualized as occurring when a valence atomic orbital of one merges with that of another atom.

• covalent bond consists of a pair of electrons in atomic orbital

• can be described by the concept of hybridization• tries to explain the bonding process; merging of valence

atomic orbitals to form an overlap region; the overlap region can accommodate maximum of 2 electrons of opposite spins to form a covalent bond.

Valence-Bond Hybridized Orbitals

Hybridization• the process of mixing of atomic orbitals of the

same atom to form degenerate orbitals • the number of hybrid orbitals formed is equal

to the number of pure atomic orbitals that combine.

Types of hybrid orbitals:• sp hybrid orbital: mixing one s and one p orbitalConsider BeCl2:• the ground state orbital diagram for Be should be• but this indicates that Be does not form covalent bonds with Cl

since Be’s electrons are already paired• one of Be’s electrons must be promoted to 2p:

• Now there are two Be atoms for bonding, but this indicates that the two Be–Cl bonds are different since one forms from a 2s orbital and other from the 2p orbital.

• But experiments indicates that the two Be–Cl bonds are equivalent• the 2s and 2p orbital hybridizes to form two equivalent sp hybrid

orbital:

sp2 hybrid orbital: mixing one s and two p orbitals

Consider BF3:• the ground state orbital diagram for B should be:• and promoting one of B’s 2s electrons to 2p gives• Now there are three B orbitals for bonding, but this

indicates that two of the B–F bonds (from 2s) should be the same but one (from 2s) should be different.

• But experiment indicates that the three B–F bonds are equivalent

• the 2s and 2p orbital hybridizes to form three equivalent sp2 hybrid orbital:

sp3 hybrid orbitals: mixing one s and three p orbitals• Consider CH4. (Draw the ground state orbital

diagram and show the hybridization process)

• sp3d hybrid orbitals: mixing of one s orbital, three p orbitals, and one d orbital

• sp3d2 hybrid orbital: mixing of one s orbital, three p orbitals, and two d orbitals

Predicting what hybrid orbitals form:

• Draw Lewis structure to determine total number of bonds on central atom

• Given the Lewis structure, we can determine what hybrid orbitals must be involved by counting the number of bonds around each atom (counting multiple bonds as one) and number of lone pairs:

Number of bonds + Lone Pairs Hybrid Orbitals

2 sp

3 sp2

4 sp3

5 sp3d

6 sp3d2

Valence Shell Electron Pair Repulsion Theory

• “The BEST arrangement of a given number of e– pairs is the one that minimizes the repulsion among them.”

• “The valence electron pairs (electron domains) surrounding an atom repels one another so the orbitals containing those electrons are oriented as far apart as possible.”

• Types of valence electron pairs: – bonding pairs –shared electrons– nonbonding pairs – lone pairs

• Types of electron repulsions:– lone pair – lone pair (lp-lp)– lone pair – bonding pair (lp-bp)– bonding pair- bonding pair (bp-bp)

Rules on Electronic Repulsion

• Types of Repulsion– Bonding pair – bonding pair < lone pair – bonding

pair < lone pair – lone pair• The increasing size and lower electronegativity

of the central atom permits the lone pairs to be drawn out farther, thus decreasing repulsion between bonding pairs.– decreasing bond angle (dec electronic repulsion):

H2O > H2S > H2Se

• Repulsions exerted by bonding pairs decreases as the electronegativity of the bonded atom increases– decreasing bond angle (decreasing repulsion): H2O

> F2O

• Presence of multiple bonds increases repulsion between bonding pairs– decreasing bond angle: HC ≡ CH > H2C = CH2 > H3C

– CH3

• In trigonal bipyramidal– double bonds prefer equatorial position– lone pairs occupy equatorial position– least electronegative atoms prefer equatorial

position• In octahedron– Any lone pair occupies any position– If 2 lone pairs are present, it must be opposite to

each other

• Multiple bonds do not affect the gross geometry of the molecule because the geometry is primarily determine by the number of sigma bonds and lone pairs.

Terms:

• electron pair geometry – arrangement of electrons pairs around an atom

• molecular geometry – arrangement of atoms in space; may be predicted from the electron pair geometry

Steps in predicting the Molecular Geometry:

• Write down the Lewis structure.• Count the total number of electron pair

around the central atom. (Note: Double bonds and triple bonds counted as one electron pair.)

• Arrange electron pairs such that repulsion is minimized.

• Predict molecular geometry based on the arrangement of bonding pairs/atoms.

Note: • Nonbonding (lone pairs) electron pairs and

electrons in multiple bonds exert greater repulsive forces on adjacent electron pairs and thus, tend to compress the angles between bonding pairs.

• lone pairs occupy hybrid orbital• multiple bonds do not occupy hybrid orbital

Writing a Lewis Structure

1. Sum the valence electrons from all the atoms in the molecule.

• For polyatomic anions, add the number of negative charges to that total.

• For polyatomic cations, we subtract the number of positive charges from this total.


2. Choose the central atom and write the skeletal structure of the compound, using chemical symbols and placing bonded atoms next to one another.• Least electronegative atom occupies the central

position.• Hydrogen and fl uorine usually occupy the

terminal (end).


3. Draw a single covalent bond between the central atom and each of the surrounding atoms.• For every used electron, subtract it from the

total number of valence electrons.


4. Complete the octets of the the central atom.• For every used electron, subtract it from the

total number of valence electrons.


5. Distribute the remaining electrons in the surrounding atoms.• For every used electron, subtract it from the total

number of valence electrons.• Remember to satisfy the octet rule.• Carbon, Nitrogen, Oxygen and Fluorine (CNOF) will

never violate the octet rule, other elements can. -If an atom violates the OCTET Rule, it usually is the central atom.


6. if the atom has fewer than eight electrons (except H and He), try adding double or triple bonds between the surrounding atoms and the central atom.

experiment 6

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