AP Chemistry Name __________________________ Ms. Ye Date ________________ Block ____

Molecular Shapes and VSEPR (Valence Shell Electron Pair Repulsion Theory) Go to bit.ly/VSEPRshapes Introduction Atoms bond to satisfy their need for more electrons. If both atoms have high electronegativities (are nonmetals), atoms will share electrons to satisfy the Octet Rule – every atom wants 8 electrons to fill the s and p orbitals in the outer energy level. But if the electronegativities are high enough and both atoms unwilling to give up electrons, sometimes atoms can deviate from and not follow) the Octet Rule. Because electrons have a negative charge and atoms occupy space, bonds and electrons will spread out as much as possible. Since we write in a two dimensional plane on paper, it is difficult to visualize the true geometry of these molecules. This activity and the program you are about to use allows us to visualize on a more 3-dimensional scale. Part 1 – Generic Molecules Use the following key to build your molecules:

A – central purple atom – cannot be removed

B – single bonded white atom

C – double bonded white atom

D – triple bonded white atom

E – Electron pairs not bonded Fill in the chart by creating the generic molecules. On your screen in the lower left corner, click on “molecule geometry.” Add atoms and electron pairs as needed to produce the generic formula. Once the molecule is assembled, click and drag the screen to spin the atom around. Click on the “Show bond angles.” In each box:

1. Draw the molecule you create in 3D to the best of your ability 2. Write the Molecule Geometry (MG) name in the box 3. Label the bond angles

Molecule 3D Drawing

DAB MG:

AB3 MG:

B2AC MG:

AB4 MG:

AB3E MG:

AB2E2 MG:

Molecule 3D Drawing

AB5 MG:

AB4E MG:

AB3E2 MG:

AB6 MG:

AB5E MG:

AB4E2 MG:

Part 2 – Real Molecules Click on the “Real Molecules” tab at the top of the page. Using the pull down menu, select the molecules below and fill in the chart. Match the molecule to the generic structure above. Fill in the generic bond angles.

Molecule Generic Formula Generic bond angles (from part 1)

True Bond Angles

H2O

CO2

CH4

NH3

BF3

Summary: 1. Looking at the real molecules in Part 2, some of the angles stayed consistent with the

generic bond angles while others did not. Compare and contrast the two groups of molecules (those with matching angle measurements to those with different measurements). What do you think is causing the angles to skew? Explain why this might be.

2. What angle is needed to spread 4 bonds as far apart as possible? Hint: look at a molecule

with four separate bonds.

Covalent Compounds & Lewis Structures

Forming Bonds

Bond= a region that forms when ____________________ from _______________________ interact with each other o The attraction between 2 or more atoms allows for the formation of a compound. o Only ______________________ electrons participate in bonding

Octet Rule o Atoms bond in order to get a full valence shell, or in general _______________________ o Exceptions: Hydrogen will bond and be happy with ___________ valence electrons o Boron will bond and be happy with __________ valence electrons

Covalent Bonds

A covalent compound is one that is made up of ___________________________________

A covalent bond involves the ________________________ of electrons Lewis Dot Structures o Show just the ________________________ electrons of an atom *Remember: _________________________________ the element is in gives you information about the _______________________________ it has! HONC Rule:

Bonds 1 Hydrogen atom needs to form in order to achieve a full valence shell: _____

Bonds 1 Oxygen atom needs to form in order to achieve a full valence shell: _______

Bonds 1 Nitrogen atom needs to form in order to achieve a full valence shell: ______

Bonds 1 Carbon atom needs to form in order to achieve a full valence shell: ______

Bonds 1 halogen atom needs to form in order to achieve a full valence shell:_______ Fill in the following blanks with the terms “single”, “double” or “triple” bond

When only 1 pair of electrons is shared between atoms (one dash), it is known as a ______

When 2 pairs of electrons are shared between atoms, it is known as a _________________

When 3 pairs of electrons are shared between atoms, it is known as a _________________

Exceptions to the Octet Rule: Expanded Octet

“Expanded octet” refers to the Lewis structures where the central atom ends up with more

than an octet. Expanded octets generally occur when there are _______________________

to fit in.

The central atom with an expanded octet MUST have an ____________________________

(beyond neon). In the discussion of hybridization, we see that when there are 5 or 6

“groups” on the central atom, ______________________________. This cannot occur with

elements that do not contain d-orbitals!

Extra electrons should be first placed on the outside atoms.

After the outside atoms have fulfilled the Octet Rule, and there are still extra electrons,

start with placing them as lone pairs on the central atom. If the central atom has a

positive charge should you move a lone pair from the outside atoms to share.

Hybrid Orbitals

To explain molecular geometries, we can

assume that the atomic orbitals on an atom

(usually the central atom) mix to form new

orbitals called hybrid orbitals.

Orbital diagram for a ground state boron atom

1s 2s 2p

The 2s orbital and two of the 2p orbitals are hybridized together to generate _____________

__________________________________________________________________________

that can each overlap with the orbitals of another atom and form covalent bonds.

Orbital diagram for a ground state carbon atom:

1s 2s 2p

The 2s orbital and the 2p orbitals are

hybridized together to generate ________

___________________________________

that can each overlap with the orbitals of

another atom and form covalent bonds.

The orbital diagram for a ground state

beryllium atom is:

1s 2s 2p

The Be atom in its ground state cannot bond with the fluorine atoms because it has no

unpaired electrons. The Be atom could form two bonds by promoting one of the 2s

electrons to a 2p orbital:

1s 2s 2p

The 2s orbital and the 2p orbital are hybridized together to generate ___________________

_____________________________________________________. These sp hybrid orbitals

can each overlap with the orbitals of another atom to form covalent bonds.

Hybridization: ________________________________

____________________________________________________

The __________________________ is determined by the

_________________________________________________

The _________________________ is determined by the

_________________________________________________

# Sets Orbital Hybridization

2

3

4

5

6

Sets= # things (atoms and lone

pairs) around central atom

A = X= E=

Shapes of Larger Molecules

Previously, only focused on a single central atom, the VSEPR model can be extend for more complex molecules such as acetic acid.

Application: Based on the molecule shown below and your knowledge of the VSEPR model, predict the bond angle for each of the following bonds.

Number of electron domains

Electron domain geometry

Predicted bond angles

Multiple Bonds Sigma Bonds (σ):

a bond formed _____________

2 elements

orbitals come together head on

Pi Bonds (π):

a bond formed

___________________ elements

orbitals come together side by side

In almost all cases, o Single bonds consist of bonds. o A double bond consists of bond and bond. o A triple bond consists of bond and bonds.

Label the bonds on the diagrams of ethylene (C2H4) and acetylene (C2H2).

For molecules that contain two or more resonance structures involving bonds, we cannot describe the bonds as individual bonds between neighboring atoms. Therefore we say that the bonds are delocalized among the atoms.

Energy Associated with Bonding

Atoms with a full valence shell are stable (“happy”) and are in a low energy state.

Breaking Bonds (NOT spontaneous): Energy is _____________________________

o Go from ____________ energy (“happy” atoms) to ______________ energy (“unhappy”

atoms)

o Ripping two atoms apart _____________________________ =

_________________________

SPONTANEOUS Bond formation = Energy is ___________________________________

Go from ___________ energy (“unhappy” atoms) to ____________ energy (“happy”

atoms)

Forming a bond is _______________________________

Energy Associated with Bonding

Atoms bonded together exist at the ________________________ energy possible

Bond energy- The energy needed to ______________________________

Bond length- _____________________between two bonded atoms to give the lowest energy

possible

The _____________________the bond length the ___________________ the bond energy

The ____________ pi bonds the ____________________________ the bond (more energy)

The ______________________ bonds between atoms (ie single bond vs double bond) the

__________________ the bond length

Example: Draw the Lewis structure for C2H6 and C2H2. Indicate which has the longest C-C bond and which has the strongest C-C bond.

C2H6

C2H2

Molecular Shapes WS Directions: For each the following compounds:

Draw the 3-D Lewis Structure

Label the bond angles

Indicate the molecular shape

Identify the number of sigma and pi bonds.

Write down the VSEPR formula and hybridization

Calculate the Bond Energy

*Note: the ones that are bolded involve an expanded octet. 1. HCN 2. CO2

3. CH4

4. BF3

5. NH3

6. H2O

7. SF6

8. AsCl5

9. BrF5

10. TeF4