Molecular Shapes and VSEPR (Valence Shell Electron Pair Repulsion Theory)

Transcription

1 AP Chemistry Ms. Ye Name 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

2 Molecule 3D Drawing DAB Molecule 3D Drawing AB5 AB3 AB4E B2AC AB3E2 AB4 AB6 AB3E AB5E AB2E2 AB4E2

3 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 True Bond Angles part 1) 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.

4 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

5 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.

6 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.

7 Hybridization: The is determined by the # Sets Orbital Hybridization The is determined by the 6 Sets= # things (atoms and lone pairs) around central atom A = X= E=

8 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

9 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.

10 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

11 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

12 Molecular Shapes WS Directions: For each the following compounds: Draw the 3-D Lewis Identify the number of Structure sigma and pi bonds. Label the bond angles Write down the VSEPR Indicate the molecular formula and shape hybridization *Note: the ones that are bolded involve an expanded octet. Calculate the Bond Energy 1. HCN 6. H2O 2. CO2 7. SF6 3. CH4 8. AsCl5 4. BF3 9. BrF5 5. NH3 10. TeF4