Molecular Geometry Cocaine To play the movies and simulations included, view the presentation in Slide Show Mode.

Review of Chemical Bonds There are 3 forms of bonding: 1) _________—complete transfer of 1 or more electrons from one atom to another (one loses, the other gains) forming oppositely charged ions that attract one another 2) _________—some valence electrons shared between atoms 3) _________ – holds atoms of a metal together Most bonds are somewhere in between ionic and covalent.

The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together.

Electronegativity Difference If the difference in electronegativities is between: 1.7 to 4.0: Ionic 0.3 to 1.7: Polar Covalent 0.0 to 0.3: Non-Polar Covalent Example: NaCl Na = 0.8, Cl = 3.0 Difference is 2.2, so this is an ionic bond!

Bonding and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. • •• H Cl shared or bond pair lone pair (LP) This is called a LEWIS structure.

Bond Formation H Cl H – Cl Note that each atom has a A bond can result from an overlap of atomic orbitals on neighboring atoms. Cl H •• • + •• H – Cl • •• Overlap of H (1s) and Cl (2p) Note that each atom has a single, unpaired electron.

Review of Valence Electrons Remember valence electrons are the electrons in the OUTERMOST energy level…

Steps for Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Why? If there is a choice, the central atom is atom of lowest affinity for electrons. (Most of the time, this is the least electronegative atom… Therefore, N is central on this one Add up the number of valence electrons that can be used. Ammonia, NH3 H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons •• = 8 electrons = 4 pairs

H N H N 3 single bonds= 3 pairs 3. Form a single bond between the central atom and each surrounding atom (each bond takes 2 electrons!) H N 3 single bonds= 3 pairs H N 4. Remaining electrons form LONE PAIRS to complete the octet as needed (or duet in the case of H). 3 BOND PAIRS + 1 LONE PAIR = 4 Pairs =8 electrons = 4 pairs

Check to make sure there are 8 electrons around each atom except H Check to make sure there are 8 electrons around each atom except H. H should only have 2 electrons. This includes SHARED pairs. H •• N 6. Also, check the number of electrons in your drawing with the number of electrons from step 2. If you have more electrons in the drawing than in step 2, you must make double or triple bonds. If you have less electrons in the drawing than in step 2, you made a mistake! = 8 electrons = 4 pairs

Carbon Dioxide, CO2 1. Central atom = 2. Valence electrons= 3. Form bonds. C 4 e- O 6 e- (x2) O’s = 12 e- Total: 16 electrons = 8 pairs 2 single bonds = 2 pairs 4. Place lone pairs on outer atoms. 6 Loan pairs = 6 pairs 5. Check to see that all atoms have 8 electrons (except for H, which can have 2.) What is wrong with our drawing?

What is wrong with our drawing? Each single bond = 1 pair (2) Each loan pair = 1 pair (2) 8 4 8 These loan pairs should actually be bonding pairs 4 single bonds = 4 pairs 4 loan pairs = 4 pairs 8 8 8 C 4 e- O 6 e- (x2) O’s = 12 e- Total: 16 electrons = 8 pairs

Double and even triple bonds are commonly observed for C, N, P, O, and S H2CO C2F4 SO3

Violations of the Octet Rule (Honors only) Common exceptions are: Be, B, P, S, and Xe. Be: 4 (not 8) B: 6 P: 8 OR 10 S: 8, 10, OR 12 Xe: 8, 10, OR 12 SF4 BF3 : Loan Pair not shown

VSEPR MOLECULAR GEOMETRY Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions. (get as far away as possible) (click me)

Arrangement of the number of electron pairs around the central atom 4 charge clouds, tetrahedral 3 charge clouds, trigonal planar 2 charge clouds, linear 6 charge clouds, Octahedral 5 charge clouds, trigonal bipyramidal electron pairs (bonding & loan pair)

Some Common Geometries Linear Trigonal Planar Tetrahedral (click me) (click me)

VSEPR charts 1 region = electron pairs (bonding & loan pair) Use the Lewis structure to determine the geometry of the molecule Electron arrangement establishes the bond angles Molecule takes the shape of that portion of the electron arrangement Charts look at the CENTRAL atom for all data! Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would only be 1 region – CHARGE CLOUD) 1 region = electron pairs (bonding & loan pair)

VSEPR Molecular Geometry bond length and angles determined experimentally where Lewis Structures  bonding geometry VSEPR Valence Shell Electron Pair Repulsion Theory octahedron 90o bond angles 6 electron pairs (bonding & loan pair) trigonal bipyramid equatorial 120o axial 180o 5 electron pairs (bonding & loan pair)

tetrahedron 109.5o 4 electron pairs (bonding & loan pair) trigonal planar 120o 3 electron pairs (bonding & loan pair) linear 180o 2 electron pairs (bonding & loan pair)

Be is an exception to the octet rule with 4 Ve- BeCl2 valence e- = 2 + (2 x 7) = 16e- Cl .. Be Be is an exception to the octet rule with 4 Ve- 8 4 8 2 valence pairs on Be = 2 bonding pairs + 0 lone pair Electron pair arrangement = linear Molecular geometry = linear

CO2 .. .. valence e- = 4 + (2 x 6) = 16e- C O 2 valence pairs on C = 2 bonding pairs + 0 lone pair C O .. 8 8 8 single and double bonds same Electron pair arrangement = linear Molecular geometry = linear

B = exception to the octet rule BF3 valence e- = 3+ (3 x 7) = 24 e- B F .. B = exception to the octet rule 3 valence pairs on B = 3 bonding pairs + 0 lone pair 120o Electron arrangement = trigonal planer Molecular geometry = trigonal planer

SO2 .. : .. : .. : valence e- = 6+ (2 x 6) = 18e- S O S O S O 3 valence pairs on S = 2 bonding pairs + 1 lone pair < 120o Electron pair arrangement= trigonal planer Molecular geometry= bent (angular)

CH4 valence e- = 4+ (4 x 1) = 8e- = 4 pairs C H 109.5o 4 valence pairs on C = 4 bonding pairs + 0 lone pair Electron pair arrangement = tetrahedral Molecular geometry = tetrahedral

NH3 : valence e- = 5+ (3 x 1) = 8e- = 4 pairs N H 4 valence pairs on N = 3 bonding pairs + 1 lone pair < 109.5o Electron pair arrangement = tetrahedral Molecular geometry = trigonal pyramidal

H2O : valence e- = 6+ (2 x 1) = 8e- = 4 pairs O H < 109.5o 4 valence pairs on O = 2 bonding pairs + 2 lone pair Electron pair arrangement = tetrahedral Molecular geometry = Bent (angular)

P = exception to the octet rule PCl5 valence e- = 5+ (5 x 7) = 40e- P Cl .. P = exception to the octet rule 90o 120o 180o 5 valence pairs on P = 5 bonding pairs + 0 lone pair Electron pair arrangement = trigonal bipyramidal Molecular geometry= trigonal bipyramidal

S = exception to the octet rule SF4 valence e- = 6+ (4 x 7) = 34e- S = exception to the octet rule S .. F : < 180o 5 valence pairs on P = 4 bonding pairs + 1 lone pair Electron pair arrangement = trigonal bipyramidal Molecular geometry= Seesaw (distorted tetrahedron)

S = exception to the octet rule SF6 valence e- = 6+ (6 x 7) = 48e- S = exception to the octet rule S F .. 90o 6 valence pairs on P = 6 bonding pairs + 0 lone pair Electron pair arrangement = Octahedral Molecular geometry= Octahedral