DO NOW Pick up notes and Electron Dot handout Do the Electron Dot handout NOW. You have ten minutes.

ELECTRON DOT DIAGRAMS

Determining the Structure of Molecules MOLECULAR GEOMETRY Determining the Structure of Molecules

Molecular Structures Molecular Formula BeH2 Electron Dot Structure H:Be:H Structural Formula H – Be – H Ball and Stick Molecular Model

MOLECULAR GEOMETRY Structural formulas, such as NH3, provide information about bonding only. It does not provide direct information about the shape of the bond or the shape of the molecule. The repulsion between charge clouds in the outer levels of atoms determines the arrangement of the orbitals. The orbital arrangement determines the shape of the molecules.

VSEPR VSEPR - Valence Shell Electron Pair Repulsion theory is based on the number of regions of high electron density around a central atom. can be used to predict structures of molecules or ions by minimizing the electrostatic repulsion between the regions of high electron density. can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons. does fail in some cases.

VSEPR In small molecules, electron pairs will spread as far apart as possible to minimize repulsive forces. Two electron pairs = 180 apart Three electron pairs = 120 apart Four electron pairs = 109.5 apart

PAIRS OF ELECTRONS In bonding, electrons can be shared with another atom or paired up within the same atom. Shared pair = a pair of electrons, one coming from each atom participating in the bond. Lone or Unshared pair = a pair of electrons in the same energy level of the same atom.

SHAPES WE WILL LEARN Linear Trigonal Planar Tetrahedral Pyramidal Bent

LINEAR Atoms are connected in a straight line. One or two bonded pairs of electrons 180 bond angles Examples: HCl CO2

TRIGONAL PLANAR Atoms are connected in a flat equilateral triangle. Three bonded pairs of electrons. 120 bond angle. Example: BCl3

TETRAHEDRAL Atoms are connected in a shape with four surfaces. Four bonded pairs of electrons 109.5 bond angles. Example: CH4

PYRAMIDAL Atoms are in the shape of a pyramid. Similar to the tetrahedral but only has three bonded pairs of electrons, not four; has one unshared/lone pair of electrons. Less than 109.5 bond angle due to unshared pair of electrons. Example: NH3

BENT Atoms are close to the shape of a tetrahedral, but the two unshared pairs of atoms exert a greater repulsive force than the two pairs in the bonds. Two shared pairs of electrons and two unshared pairs of electrons 105 bond angle Example: H2O

OTHER SHAPES

VSEPR TABLE

HOW TO DETERMINE THE GEOMETRY OF A MOLECULE USE ELECTRON DOT DIAGRAMS 1. Look at the chemical formula. Figure out the location of the atoms Hydrogen is always on the outside The least electronegative atom is the central atom (most “electropositive”). 2. Draw the Electron Dot Diagram for each atom. Count up the total amount of valence electrons for all the atoms involved.

HOW TO DETERMINE THE GEOMETRY OF A MOLECULE 3. Determine the number of bonding pairs of electrons by dividing the total # of electrons by two. Remember that each atom has to be bonded at least one other atom. 4. Arrange a skeletal diagram of the molecule by placing the other atoms around the central atom. Place a bonding pair of electrons (2) between the central atom and each of the terminal atoms.

HOW TO DETERMINE THE GEOMETRY OF A MOLECULE 5. Subtract the number of bonded pairs of electrons for the central atom from the electron total. If there are any electrons left over, these electrons are lone pairs to be placed around the central atom.

HOW TO DETERMINE THE GEOMETRY OF A MOLECULE 6. If the terminal (outside) atoms do not have a full octet, place lone pairs of electrons around them. The rest go around the central atom. 7. If the central atom does not have four pairs of electrons around it (and it had a least four valence electrons to begin with), try converting some of the lone pairs to double or triple bonds. (Carbon, nitrogen, oxygen, and sulfur like to form multiple bonds)

EXCEPTIONS There are exceptions to the OCTET rule: Atoms with less than an Octet: Hydrogen – only 2 valence electrons Group 2A – only 4 valence electrons Group 3A – only 6 valence electrons Atoms with more than an Octet: Sulfur and phosphorus – 10+ valence electrons Krypton, xenon, iodine, and others with “d” orbitals will accept more than 8.

PRACTICE CF4 BBr3

PRACTICE MgI2 CO2

PRACTICE Fill in the following chart and predict the molecular shape for the following substances: Molecule E- Dot diagram # of shared/ # of lone/ Electron Dot Shape of Bond Type Formula all elements bonded e- unshared e- Structure (e-negativity)  

TO DO Molecular Geometry – fill in the first column: H2O SiCl4 NH3 Cl2 GaF3