VSEPR Theory Demos sp3 Bonding Balloons Videodisk Unit 3

Valence Shell Electron Pair Repulsion VSEPR Theory Valence Shell Electron Pair Repulsion Electrons repel each other in a molecule The structure around a given atom is determined by minimizing electron-pair repulsions (getting pairs of electrons as far apart as possible) Think about 300 students in the gym, and everyone swinging their arms to make sure they have space on all sides in every direction Helps to predict the 3-D geometry of molecules sp3 Bonding Balloons Demo 4 balloons tied together.

Steps for Determining Shape Draw the dot diagram for the compound Most compounds follow the octet rule but not all of them (note: only the central atom can break the octet rule. Boron breaks the rule) Determine how many total pairs of electrons are on the CENTRAL atom Determine how many bonds the central atoms is involved in. (each bond, whether single, double or triple, counts as one shared pair) Look at number of bonds (shared pairs) versus unshared (lone) pairs of electrons and assign shape

Linear CO2 O C O Total Pairs = 4 Shared pairs = 4 Unshared (lone) pairs = 0 *A linear shape occurs any time only 2 atoms bond

BH3 H B H H Trigonal Planar Total Pairs = 3 Shared Pairs = 3 Unshared (lone) Pairs = 0

Tetrahedral CH4 H H C H H Total Pairs = 4 Shared Pairs = 4 Unshared (lone) Pairs = 0

NH3 H N H H Trigonal Pyramidal Non-bonding pairs push harder then bonding pairs, thus, bond angles get smaller. In trigonal pyramidal, the bond angles are 107o instead of 109.5o. Total Pairs = 4 Shared pairs = 3 Unshared (lone) pairs = 1

H2O H O H Bent Total Pairs = 4 Shared Pairs = 2 In bent, the bond angles are 105o instead of 109.5o. Total Pairs = 4 Shared Pairs = 2 Unshared (lone) pairs = 2

Bond Angles Bond angles are determined by the number of non-bonding pairs of electrons which push the bonding pairs closer together Linear 180° Trigonal Planar 120° Tetrahedral 109.5° Trigonal Pyramidal 107.3° Bent 104.5°

Polar Covalent Bonds

Covalent Bonding A covalent bond is a shared pair of electrons electrostatically attracted to the positive nuclei of two atoms. - + - + The atoms achieve a stable outer electron arrangement (a noble gas arrangement) by sharing electrons.

Covalent Bonding Both nuclei try to pull the electrons towards themselves - + - + This is like a tug-of-war where both sides are pulling on the same object. It creates a strong bond between the two atoms.

Covalent Bonding Picture a tug-of-war: If both teams pull with the same force the mid-point of the rope will not move.

Pure Covalent Bond H e H This even sharing of the rope can be compared to a pure covalent bond, where the bonding pair of electrons are held at the mid-point between the nuclei of the bonding atoms.

Polar Covalent Bonding What if it was an uneven tug-of-war? The team on the right are far stronger, so will pull the rope harder and the mid-point of the rope will move to the right.

Polar Covalent Bond A polar covalent bond is a bond formed when the shared pair of electrons in a covalent bond are not shared equally. This is due to different elements having different electronegativities.

Polar Covalent Bond I δ- δ+ H e.g. Hydrogen Iodide e If hydrogen iodide contained a pure covalent bond, the electrons would be shared equally as shown above. However, iodine has a higher electronegativity and pulls the bonding electrons towards itself (winning the tug-of-war)

Polar Covalent Bond I δ- δ+ H e.g. Hydrogen Iodide e This makes iodine slightly negative and hydrogen slightly positive. This is known as a dipole.

Polar Covalent Bond C Cl δ+ δ- In general, the electrons in a covalent bond are not equally shared. δ- δ+ e.g. C Cl 2.5 3.0 Electronegativities Polar Bonds (1.5 minutes) δ- indicates where the bonding electrons are most likely to be found.

Polar and Nonpolar Covalent Bonds Although all covalent bonds involve sharing of electrons, they differ widely in the degree of sharing We divide covalent bonds into nonpolar covalent bonds & polar covalent bonds Bonds are polarized along a continuum from covalent to ionic depending on the difference in electronegativity. A polar covalent bond has some amount of partial + and – charges at either end. A-A δ+A-B δ- A+B- |____________________________________________________________| Covalent Polar Covalent Ionic

Polar Covalent Bond Consider the polarities of the following bonds: Electronegativities Difference C Cl 2.5 3.0 0.5 P H 2.2 2.2 O H 3.5 2.2 1.3 C Cl δ- δ+ O H δ- δ+ P H Increasing Polarity Complete a similar table for C-N, C-O and P-F bonds.

Polar Vs. Non Polar Properties Polar Covalent Pure (non-polar) Covalent Soluble in water (sugar), or other polar solutes Think about sugar and water. Sugar dissolving video Insoluble in water or other polar solutes Think about oil and water

Network Solids A compound with all covalent bonds Highest m.p. of all substances Examples: Carbon exhibits the most versatile bonding of all the elements: diamond structure, graphite structure, fullerene structure, nanotubes

Network Solids Diamond structures consists of tetrahedral carbons in a 3-dimensional array Graphite structures consist of trigonal planar carbons in a 2-dimensional array Fullerenes consist of 5 and 6 member carbon rings fused into icosahedral spheres of at least 60 carbons Nanotubes are long hollow tubes constructed of fused C6 rings

Network Solids Other examples include Silicates: covalent atomic solids of Si and O; includes quartz, pyroxene, asbestos, talc, mica Boron: metalloid almost always found in compounds with O borax = Na2[B4O5(OH)4]8H2O kernite = Na2[B4O5(OH)4]3H2O colemanite = Ca2B6O115H2O Pyrex

Molecular Solids A crystalline molecular solid has molecules arranged in a particular conformation held together by intermolecular forces (van der Waals forces). In water, H2O, the molecules are arranged in a regular three-dimensional structure for ice. Other examples of crystalline molecular solids are table sugar, C12H22O11, and sulfur, S8. Chapter 13