Chemical Bonding Chapters 8 & 9

Bonding occurs to lower the energy of the system. ionic bonding - transfer of electrons; bonding occurs due to the attraction of oppositely charged ions. covalent bonding - sharing of electrons; If the electrons are shared unequally it is a polar covalent bond.

polarity of water:

Electronegativity - the ability of an atom in a molecule to attract shared electrons to itself. Periodic trend: increases across a period (excluding noble gases), decreases down a group

Polar molecules are called dipoles. They have a dipole moment. examples:

Bond Energy - the enthalpy change required to break a bond in a mole of a gaseous substance. Breaking bonds is ALWAYS endothermic. Cl 2(g) --> 2 Cl (g) ΔH = 242 kJ As the number of bonds between two atoms increases, the bond grows shorter and stronger. Reaction enthalpy can be ESTIMATED from bond energy data. ΔH = sum of the bonds broken - sum of the bonds formed (chart p. 326) Simulation

ex - Estimate ΔH for the following reaction. N 2 H 4(g) --> N 2(g) + 2 H 2(g) Simulation

Writing Lewis Structures: (see rules handout) Write the Lewis structure of NO 2 -. Resonance forms - equivalent Lewis structure. The true structure is an average of the Lewis structures.

Formal Charge - helps to decide between two possible, nonequivalent Lewis structures. Remember: formal charges are not real charges. Assign electrons: 1. All bonding electrons are divided equally between the atoms that form bonds. 2. All nonbonding electrons are assigned entirely to the atom on which they are found. 3. The structure with the lowest formal charge on each atom is the best structure.

ex NCO - Simulation

VSEPR- Valence Shell Electron Pair Repulsion The repulsion of the electron pairs - both bonding and nonbonding - determines the shape of the molecule. return to worksheet - 2nd and 3rd columns - use handouts on molecular geometry. Then build and look at the symmetry to complete column 4

Hybridization: When s orbitals overlap with s or p orbitals a sigma bond is formed. When p orbitals overlap with each other a pi bond is formed. Single bonds are always sigma bonds. Double bonds are 1 sigma and 1 pi. Triple bonds are one sigma and 2 pi.

Hybridization counts the orbitals holding electrons (electron domains). 2 electron domains = sp 3 electron domains = sp 2 4 electron domains = sp 3 5 electron domains = dsp 3 6 electron domains = d 2 sp 3

ex Acetonitrile, CH 3 CN a. Draw the Lewis structure. b. Predict the bond angles around each carbon atom. c. Give the hybridization of each carbon atom. d. Determine the total sigma and pi bonds i. on each carbon atom. ii. in the molecule Watch this simulation of hybridization

Lattice energy: The attractions that hold ionic compounds together. They are governed by Coulomb's Law:

The greater the charges of the ions, the greater the attraction and the stronger the lattice (more negative lattice energy). The smaller the atoms, the greater the attraction and the stronger the lattice energy.

Notes Quiz B2ZXdtNlZkWUhFNzJ4aXlZWlY4MHc6MQ