CHEMICAL BONDING Set 1 Cocaine To play the movies and simulations included, view the presentation in Slide Show Mode. SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")!

Credits Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website www.chemistrygeek.com

Chemical Bonding Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties?

Review of Chemical Bonds There are 3 forms of bonding: ionic—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 covalent—some valence electrons shared between atoms metallic – holds atoms of a metal together Most bonds are somewhere in between ionic and covalent.

Why do atoms bond in these ways? A bond will form if the energy of the resulting molecule is lower than that of the separated atoms. Bond energy: the energy required to break a bond Bond length: the distance where the energy is minimal Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

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 0.45 to 1.7: Polar Covalent (uneven sharing of electrons) 0.0 to 0.45: Non-Polar Covalent (even sharing of electrons) Example: NaCl Na = 0.8, Cl = 3.0 Difference is 2.2, so this is an ionic bond!

Learning Check: Relative Bond Polarities Order the following bonds according to increasing polarity: H-H, O-H, Cl-H, S-H, and F-H.

Ionic Bonds All those ionic compounds were made from ionic bonds. We’ve been through this in great detail already. Positive cations and the negative anions are attracted to one another (remember the Paula Abdul Principle of Chemistry: Opposites Attract!) Therefore, ionic compounds are usually between metals and nonmetals (opposite ends of the periodic table).

Ions: Electron Configurations and Sizes Atoms in stable compounds usually have a noble gas electron configuration When two nonmetals react to form a covalent bond, they share electrons in a way that completes the valence electron configurations of both atoms. That is, both nonmetals attain noble gas electron configurations. When a nonmetal and a metal react to form a binary ionic compound, the ions form so that the valence electron configuration of the nonmetal achieves the electron configuration of the next noble gas atom and the valence orbital of the metal are emptied. In this way, both ions achieve noble gas electron configurations. Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

Predicting Formulas of Ionic Compounds It should be emphasized that when chemists use the term ionic compound, they are usually referring to the solid state of the compound In the solid state of an ionic compound, the ions are relatively close together and many ions are simultaneously interacting In the gas phase of an ionic substance the ions would be relatively far apart and would not contain large groups of ions Solid ionic compounds contain a large collection of positive and negative ions packed together in a way that minimizes the (-)(-) and (+)(+) repulsions and maximizes the (+)(-) attractions. Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

Homework Questions 1) Explain the difference between the following pairs of terms. A. electronegativity and electron affinity B. covalent bond and polar covalent bond C. polar covalent bond and ionic bond 2) Some of the important properties of ionic compounds are as follows: Low electrical conductivity as solids and high conductivity in solution or when molten Relatively high melting and boiling points Brittleness Solubility in polar solvents (e.g., water) How does the concept of ionic bonding discussed in previous units account for these properties?

Homework Questions Cont’d 3) Predict which bond in each of the following groups will be the most polar. A. C-F, Si-F, Ge-F B. P-Cl, or S-Cl C. S-F, S-Cl, S-Br D. Ti-Cl, Si-Cl, Ge-Cl