CHEMICAL BONDINGCHEMICAL BONDINGSet 1Set 1

CHEMICAL BONDINGCHEMICAL BONDINGSet 1Set 1

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Credits

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

Chemical Chemical BondingBonding

Problems and questions —Problems and questions —How is a molecule or How is a molecule or

polyatomic ion held polyatomic ion held together?together?

Why are atoms distributed at Why are atoms distributed at strange angles?strange angles?

Why are molecules not flat?Why are molecules not flat?Can we predict the structure?Can we predict the structure?How is structure related to How is structure related to

chemical and physical chemical and physical properties?properties?

Review of Chemical BondsReview of Chemical Bonds• There are 3 forms of bonding:There are 3 forms of bonding:• ionicionic—complete —complete transfer transfer of 1 or of 1 or

more electrons from one atom to more electrons from one atom to another (one loses, the other another (one loses, the other gains) forming oppositely gains) forming oppositely charged ions that attract one charged ions that attract one anotheranother

• covalentcovalent——some valence some valence electrons electrons sharedshared between atoms between atoms

• metallicmetallic – holds atoms of a metal – holds atoms of a metal togethertogether

Most bonds are Most bonds are somewhere in between somewhere in between ionic and covalent.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 The type of bond can usuallyusually be calculated by be calculated by finding the difference in electronegativity of the two finding the difference in electronegativity of the two

atoms that are going together.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: NaClNa = 0.8, Cl = 3.0Difference is 2.2, sothis 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 BondsIonic BondsIonic BondsIonic BondsAll those ionic compounds were made All those ionic compounds were made

from ionic bonds. We’ve been through from ionic bonds. We’ve been through this in great detail already. Positive this in great detail already. Positive cations and the negative anions are cations and the negative anions are attracted to one another (remember attracted to one another (remember the Paula Abdul Principle of the Paula Abdul Principle of Chemistry: Opposites Attract!)Chemistry: Opposites Attract!)

Therefore, ionic compounds are usually Therefore, ionic compounds are usually between metals and nonmetals (opposite between metals and nonmetals (opposite ends of the periodic table).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 Questions1) 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