1 Section 8.1The Covalent Bond Section 8.2 Naming Molecules Section 8.3 Molecular Structures Section 8.4 Molecular Shapes (Hybridization and VSEPR model) Section 8.5Electronegativity and Polarity Chapter 8 Covalent Bonding

2 Hybridization is a process in which atomic orbitals mix and form new, identical hybrid orbitals. Example: CH 4 Hybridization of one s orbital and three p orbitals creates four sp 3 orbitals Tetrahedral shape minimizes electron repulsion Section 8.4

3 =SJdllffWUqg&noredirect=1 Example sp 2 -hybridization “triangle” trigonal planar and sp-hybridization (one s orbital and one p orbital) creates two orbitals along one axis, with the major lobes pointing in opposite directions

4 Sigma bond and lone pairs can be in hybrid orbitals, but  bonds (in double or triple bonds) cannot!! Example C 2 H 4  Bonds made from pure p orbitals

5 Example carbon dioxide CO 2 Carbon has two sigma bonds with sp hybridized orbitals and p y and p z orbitals for the two  bonds CO 2

6 The Valence-shell electron-pair repulsion model or VSEPR theory of molecular geometry states that, generally, molecules adjust their three-dimensional shapes so the valence-electron pairs are as far apart as possible.

7 pyramidal tetrahedral

8 bent linear

9 Common molecule shapes

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14 Section 8.5 Electronegativity and Polarity Electron affinity measures the tendency of an atom to accept an electron. [amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion] Chemists like to use electronegativity to evaluate the electron affinity for specific atoms in compounds Electronegativity is the relative ability of an atom to attract electrons in a chemical bond How each atom fares in a tug-of-war for shared electrons is determined by comparing the electronegativities of the two bonded atoms.

15 Noble gases are not listed because they generally do not form compounds. highest lowest

16 Electronegativity Values of Chemical Elements Noble gases are not listed because they generally do not form compounds.

17 Difference in Electronegativity  EN and Bond Character Dipole vector  EN = 0.96

18 Polar covalent bonds are called polar because the unequal electron sharing creates two poles across the bond. Like magnets have separate positive and negative poles so do polar covalent bonds. The negative pole is centered on the more electronegative atom in the bond. Electrons closer to higher EN  more negative

Molecules with polar bonds can be polar or nonpolar (if dipole vectors cancel out) Polar and Nonpolar Molecules Water has two polar bonds and is a polar molecule Surface tension due to polarity

20 Compare water, H 2 O, and CCl 4 Both bonds are polar. The molecular shapes, determined by VSEPR, is bent and tetrahedral O – H bonds are “asymmetric” in water, so has a definite positive end and definite negative end. Thus, water is polar. The C – Cl bonds are symmetrical in CCl 4. The electric charge measured at any distance from the center is identical on all sides and partial charges are balanced. Thus CCl 4 nonpolar.

21 Note: If bonds are polar, “asymmetrical” molecules are polar and “symmetrical” molecules are nonpolar. !!!!!!! (arrows cancel out)

22 “Like dissolves like” Nonpolar compounds dissolve (only) in nonpolar solvents (oil, gasoline, petrol ether, lighter fluid etc.) Polar and ionic compounds are usually soluble in polar solvents like water. Oil spill Solubility is the property of a substance’s ability to dissolve in another substance.

23  The weak attraction forces (intermolecular forces) between molecules are known as van der Waals forces.  They vary in strength but are weaker than the bonds in a molecule or ions in an ionic compound  Non-polar molecules exhibit a weak dispersion force, or induced dipole.  The force between two oppositely charged ends of two polar molecules is a dipole-dipole force.  A hydrogen bond is an especially strong dipole-dipole force between a hydrogen end of one dipole and a fluorine, oxygen, or nitrogen atom on another dipole. Intermolecular Forces

24 Many physical properties are due to intermolecular forces: Weak forces result in the relatively low melting and boiling points of molecular substances. Many covalent molecules are relatively soft solids (ex. wax). Molecules can align in a crystal lattice, similar to ionic solids but with less attraction between particles. Compare: Nonpolar covalent CO 2 and CH 4 are gases at room temperature, while H 2 O, polar covalent is a liquid, and BeO (ionic) is a brittle solid. (Covalent Network Solids (special case) Are some solids composed of only atoms interconnected by a network of covalent bonds and are very hard and brittle. Quartz and diamonds are two common examples of network solids) [not on test]