1. Chapter 8
Molecular Compounds

2. Molecular Compounds
Covalent bonds-Atoms held together by sharing electrons
Molecule- is a neutral group of atoms joined together by covalent bonds.
Ex. Air contains oxygen molecules, each oxygen molecule consist of two oxygen atoms joined by a covalent bond.
Diatomic molecule-is a molecule consisting of two atoms.
Ex. Oxygen molecule is a diatomic molecule

3. Molecular Compounds
Molecular compound-A compound composed of molecules
Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds.
Many molecular compounds are liquids or gases at room temperature.
Ionic compounds are formed from a metal combined with a nonmetal; most molecular compounds are composed of atoms of two or more nonmetals.
Ex. CO carbon monoxide

4. Molecular Compounds
Molecular formula- is the chemical formula of a molecular compound
A molecular formula shows how many atoms of each element a molecule contains.
Water H2O CO Ethanol C2H6O

5. Molecular Compounds
In forming covalent bonds electron sharing usually occurs so that atoms attain the electron configuration of noble gases.
Combinations of atoms of the nonmetallic elements in Groups 4A,5A,6A, and 7A of the periodic table are likely to form covalent bonds. Atoms usually acquire a total of eight electrons.
Single covalent bond-two atoms held together by sharing a pair of electrons
Ex. Hydrogen molecule

6. Molecular Compounds
An electron dot structure such as H:H represents the shared pair of electrons of a covalent bond by two dots.
Structural formula-represents the covalent bond by dashes and shows the arrangement of covalently bonded atoms. Ex. H-H Fluorine molecule F-F
Unshared pair- A pair of valence electrons that is not shared between atoms
Ex. Water molecule

7. Molecular Compounds
Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons.
Double covalent bonds-A bond that involves two shared pairs of electrons
Ex. O=O
Triple covalent bonds-A bond formed by sharing three pairs of electrons
Ex. N=N

8. Molecular Compounds
Coordinate covalent bond-is a covalent bond in which one atom contributes both bonding electrons
C O
Polyatomic ion-a tightly bound group of atoms that has a positive or negative charge and behaves as a unit.
SO3-2
Bond dissociation energy-The energy required to break the bond between two covalently bonded atoms
A large bond dissociation energy corresponds to a strong covalent bond
Resonance structure-is a structure that occurs when it is possible to draw two or more electron dot structures that have the same number of electron pairs for a molecule or ion.
Ex. O3

9. Exceptions to the Octet Rule
The octet rule cannot be satisfied in molecules whose total number of valence electrons is an odd number. There are also molecules in which an atom has fewer, or more, than a complete octet of valence electrons
A few atoms, especially phosphorus and sulfur sometimes expand the octet to include ten or twelve electrons.
Ex.PCl5 or SF6

10. Bonding Theories
Molecular orbitals-orbitals that apply to the entire molecule
Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole.
Bonding orbital- a molecular orbital that can be occupied by two electrons of a covalent bond.

11. Bonding Theories
Sigma bond-orbitals combine to form a molecular orbital that is symmetrical around the axis connecting two atomic nuclei
Pi bond – the bonding electrons are most likely to be found in sausage shaped regions above or below the bond axis of the orbitals.

12. Bonding Theories
Orbital hybridization provides information about molecular bonding and molecular shape.
Hybridization-several atomic orbitals mix to form the same total number of equivalent hybrid orbitals. pg.234

13. VSEPR Model
Valence shell electron pair repulsion theory VSEPR theory- the repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible.
Geometric shape of molecule
Tetrahedral angle degrees All of the H-C-H angles in CH4
Linear triatomic-180 degrees CO2
Trigonal planar-120 degrees C2H4
Bent triatomic degrees H20
Pyramidal degrees NH3

14. Polar Bonds and Molecules
Nonpolar covalent bond-Atoms in the bond are shared equally in a bond.
This occurs when identical atoms are bonded together. Ex. H-H or O=O
Polar bond-is a covalent bond between atoms in which the electrons are shared unequally.
The more electronegative atom attracts electrons more strongly and gains a slightly negative charge. The less electronegative atom has a slightly positive charge.
Ex. H-Cl , H2O
Table on pg238, and 177

15. Polar Bonds and Molecules
Polar molecule-one end of the molecule is slightly negative and the other end is slightly positive.
Dipole -is a molecule that has two poles is called a dipolar molecule.
When polar molecules are placed between oppositely charge plates, they tend to become oriented with respect to the positive and negative plates. 
Ex. CO2 linear form non polar molecule.
Intermolecular attraction are weaker than either ionic or covalent bonds.

16. Polar Bonds and Molecules
Van der Waals forces-the two weakest attractions between molecules.
1. Dipole interactions-attachments of polar molecules
2. Dispersion forces-Caused by motion of electrons, momentary polar force
Hydrogen bonds-attractive forces in which a hydrogen is covalently bonded to a very electronegative atom, and also weakly bonded to an unshared electron pair of another electronegative atom.
Network solids-a solid in which all of the atoms are covalently bonded to each other.
ex. Diamond is a network solid
Melting point very high if at all. Diamond doesn’t melt at all it vaporizes to gas at 3500 degrees C.
Melting a network solid requires breaking covalent bonds throughout the solid.
Table 8.4 pg244 Comparison of ionic and covalent compounds.