CHEMICAL BONDING CHAPTER 12

COVALENT BONDS

A covalent bond results from the sharing of electrons between two nonmetal atoms.
The electrons belong to both nonmetal atoms.
Each atom uses the bonding electrons to complete an octet.

The octet rule states that atoms bond is such a way so that each atom acquires eight electrons in its outer shell.

Guidelines for Drawing Electron Dot Formulas

1. Calculate the total number of valence electrons (should be an even #).

2. Divide the total number of valence electrons by 2 to find the # of electron pairs in the molecule.

3. Surround the central atom with four electron pairs. Then move the other remaining atoms around the central atom. Use the remaining electron pairs to complete an octet around each of the other atoms.

4. If there are not enough electron pairs to provide an octet for each atom, move a nonbonding electron pair between two atoms that already share an electron pair.

Write the electron dot formula for SO₃
Sulfur has 6 valence electrons, each oxygen has 6 valence electrons.
6+3(6)=24 e- 24e- and 12e- pairs

POLAR COVALENT BONDS

Covalent bonds result from the sharing of valence electrons. In many instances one of the two atoms holds the electron pair more tightly.

When the electrons are drawn more closely to one of the atoms, the bond is said to be polarized.

We identify a polar bond using the Greek letter delta ().

ELECTRONEGATIVITY

Each element has an innate ability to attract valence electrons. The ability of an atom to attract electrons in a chemical bond is referred to as its electronegativity.

How does soap work?

METALLIC BONDING

Metallic bonding represents a fourth major type of chemical bonding. Metal atoms are closely packed, and in most cases, the outmost electron shell of one metal atom overlaps with a large number of neighboring atoms, and as a result, the valence electrons move freely form one atom to another.

Valence electrons are not associated with any specific atom or pair of atoms, but wander freely among adjacent metal atoms. Rather than being attracted to a specific pair of electrons, each positive metal ion is attracted to all of the electrons in its vicinity

Metals can be described as positive nuclei (cations) immersed is a sea of electrons. The free-electron character of metals explains why the are excellent conductors of electricity and heat.

Since the binding forces in metals are non-direction, metals can be reshaped easily and do not fracture when stretched or hammered

	Ionic	Covalent	Metallic	Intermolecular
Bond strength	strong	very strong	moderate and variable	weak
Hardness	moderate to high	very hard, brittle	low to moderate; ductile, malleable	soft and plastic
Electrical conductivity	conducts by ion transport only when dissociated	insulator in solid and liquid	good conductors; by electron transport	insulators in solid and liquid states
Melting point	moderate to high	low	generally high	low
Solubility	soluble in polar solvents	very low solubilities	insoluble	soluble in organic solvents
Examples	most minerals	diamond, oxygen, organic molecules	Cu, Ag, Au, other metals	ice, organic solids (crystals)

SHAPES OF MOLECULES

In the 1950s, a simple theory was proposed to explain the shapes of molecules.

Valence shell electron pair repulsion (VSEPR) theory states that the electron pairs surrounding an atom tend to repel each other

Electron pair geometry indicates the arrangement of electron pairs about the central atom.

Molecular shape indicates the arrangement of atoms about the central atom.

The angle formed by any two atoms bonded to the central atom is referred to as the bond angle.

Summary of VSEPR Geometries

*Bonding groups (pairs)**	*Nonbonding groups (pairs)**	*Electron Pair Goemetry**	Molecular Shape	Bond Angle
4	0	tetrahedral	tetrahedral	109.5^o
3	0	equilateral triangle	triagular planer	120^o
3	1	tetrahedral	triangular pyramidal	107^o
2	2	tetrahedral	angular (bent)	104.5^o
2	0	linear	linear	180^o
1	3	tetrahedral	linear	----
2	1	equilateral triangle	bent	----