COVALENT BONDING

Chapter 9

THE COVALENT BOND

Remember that atoms bond to gain stability, usually meaning an octet of electrons.

Sometimes atoms who need to gain electrons will reach their octet by sharing electrons.

The bond formed by sharing electrons is called a covalent bond.

When two or more atoms bond covalently, a molecule is formed.

In covalent bonds the atoms move close enough together so that the repulsive force of like charged particles is balanced by the attractive forces between oppositely charged particles.

EXAMPLES OF MOLECULES

Examples of molecules include diatomic elements:

… H2

… N2

… O2

… F2

… Cl2

… Br2

… I2

LEWIS STRUCTURES

A Lewis structure is a representation used when electrons are shared.

These structures show how the electrons are arranged in the molecules.

In the Lewis structure shared electron pairs, often called bonding pairs, are represented either by either a pair of dots or a line.

For example, the hydrogen molecule can be represented as:

H:H or H—H .

BOND TYPE

In the formation of covalent bonds there are several types of bonds that can form.

Single bonds are always sigma bonds, in which the electron pair is shared in an area centered between two atoms.

These bonds can form by the overlap of two s orbitals, an s and a p orbital, or two p orbitals.

MULTIPLE BONDS

Sometimes atoms fain stability by sharing more than one pair of electrons forming a multiple covalent bond.

For example, in a double covalent bond, two pairs of electrons are shared.

In a triple covalent bond, three pairs of electrons are shared.

A multiple covalent bond always consists of a sigma bond and at least one pi bond, a bond in which parallel orbitals overlap.

A pi bond occupies space above and below the line that represents where two atoms are joined.

BOND LENGTH AND STRENGTH The distance between the nuclei of two bonded

atoms is called bond length.

Energy is absorbed when a bond breaks and released when a bond forms.

The amount of energy required to break a covalent bond is called the bond dissociation energy and is always positive.

The stronger the bond, the greater the bond dissociation energy, and the more difficult to break the bond.

Double bonds are stronger than single bonds; triple bonds are stronger than double bonds.

Double bonds are shorter than single bonds and triple bonds are shorter than double bonds.

TOTAL ENERGY CHANGE

The total energy change in a chemical reaction is determined from the energy of the bonds that have to be broken and formed.

In an endothermic reaction, more energy is required to break existing bonds in the reactants than is released when the new product bonds are formed.

In an exothermic reaction, more energy is released in the formation of the new products than is required to break bonds in the reactants.

NAMING MOLECULES

Section 9.2

NAMING BINARY MOLECULAR COMPOUNDS

Binary molecular compounds only contains two different elements.

In naming such compounds, given their formulas, use the following rules:

1. Name the first element in the formula using its name unchanged.

2. Name the second element, using the root of its name and adding the suffix –ide.

3. Use the following prefixes, given the number of each type of atom present:

COVALENT PREFIXES

Number Prefix

1 Mono-

2 Di-

3 Tri-

4 Tetra-

5 Penta-

6 Hexa-

7 Hepta-

8 Octa-

9 Nona-

10 Deca-

****Mono- is not used as a prefix for the name of the first element.

PROBLEMS

Name the following binary molecular compounds.

1. SO2

2. P4O10

3. N2O3

4. SiF6

NAMING BINARY ACIDS

A binary acid contains hydrogen and one other element.

To name the acid, use the prefix hydro- to name the hydrogen part of the mole. Then to the root of the second element add the suffix –ic. Finally, add the word acid.

OXYACIDS

An oxyacid is a polyatomic ion that contains oxygen.

An oxyacid is an acid that contains hydrogen and an oxyanion.

To name an oxyacid, first write a form of the root of the name of the oxyanion. If the oxyanion name ends in –ate, name the acid by adding the suffix –ic to the root. If the oxyanion name ends in –ite, name the acid by adding the suffix –ous to the root. Then add the word acid.

PROBLEMS

Name the following acids.

1. H2Se

2. HBrO3

3. H2CO3

4. HI

5. HClO4

6. H2SO3

FORMULAS FROM NAMES

Given the name of a molecular compound, you can write its formula by analyzing the name in terms of the naming rules. Use the prefixes that indicate number to write the proper subscripts.

PROBLEMS

Write the formulas for the following molecular compounds.

1. Disulfur dichloride

2. Dinitrogen tetroxide

3. Hydrosulfuric acid

4. Sulfuric acid

MOLECULAR STRUCTURES

Section 9.3

MOLECULAR MODELS

There are several types of molecular structures that illustrate the positions of the atoms in a molecular compound.

Those structures include:

… Molecular formula

… Space filling model

… Lewis structure

… Ball and stick model

… Structural formula

STRUCTURAL FORMULAS

A structural formula is a molecular model that uses letter symbols and bonds to show relative positions of the atoms.

It can be predicted using the Lewis structure.

LEWIS STRUCTURE

The steps for drawing Lewis structures are as follows:

1. Predict the location of certain atoms. Hydrogen is always a terminal atom. The atom with the least attraction for electrons is the central atom.

2. Find the total # of e- available for bonding (valence e-). If the structure is to represent a positive or negative polyatomic ion, the ion charge must be subtracted or added, respectively.

3. Divide the total # of available e- by 2 to obtain the # of bonding pairs.

4. Place one bonding pair between the central atom and each terminal atom.

LEWIS RULES (CONT’D)

5. To find the total # of lone pairs and pairs available for multiple bonding, subtract the # of bonding pairs used in step 4 from the # of bonding pairs determined in step 3. Place lone pairs around the terminal atoms to satisfy the octet rule. Assign remaining pairs to the central atom.

6. If the central atom is not surrounded by four e- pairs, convert one or two lone pairs to the terminal atoms to a double or triple bond to the central atom.

PROBLEM

Draw the Lewis structures for each of the following.

1. CF4

2. CO

3. SiS2

4. NH4+

RESONANCE STRUCTURES

Resonance occurs when more than one valid Lewis structure can be written for a molecule or an ion.

For example, three resonance structures can be written for the NO3

- ion because the double bond can be placed between the central N atom and any of the three O atoms.

PROBLEM

Draw the three resonance structures for the carbonate ion (CO3

2-). (Hint: Each structure contains one double bond.)

EXCEPTIONS TO THE OCTET RULE

Sometimes there are exceptions to the octet rule.

This may be because there are an odd # of total e-, as in ClO2.

Sometimes the central atom may have more or fewer than eight e-.

In the latter, an atom with a lone pair may attach to it by means of a coordinate covalent bond, a bond in which both shared e- are donated by only one of the atoms.

PROBLEM

State why each of the following is an exception to the octet rule.

1. NO2

2. BCl3

3. PF5

MOLECULAR SHAPE

Section 9.4

VSEPR MODEL

Valence Shell Electron Pair Repulsion

The VSEPR model is used to determine molecular shape.

It assumes arrangements that minimize repulsion of electron pairs around the central atom.

Learn the molecular shapes in the table on page 260.

HYBRIDIZATION

During bonding, atomic orbitals can undergo hybridization, or mixing to form new identical hybrid orbitals.

An s orbital and three p orbitals can hybridize to form four identical sp3 hybrid orbitals.

These hybridized orbitals will form varying molecular shapes based on the number of lone pairs present.

PROBLEM

Use VSPER model and the concept of hybridization to describe the hybrid orbitals and the shape of each of the following molecules.

1. H2S

2. BeF2

3. CBr4

4. NF3

5. BI3