chapter 8

Chapter 8Covalent Bonding

VSEPR Theory

Molecular Shape

Polar or NonPolar

Properties of Molecular Substances

Why Do Atoms Bond? The stability of an atom, ion or compound is related

to its energy– lower energy states are more stable.

Metals and nonmetals gain stability by transferring electrons (gaining or losing) to form ions that have stable noble-gas electron configurations.– Ionic Bonding

Another way atoms can gain stability is by sharing valence electrons with other atoms, which also results in noble-gas electron configurations.– Covalent Bonding

The most stable arrangement of atoms exists at the point of maximum net attraction, where the atoms bond covalently and form a molecule.

The Covalent Bond Atoms will share electrons in order to

form a stable octet. Covalent bond : the chemical bond

that results from the sharing of valence electrons

also called a molecular bond

The Molecule formed when two or more atoms bond

covalently The smallest piece in a covalent compoundFormed when the proton of one atom is

attracted to the electron cloud of another atom.

Models Molecules

Single Covalent Bonds

In a single covalent bond a single pair of electrons is shared

This can be represented with a Lewis structure

A single line represents a single covalent bond

A single pair of electrons

• Bonding pair: a pair of electrons shared by two atoms

• Lone pair: an unshared pair of electrons on an atom

Formation of Water

Group 17 elements will form one covalent bond.

Group 16 elements will form two covalent bonds.

Group 15 elements will form three covalent bonds.

Group 14 elements will form four covalent bonds.

Sigma Bonds • Single covalent bonds are also called

sigma bonds:• the electron pair is centered between

two atoms.

 Multiple Covalent Bonds    

When more than one pair of electrons is shared, a multiple covalent bond is formed

Multiple bonds are made up of sigma bonds and pi bonds: formed when parallel orbitals share electrons.

Double Covalent Bond Two pairs of electrons are shared Contains one sigma and one pi bond.

Triple Covalent Bond Three pairs of electrons are shared Has one sigma and two pi bonds.

Strength of Covalent Bonds

The strength of covalent bonds is determined by the bond length:– distance between the bond

nuclei Bond length is determined by:

– The size of the atoms involved—larger atoms have longer bond lengths

– How many pairs of electrons are shared—the more pairs of electrons shared, the shorter the bond length is.

Bond Dissociation Energy the amount of energy

required to break a bond Indicates the strength of a

covalent bond When a bond forms, energy

is released; When a bond breaks, energy

must be added Each covalent bond has a

specific value for its bond

dissociation energy.

Bond Energy and Bond Length A direct relationship exists between bond

energy and bond length– Shorter Bond– Stronger Bond – Higher Bond Dissociation Energy – Longer Bond– Weaker Bond – Lower Bond Dissociation Energy

Energy Changes An endothermic reaction is one where

a greater amount of energy is required to break a bond in reactants than is released when the new bonds form in the products.

An exothermic reaction is one where more energy is released than is required to break the bonds in the initial reactants.

Naming Molecules Molecular Formula

Shows what atoms and how many are in a molecule

Examples: Nonmetal-Nonmetal Combinations

Naming (Binary Compounds) The first element is always named first using

the entire element name The second element is named using its root

and adding the suffix -ide Prefixes are used to indicate the number of

atoms of each element that are present in the compound

Common Names Many compounds were discovered and

given common names long before the present naming system was developed (water, ammonia, hydrazine, nitric oxide).

Binary Acids An acid that contains hydrogen and one other

element Ex. HCl ion ends –ide. Name the acid with hydro-root of the anion-ic HCl (hydrogen and chloride ) becomes

hydrochloric. HCl in a water solution is called hydrochloric

acid.

Naming Acids Acids contain hydrogen as the first

element Binary Acids: H bonded to one other

element An ion that ends –ide Name the acid with hydro-root-ic Example: HCl

Hydrogen ion and chloride ion Hydrochloric acid

Oxyacids An acid that contains both a hydrogen atom and an

oxyanion. Example: HNO3 Identify the oxyanion present. name ends with the suffix –ate, replace it with the

suffix –ic. If the name ends with suffix –ite, replace it with suffix

–ous, NO3 is the nitrate ion so the acid is nitric acid.

Acid Naming Summary Anion name Acid name

--ide Hydro-root-ic

--ite Root---ous

--ate Root-ic

Structural Formulas A structural formula uses letter symbols and bonds to

show relative positions of atoms.

Lewis Structures

Used to predict the structural formula Show arrangement of the atoms and

un-bonded electrons

Five steps to draw Lewis structures:1. Count the total number of valence electrons

in all atoms involved.

2. Decide how the elements are arranged in the structure and draw it out.

1. Hydrogen is always an end atom.

2. Central atom is usually written first in compound

3. Central atom has least attraction for the electrons

4. Usually closer to left on periodic table

3. Subtract the # of electrons used in the bonds.

4. Satisfy the octets of the terminal atoms.

5. Place any remaining electrons around the central atom to satisfy its octet. If the central atom cannot be satisfied, make a multiple bond using a lone pair from the terminal atoms.

6. Check your work 

Examples

Drawing Lewis structures for polyatomic ions is very similar to drawing Lewis structures for covalent

compounds EXCEPT in finding the number of electrons available for bonding

Count the total number of valence electrons in all atoms involved.

If the polyatomic ion is negatively charged, ADD the charge to the number of valence electrons.

If the ion is positively charged, SUBRACT the charge from the number of valence electrons.

Follow the rest of the steps to drawing Lewis structures.

Examples

Resonance Structures When a molecule or polyatomic ion has

both a double bond and a single bond, it is possible to have more than one correct Lewis structure:

Resonance a condition that occurs when more than

one valid Lewis structure can be written for a molecule or ion.

The structures are called resonance structures.

A molecule that undergoes resonance behaves as if it has only one structure.

Exceptions to the Octet Rule  

Three Ways Molecules Might Violate the Octet Rule

Odd Number of Valence Electrons Some molecules have an odd number of valence

electrons and cannot form an octet around each atom Example: NO2

Sub Octet Some compounds form with fewer than 8 electrons present around an atom. Boron BF3

Coordinate covalent bond when one atom donates an entire pair of electrons to be

shared with atoms or ions that need two more electrons. Boron compounds often do this

Expanded Octet Some elements can have more than eight electrons in

their valence shell Because of d-level electrons PCl5

How? The d orbital starts to hold electrons.This occurs in atoms in Period 3 or higher.  When you draw Lewis structures for these

compounds, extra lone pairs are added to the central atom OR the central atom will form more than four bonds.

Things to Remember Any exceptions to the Octet Rule are on

the central atom

Molecular Shape How a molecule “looks” Determines properties The shape of a molecule determines

whether or not two molecules can get close enough to react

We describe shape using the VSEPR model

VSEPR This model is based on the fact that

electrons pairs will stay as far away from each other as possible

Valence

Shell

Electron

Pair

Repulsion

How to apply VSEPR 1. Draw the Lewis Structure for a

Molecule

2. Count the pairs of bonded electrons

3. Count the pairs of unbonded electrons

4. Match the information with the VSEPR chart to classify the shape of the molecule

Atoms will assume certain bond angles: the angle formed by any two terminal atoms and the central atom

Lone pairs take up more space than bonded pairs do.

http://www.chem.purdue.edu/gchelp/vsepr/structur.html

Molecular Polarity Molecules are either polar or nonpolar

depending on the bonds in the molecule.

We must look at the shape (geometry) of a molecule to determine polarity.

Symmetric molecules are nonpolar. Asymmetric Molecules are Polar

Polar or NonPolar Determine if a molecule is polar or

nonpolar by– Looking at a model of the molecule– Looking at a Lewis Structure of the

molecule

Solubility of Polar Molecules Bond type and shape of the molecule

determine solubility Polar substances and ionic substances

will dissolve in polar solvents Nonpolar substances will only dissolve

in nonpolar substances

Intermolecular Forces Nonpolar Molecules: Van der Waals intermolecular

Forces Very Weak forces between molecules

Polar Molecules: have dipole-dipole intermolecular bonding. Stronger intermolecular Forces

Polar Molecules with Hydrogen Bonding: hydrogen bonded to nitrogen, oxygen or fluorine, it will have hydrogen bonding between molecules. A very strong dipole-dipole interaction Very strong intermolecular Forces High boiling points, high melting points