Chapter 7.

Chemical Bonds

Chemical bonds are the attractive forces that hold atoms together in a

complex unit.

Types of Bonds

Ionic bonds are electrostatic forces between ions.

Oppositely charged ions attract one another.

Covalent bonds form when atoms share electrons.

Bonds give shapes to molecules

Atoms are not charged

Valence Electrons

Valence electrons are the electrons in the outermost s sub-shell and electrons in any other unfilled subshell.

Valence electrons are the least tightly held electrons in an atom, and they are the ones that "do chemistry".

An element's position on the Periodic Table shows how many valence electrons it has.

Valence Electrons

Representative elements (Groups 1A – 7A) have 1 to 7 electrons.

We can represent them as dots around the element's symbol.

These are called Lewis Symbols.

Lewis Symbols of 1st 20 Elements

Stable Electron Configurations

Atoms are most stable when they have filled valence shells.

For representative elements, each atom wants 8 electrons surrounding it.

Transition metals are most stable with filled or half-filled outer s and d subshells.

Atoms will lose, gain or share electrons to reach these stable configurations.

The Octet RuleRepresentative elements want 8 electrons in

their valence shells.

Noble gases have filled valence shells.

Group 6A and 7A nonmetals gain elec-trons to fill their valence shells.

Become anions

Group 1A, 2A, and 3A metals lose elec-trons to fill their valence shells.

Become cations

Predict Charges

For ions of the following metals:

Na, Ca, Al

For ions of the following nonmetals:

F, O, P

Ionic Bonds

Ionic bonds are chemical bonds that result from the attraction of positive and negative ions for each other.

Metal cation(s) and nonmetal anion(s)

Higher charges, stronger bonds

Ionic Bonds

Ionic compounds are chemical compounds characterized by ionic bonds between atoms.

An ionic compound will have a formula such that charge neutrality is achieved.

Na1+ + Cl1- NaCl

Mg2+ + 2 Cl1- MgCl2

Al3+ + 3 Cl1- AlCl3

Structures of Ionic Compounds

Ionic bonds are non-directional electrostatic forces

Ions arrange themselves in an array (usually a crystal lattice) that

Maximizes interactions between oppositely charged ions

Minimizes interactions between ions of like charge

Structure of NaCl(2 dimensions)

Structure of NaCl(3 dimensions)

Other Crystal Structures

Details about Ionization

Ionization Energy

Electron Affinity

Covalent Bonds

Covalent bonds are chemical bonds that result when two nuclei attract and share the same electrons.

Covalent bonds form between atoms of nonmetals.

Covalent Bonds

Covalent compounds are characterized by covalent bonds between atoms.

A covalent compound will have a formula such that all the atoms share electrons in such a way that the octet rule is satisfied.

Lewis Structures

Lewis Structures are groupings of Lewis symbols that show transfer of electrons in ionic compounds or sharing of electrons in covalent compounds.

Lewis Structure of H2

Bond Distance

Lewis Structures of Diatomic Molecules

Lewis Structure for Ammonia (NH3)

Multiple Covalent Bonds

Two atoms can share more than two electrons.

Drawing Lewis Structures

Determine the number of electrons

A. Needed to give each atom an octet

except hydrogen, which needs two

B. Available (all valence electrons for

all atoms in structure)

C. Shared = Needed Available

Drawing Lewis Structures

Choose the central atom for the structure

A. NEVER hydrogen

B. CARBON if it's in the formula

C. Choose atom furthest left and/or down

on the Periodic Table

Drawing Lewis Structures

Draw structure

A. Write symbol of central atom

B. Arrange other atoms around it

C. Show shared electrons as dots between atoms

D. Show other available electrons as dots around atoms

Drawing Lewis Structures

Check structure

A. Is the right atom in the center?

B. Have the correct number of electrons

been shared?

C. Are all available electrons shown?

D. Is the octet rule satisfied?

Drawing Lewis Structures

Examples:

H2O

CO2

HCN

SO3

Drawing Lewis Structures

Resonance Structures are two or more Lewis Structures for the same species that differ only in the position of the electrons.

There must be at least one double bond in the structure for there to be resonance.

Drawing Lewis Structures

Polyatomic Ions are charged groups of atoms, held together by covalent bonds.

They often appear in parentheses in chemical formulas, e.g. Al2(SO4)3.

Draw their structures in the usual way, but account for the charge in the available electrons.

Examples:

PO43 NH4

1+ CO32

Drawing Lewis Structures

Acids of Polyatomic Anions:

Draw the structure of the anion, with hydrogen atoms bonded to the oxygen atoms.

Examples:

H3PO4

H2CO3

Molecular Geometry

Lewis Structures give numbers and types of bonds in molecules and polyatomic ions.

Lewis Structures do not convey information about the shapes of the molecules and ions.

Molecular Geometry

VSEPR:

Valence Shell Electron Pair Repulsion Theory.

VSEPR theory is an explanation of the shapes of simple molecules that uses Lewis Struc-tures. It is based on the fact that electrons repel each other, and groups of electrons will get as far away from each other as possible in a molecule or polyatomic ion.

Molecular Geometry

VSEPR electron groups are groups of valence electrons that are present in a localized region in a molecule.

Each bond is a group (single or multiple)

Each non-bonded pair is a group

If the central atom follows the octet rule, there can be 2, 3, or 4 electron groups around it.

Molecular Geometry

Electronic geometries are descriptions of the arrangement of electrons about the central atom in a molecule or ion.

# of Electron Regions Electronic about Central Atom Geometry

2 Linear

3 Trigonal Planar

4 Tetrahedral

Molecular Geometry

The Three Possible Electronic Geometries (where the electrons are)

Molecular GeometryWhat? That's not where you told me the

atomic orbitals were!

Nope! Now we're looking at molecules, with hybrid orbitals.

Hybrid orbitals are combinations of atomic orbitals. These give rise to the shapes of molecules and polyatomic ions.

A specific type of hybrid orbital is associated with each electronic geometry.

Linear Electronic Geometry, sp Hybridization

Trigonal Planar Electronic Geometry, sp2 Hybridization

Tetrahedral Electronic Geometry, sp3 Hybridization

Molecules with Linear Electronic Geometry, sp Hybridization

Molecular Geometry is Linear

CO2 (carbon dioxide)

HCN (hydrogen cyanide)

C2H2 (acetylene)

Molecules with Trigonal Planar Electronic Geometry, sp2

Hybridization

Molecular Geometry

SO3 (sulfur trioxide) Trigonal Planar

SO2 (sulfur dioxide) Angular/Bent

Molecules with Tetrahedral Electronic Geometry, sp3 Hybridization

Molecular Geometry

CH4 (methane) Tetrahedral

NH3 (ammonia) Trigonal Pyramidal

H2O (water) Angular/Bent

Molecular Geometry

Examples:

CCl4 (carbon tetrachloride)

H3O1+ (hydronium ion)

C2H4 (ethylene)

Types of Bonds (sigma) bonds result from end-to-end overlap

of orbitals, electron density is on bond axis.

Types of Bonds (pi) bonds result from parallel overlap of orbit-

als; there is no electron density on bond axis.

Types of Bonds

There can be more than one bond between two atoms.

Types of BondsResonance is really a bond distributed over

several bonds, as in the carbonate anion

Bond PolarityWe've looked at ionic bonds and covalent

bonds. Actually, there's a continuum, and most bonds fall in the middle.

Bond Polarity

Representing bond polarity:

+

Electronegativities

Bond Classification

Look up electronegativities of atoms, calculate difference between them

Difference: Bond Type:

0.0 to 0.4 Nonpolar covalent

0.5 to 1.9 Polar covalent

2.0 Ionic

Polarity of Molecules

A nonpolar molecule or ion has a symmet-rical distribution of electrical charge.

A polar molecule or ion has an unsymmet-rical distribution of electrical charge.

Look at polarities of individual bonds. If the polarities cancel out, the molecule is not polar. If they don't, the molecule is polar.

Polarity of Molecules

Rule:

If all the electron regions around the central atom in a simple structure are bonds,

and

all the substituent (surrounding) atoms are the same

the molecule or ion is not polar.

In any other case, it is polar.

Polarity of Molecules

Polarity of Molecules

Polarity of Molecules

Examples:

SO3 SO2

NH3 H2O

CH2O NO31