Ch. 8 Covalent Bonding

1
Ch. 8 Covalent Bonding

• Pre AP Chemistry

2
I. Molecular Compounds

• A. Molecules Molecular Formulas
• 1. Another way that atoms can combine is by
  sharing electrons.
• 2. Bonds formed by sharing electrons are called
  covalent bonds.
• 3. A neutral group of atoms joined together by
  covalent bonds are called molecules.
• 4. Atoms of the same element that are bonded
  covalently are called diatomic molecules.
• 5. Compounds composed of molecules are called
  molecular compounds and are represented by
  molecular formulas.

3
I. Molecular CompoundsB. Comparing Molecular
Ionic Compounds

• 1. Ionic compounds have high electronegativity
  differences while molecular compounds have small
  differences in electronegativity.
• 2. Ionic compounds tend to be crystalline solids
  with high melting points and are nonconductive,
  however, in the molten (liquid) state, ionic
  compounds will conduct electricity.
• 3.Molecular compounds are mostly liquids and
  gases at room temperature and do not conduct
  electricity.
• 4. Ionic compounds are soluble in water while
  molecular compounds are not soluble in water.

4
II. The Nature of Covalent BondingA. The Octet
Rule in Covalent Bonding

• In covalent bonds, electron sharing usually
  occurs so that the atoms attain a noble gas
  configuration.
• For all atoms that form covalent bonds, except
  for hydrogen, eight electrons represents a full
  outer level.
• In a Lewis dot structure, the outer shell
  electrons are arranged as dots around the
  element symbol.
• There are two types of electron pairs
• a. Shared pairs - electrons involved in
  bonding.b. Unshared pairs - electrons not
  involved in bonding.
• A single shared pair form a single bond.
• Pairs of unshared electrons are also called lone
  pairs.
• In order to describe the shape of a molecule,
  you need to draw a Lewis dot structure.

5
II. The Nature of Covalent BondingA. The Octet
Rule in Covalent Bonding

• Atoms sometimes share more than one pair of
  electrons to attain noble gas configuration.
• There are two types of bonds
• a. Double bonds.b. Triple bonds.
• Double bonds contain two shared pairs of
  electrons.
• Triple bonds contain three shared pairs of
  electrons.
• Double bonds consist of four electrons in between
  the bonded atoms.
• Triple bonds consist of six electrons in between
  the bonded atoms.
• Double and triple bond compounds are more
  reactive than compounds with only single bonds.

6
II. The Nature of Covalent BondingB. Coordinate
Covalent Bonds

1. It is possible for atoms to achieve the noble gas
   configuration by type of bonding called
   coordinate covalent bonding.
2. A covalent bond in which one atoms contributes
   both bonding electrons is a coordinate covalent
   bond.
3. In a coordinate covalent bond, the shared
   electron pair comes from one of the bonding
   atoms.
4. This type of bonding is evident in polyatomic
   ions which are a tightly bound group of atoms
   that has a positive or negative charge and behave
   as a unit.

7
II. The Nature of Covalent BondingC. Exception
to the Octet Rule

1. For some molecules or ions, it is impossible to
   draw structures that satisfy the octet rule.
2. The octet rule cannot be satisfied in molecule
   whose total number of valence electrons is an odd
   number.
3. There are also molecules in which an atom has
   less, or more, than a complete octet of valence
   electrons.
4. An unpaired electron that is left over or an atom
   that does not aquire less than an octet rule
   example of not meeting the octet rule.

8
II. The Nature of Covalent Bonding D. Bond
Dissociation Energies

1. The total energy required to break the bond
   between two covalently bonded atoms is known as
   bond dissociation energies.
2. Compounds with only single C-H and C-C bonds are
   quite unreactive because the dissiociation
   energies are high.

9
II. The Nature of Covalent Bonding E. Resonance

1. Resonance structures are structures that occur
   when it is possible to write two or more valid
   electron dot formula that have the same number of
   electrons pairs for a molecule or ion.
2. Electron pairs do not resonate back and forth
   between the different structures but are actually
   a hybrid.

10
III. Bonding TheoriesA. Molecular Orbitals

1. When two atoms combine, this model assumes that
   the atomic orbitals overlap to produce molecular
   orbitals, which are orbitals that apply to entire
   molecule.
2. There are two types of bondsa. sigma bonds.b.
   pi bonds.
3. When two orbitals (s or p) form a bond that lies
   directly along the axis, it is called a sigma
   bond ( s ).
4. When p orbitals overlaps, they tend to be
   sideways (parallel), they form a pi bond ( p ).
5. 5. Pi bonds hybridize to sp2 orbitals.

11
III. Bonding Theories B. VSEPR Theory

• The behavior of molecules depends on their
  structural characteristics and electron
  configurations.
• There are two ways in which structure can account
  for the shape of a molecule
• repulsive forces
• atomic orbital overlap.
• A molecular shape considers the different ways s
  and p orbitals can overlap when electrons are
  shared.
• The resulting electron cloud will occupy more
  space than a single bond.
• Thus, the bond angle between the other atoms will
  decrease to allow room for the multiple bond.

12
III. Bonding Theories B. VSEPR Theory

• Electrons carry negative charges.
• Electron forces spread as far apart as possible
  to minimize repulsive forces.
• The repulsion between two unshared pairs is
  greater than the repulsion of an unshared pair
  and a shared pair.
• The repulsion between an unshared pair and a
  shared pair is greater than the repulsion of two
  shared pairs.
• There are six common shapes
• a. Linear triatomic
• b. Bent triatomic
• c. Trigonal planar
• d. Pyramidal
• e. Tetrahedral
• f. Trigonal bipyramidal

13
III. Bonding Theories C. Hybrid Orbitals

1. VSPER theory works well when accounting for
   molecular shapes, but it does not help much in
   describing the types of bonds formed.
2. Orbital hybridization provides information about
   both molecule bonding and molecular shape.
3. Several atomic orbitals mix to form the same
   total number of equivalent hybrid orbitals.
4. When an s orbital and p orbital hybridize, they
   form four identical sp3 orbitals which form
   single bonds.
5. When a 2s orbital and two 2p orbitals combine,
   they form two sp2 orbitals that form double (p)
   pi bonds.
6. When a 2s orbital and a 2p orbital combine, they
   form one sp orbital which form triple p pi bonds.

14
IV. Polar Bonds and MoleculesA. Bond Polarity

1. An atoms electronegativity is its ability to
   attract the electrons involved in bonding.
2. Because of the differences in electronegativity
   of elements, in a covalent bond between different
   elements, one atom attracts the shared pair of
   electrons much more than the other atom.
3. This type of molecule is polar covalent.
4. When the two atoms share the electrons equally,
   the bond is a nonpolar covalent bond.
5. The presence of a polar bond in a molecule often
   makes the entire molecule.
6. Because polar molecule have a positive and
   negative pole, a polar molecule is called a
   dipole ( or has a dipole moment).
7. The dipole moment is the measure of the strength
   of the dipole and is the result of the
   asymmetrical charge distribution in a polar
   molecule.

15
IV. Polar Bonds and MoleculesB. Attractions
Between Molecules

1. Weak forces are involved in the attraction of
   electrons and protons of atoms.
2. Intermolecular forces account for a wide range of
   properties among covalent molecules.
3. These forces are called Van der Waals forces.
4. These forces are weaker than chemical bonds.
5. There are several types of Van der Waals
   forcesa. Dipole - Dipole forcesb. Induced
   dipole forcesc. Dispersion forces.

16
IV. Polar Bonds and MoleculesB. Attractions
Between Molecules

• Dipole - dipole forces are when two molecules of
  the same of different substances that are both
  permanent dipoles and are attracted to each
  other.
• When a dipole approaches a molecule that is not a
  dipole, it induces a dipole in the molecule and
  the molecule is now attracted to the dipole.
• Dipoles occur in polar covalent molecules with
  higher boiling and melting points than molecules
  of the same size.
• Dispersion forces occur when a nonpolar molecule
  forms a temporary dipole and induces a dipole in
  another molecule a. They are also called London
  Forces. b. Dispersion forces occur in nonpolar
  substances with low melting and boiling points.

17
IV. Polar Bonds and MoleculesB. Attractions
Between Molecules

1. Liquids that have hydrogen in the molecular
   structure experience hydrogen bonding.
2. This occurs when the hydrogen of one of the
   molecules is attracted to another atom on
   another molecule with unshared pair of electrons.
3. The attractive force of the hydrogen bond is not
   as strong as a chemical bond.

18
IV. Polar Bonds and MoleculesC. Intermolecular
Attractions and Molecular Properties

• The physical properties of a compound depend on
  the type of bonding it displays whether it is
  ionic or covalent.
• The melting and boiling points of most molecular
  compounds are low.
• However, a few molecular solids do not melt until
  the temperature reaches 1000C or higher or they
  decompose without melting.
• These are called network solids or network
  crystals and are very stable.