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Ch. 8 Covalent Bonding

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Title: 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.
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