Chapter 5 Molecular Compounds

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Chapter 5 Molecular Compounds Covalent
Bonding Shapes of molecules
Electronegativity and polarity
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Covalent Bonds
A bond where electrons are shared between atoms
is a covalent bond.


Previously, it was shown that atoms could attain
a stable octet of electrons through ionization.
The combination of oppositely charged ions
results in an ionic compound. In ionic bonds,
valence electrons on the anion remain localized,
and there is very little sharing of electrons
with the cation.





Na Cl ? Na Cl-









The bonding between sodium cations and chloride
anions is almost purely electrostatic (based on
the attraction of opposite charge).

H H ? H H



The bonding between hydrogen atoms is covalent.
Each atom contributes its valence electron to the
covalent bond. The electrons count towards both
of the hydrogen atoms in filling the valence
electron shell.
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Covalent Bonds
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Covalent Bonds
Why do covalent bonds occur? The electrons
occupy a larger cloud in the covalent bond than
in the individual atomic orbitals. This larger
cloud is more stable than the smaller clouds.
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Covalent Bonds

• Covalent bonds result when electrons in atomic
  orbitals are combined into a bonding orbital.
• Each bonding orbital can contain two electrons.
• Each atom has a stable octet of electrons.

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Covalent Bonds
In molecules, the bonding is covalent. The
stable octet configuration of electrons is
achieved by sharing electrons. How many
electrons will an atom contribute to form
covalent bonds?
The metals are not represented Remember that
metals have low ionization energies, and readily
form cations (and often form ionic compounds with
other elements). Metals do form covalent bonds
however, their treatment is reserved for a later
discussion.
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Covalent Bonds
Examples combine hydrogen atoms with oxygen,
nitrogen, or carbon atoms to form stable
molecules
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Exceptions to the Octet Rule
Elements on the 3rd row or beyond often violate
the octet rule. These elements have empty
d-orbitals that can participate in
bonding. Boron molecules often have empty atomic
orbitals, and do not have a stable octet of
electrons.
(draw the Lewis dot structures)
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Multiple Covalent Bonds
In some cases, stable molecules are formed by
sharing more than two electrons between two
atoms. Consider the oxygen molecule (O2) , which
we all breathe in and depend on for life
double bond
lone pair

The molecule O2 contains a double bond that is
represented with two lines connecting the atoms.
Each bond contains two electrons, so a total of
four electrons are counted in this bond. Each
oxygen atom has a stable octet of electrons.
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Multiple Covalent Bonds
Ethylene (C2H4) two carbon atoms, each
contribute four valence electrons four hydrogen
atoms, each contribute one valence electron
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Coordinate Covalent Bonds
A covalent bond that results when a lone pair of
electrons is donated to a vacant orbital is a
coordinate covalent bond.
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Chemical Formulas
A molecular formula indicates the atoms and
numbers of each atom that contribute to form the
molecule. The molecular formula does not provide
information about the connectivity of the atoms
in the molecule. Structural formulas show how
atoms are connected in the molecule. H2O
vs. A molecular formula differs from a
formula unit (for ionic compounds) since it is
not the simplest ratio of atoms that make up the
substance.
Ex The molecule ethylene contains two carbon
atoms and four hydrogen atoms in each molecule.
Its molecular formula is C2H4, not CH2
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Formal Charge
Given a Lewis dot representation for a molecule,
one can readily count the number of electrons
that originate from each atom. If the number
of electrons that originate from an atom is equal
to the number of valence electrons for the
neutral atom, then the formal charge of that atom
is zero. If the number of electrons that
originate from an atom is less than the number of
valence electrons of the neutral atom, then the
formal charge ( valence e- of neutral atom -
e- that originate from the atom in the
molecule) If the number of electrons that
originate from an atom is more than the number of
valence electrons of the neutral atom, then the
formal charge -( e- that originate from the
atom in the molecule - valence e- of neutral
atom)
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Formal Charge
Consider water, hydroxide ion, and hydronium
ion H2O OH- H3O
What is the formal charge of the oxygen atom in
these structures? water hydroxide hydronium va
lence e- neutral O atom 6 6 6 e-
originating from O in the
6 7 5 structure formal charge
0 -1 1 of O
lone pair e- ½ bond pair e- unpaired e-
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Lewis Dot Structures
Lewis dot structures are a way to write a
structural formula where all of the valence
electrons are represented (including shared and
unshared electrons).

• A general guide
• Count the total number of valence electrons of
  all atoms in the molecule or ion.
• Draw a line between each pair of connected atoms
  to represent the two electrons in a covalent
  bond.
• Add lone pairs so that each peripheral atom
  (except H) connected to the central atom gets an
  octet.
• Place all remaining electrons in lone pairs on
  the central atom.
• If the central atom does not yet have an octet
  after all electrons have been assigned, take a
  lone pair from a neighboring atom and form a
  multiple bond to the central atom.
• Count the electron contribution from each atom to
  determine its formal charge. The formal charge
  of all atoms should be as close to zero as is
  possible.

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Lewis Dot Structures
trichlorophosphine, PCl3
ozone, O3
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Lewis Dot Structures
carbon dioxide, CO2
nitrate ion, NO3-
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Shapes of Molecules
Molecules are three-dimensional! Shape is an
important contributor to the chemical properties
of a molecule. VSEPR Theory Valence-shell
electron-pair repulsion Electron pairs (whether
they are lone pairs or bonding pairs) repel one
another, and therefore occupy points in space
directionally as far apart as is possible within
the coordination sphere of the central atom.
Imagine this ice-skater is holding negative
charge (electron pairs) in her hands and feet.
She looks very much like a tetrahedral molecule.
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VSEPR

• Draw a Lewis dot structure of the molecule, and
  identify the atom whose geometry is of interest.
• Count the number of electron charge clouds
  surrounding the atom of interest.
• lone pairs
• bond pairs (a single bond counts as one electron
  charge cloud, a double bond counts as one
  electron cloud, a triple bond counts as one
  electron cloud)
• unpaired electron
• Predict the molecular shape by assuming that the
  charge clouds orient in space so that they are as
  far away from one another is possible.

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VSEPR
At what point in space will an electron charge
cloud occupy in order to be as far apart from the
other electron clouds? Revisit geometry
3 charge clouds take up positions at the ends of
an equilateral triangle they are co-planar
2 charge clouds (blue) take up positions at
opposite ends of an axis through the central atom
(red)
5 charge clouds occupy the corners of a trigonal
bipyramid
6 charge clouds occupy the corners of an
octahedron
4 charge clouds take up positions at the corners
of a tetrahedron
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VSEPR
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VSEPR
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Sharing of electrons in covalent bonds
If the atoms involved in the covalent bond are
identical, then the sharing of electrons is
equal. If the atoms in the covalent bond are
different, then the distribution of the electron
density will be shifted towards one of the
atoms. In the latter case, the electrons
are attracted to one atom more than the other,
and the bond is polar.
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Electronegativity
Electronegativity is the power of an atom in a
molecule to attract electrons to itself. An
element with higher electronegativity strongly
attracts electrons, while elements with low
electronegativity weakly attract electrons. If a
covalent bond is formed between a highly
electronegative element and an element with low
electronegativity, then the electron density in
the bond will be concentrated toward the more
electronegative element. This result is called
polarization of the bond (polar covalent bond).
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Electronegativity
Periodic trends in electronegativity
increasing electronegativity
decreasing electronegativity
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Bond Polarity
The difference in the electronegativity of the
atoms that participate in bonding can be a useful
guide in classification of the bond If the
difference of electronegativity is lt 0.5, then
the bond is non-polar (to slightly polar)
covalent If the difference of
electronegativity is between 0.5 and 1.9, then
the bond is highly polar with significant
covalent and ionic character If the difference
of electronegativity is 2.0 or higher, then the
bond is predominantly ionic . This is a rough
guide. The division between ionic and covalent
is not a clear-cut line. In reality, chemical
bonds fall within a spectrum ranging from
completely covalent to completely ionic, having
partial covalent and ionic character.
Difference in Electronegativity Ionic Character
0 0.5 1.0 1.5 2.0
2.5 3.0
0 6 22 43 63
79 89
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Bond Polarity
The CO bond is more polar than the CN bond.
Examples
? ?-
? ?-
HH CN CO
EN of H 2.1 2.1 2.1 0 non-polar covalent
EN of C 2.5 EN of C 2.5 EN of N 3.0 EN of
O 3.5 3.0 2.5 0.5 3.5 2.5 1.0 polar
covalent polar covalent
NaCl- RbF-
EN of Na 0.9 EN of Rb 0.8 EN of Cl 3.0 EN
of F 4.0 3.0 0.9 2.1 4.0 0.8
3.2 ionic almost completely ionic
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Electrostatic Potential Map
Uses color to portray the calculated electron
distribution in a molecule.
Consider the molecule, hydrogen chloride (HCl)
Electronegativities H 2.1, Cl 3.5
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Polar bonds and Polar molecules
HCl
The bond is polar. The molecule is polar.
HCl
The molecule has a dipole moment (represented
with the arrow). The dipole moment is the sum
of all of the individual bond dipoles.
Each CCl bond is polar, but the four bond
dipoles cancel each other out directionally
all bond dipoles are of the same value the
directions of the bond dipoles exactly cancel
There is no net dipole in the molecule this
molecule is not polar.
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Polar Molecules
If a molecule has a dipole moment, it is
polar. One can attempt to visualize polarity in
molecules by the following if it is possible to
slice a molecule across a plane that separates an
electron rich section from an electron poor
section, then the molecule is polar.
Water is a polar molecule.
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Polarity of Molecules
Are the following molecules polar?
carbon dioxide, CO2
carbon tetrachloride, CCl4
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Naming Binary Molecular Compounds
Binary molecular compounds contain only two types
of atoms. Step 1 Name the first element in the
formula, using a prefix if needed to indicate the
number of atoms. Step 2 Name the second
element in the formula, using an ide ending as
for anions, using a prefix as needed.
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Naming Binary Molecular Compounds
Try it! N2O3 GeCl4 PCl5
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There are exceptions to these very general
characteristics.