CHEMICAL BONDS

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CHM 138 BASIC CHEMISTRY

• Chapter 6

CHEMICAL BONDS
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Valence electrons are the outer shell electrons
of an atom. The valence electrons are the
electrons that participate in chemical bonding.
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Lewis Dot Symbols for the Representative Elements
Noble Gases

• Lewis dot symbol consists of the symbol of an
  element and one dot for each valence electron in
  an atom of the element.

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The Ionic Bond

• Ionic bond the electrostatic force that holds
  ions together in an ionic compound.
• Atoms of the elements with low ionization
  energies tend to form cation alkali metals and
  alkaline earth metals
• Atoms of the elements with high electron
  affinities tend to form anion halogens and
  oxygen

(LiF)
1s22s1
1s22s22p5
1s2
1s22s22p6
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A covalent bond is a chemical bond in which two
or more electrons are shared by two atoms.
Why should two atoms share electrons?
Lewis structure of F2
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• A Lewis structure
• - a representation of covalent bonding in
  which shared electron pairs are shown either as
  lines or as pairs of dots between two atoms, and
  lone pairs are shown as pairs of dots an
  individual atoms.
• Only valence electrons are shown.
• The formation of the molecules illustrates the
  octet rule.
• - Octet rule An atom other than hydrogen
  tends to form bonds until it is surrounded by
  eight valence electrons.

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Lewis structure of water


Double bond two atoms share two pairs of
electrons
or
double bonds
Triple bond two atoms share three pairs of
electrons
or
triple bond
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Lengths of Covalent Bonds
Bond Lengths Triple bond lt Double Bond lt Single
Bond
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COMPARISON OF GENERAL PROPERTIES OF IONIC
COMPOUND AND COVALENT COMPOUND
IONIC COMPOUND COVALENT COMPOUND
Solid at room temperature, high melting points Gases, liquids, low melting solids
Soluble in water Insoluble in water
Aqueous solution conduct electricity Aqueous solution do not conduct electricity
Strong electrolytes Nonelectrolytes
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Writing Lewis Structures

1. Count total number of valence e-. Add 1 for each
   negative charge. Subtract 1 for each positive
   charge.
2. Draw skeletal structure of compound showing what
   atoms are bonded to each other. Put least
   electronegative element in the center.
3. Complete an octet for all atoms except hydrogen
4. If structure contains too many electrons, form
   double and triple bonds on central atom as needed.

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Write the Lewis structure of nitrogen trifluoride
(NF3).
Step 1 N is less electronegative than F, put N
in center
Step 2 Count valence electrons N - 5 (2s22p3)
and F - 7 (2s22p5)
5 (3 x 7) 26 valence electrons
Step 3 Draw single bonds between N and F atoms
and complete octets on N and F
atoms.
Step 4 - Check, are of e- in structure equal
to number of valence e- ?
3 single bonds (3x2) 10 lone pairs (10x2) 26
valence electrons
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Write the Lewis structure of the carbonate ion
(CO32-).
Step 1 C is less electronegative than O, put C
in center
Step 2 Count valence electrons C - 4 (2s22p2)
and O - 6 (2s22p4) -2 charge 2e-
4 (3 x 6) 2 24 valence electrons
Step 3 Draw single bonds between C and O atoms
and complete octet on C and O
atoms.
Step 4 - Check, are of e- in structure equal
to number of valence e- ?
3 single bonds (3x2) 10 lone pairs (10x2) 26
valence electrons
Step 5 - Too many electrons, form double bond
and re-check of e-
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• Write the Lewis structure for
• i) NO -2
• ii) CS2
• iii) SO3

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FORMAL CHARGE
An atoms formal charge is the difference between
the number of valence electrons in an isolated
atom and the number of electrons assigned to that
atom in a Lewis structure.
total number electron assigned to atom
The sum of the formal charges of the atoms in a
molecule or ion must equal the charge on the
molecule or ion.
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Examples
formal charge on C
-1
formal charge on O
1
formal charge on C
0
formal charge on O
0
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Formal Charge and Lewis Structures

1. For neutral molecules, a Lewis structure in which
   there are no formal charges is preferable to one
   in which formal charges are present.
2. Lewis structures with large formal charges are
   less plausible than those with small formal
   charges.
3. Among Lewis structures having similar
   distributions of formal charges, the most
   plausible structure is the one in which negative
   formal charges are placed on the more
   electronegative atoms.

Which is the most likely Lewis structure for CH2O?
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Resonance Structure
A resonance structure is one of two or more Lewis
structures for a single molecule that cannot be
represented accurately by only one Lewis
structure.
What are the resonance structures of the
carbonate (CO32-) ion?
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Exceptions to the Octet Rule
The Incomplete Octet
BeH2
BF3
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Exceptions to the Octet Rule
Odd-Electron Molecules
NO
The Expanded Octet (central atom with principal
quantum number n gt 2)
SF6
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Dative Covalent Bond / Coordinate Covalent Bond

• A covalent bond in which one of the atoms donates
  both electrons.
• Examples
• - NH4, NH3AlCl3, NH3BF3

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Hybridization mixing of two or more atomic
orbitals to form a new set
of hybrid orbitals.
Hybridization

• Mix at least 2 nonequivalent atomic orbitals
  (e.g. s and p). Hybrid orbitals have very
  different shape from original atomic orbitals.
• Number of hybrid orbitals is equal to number of
  pure atomic orbitals used in the hybridization
  process.
• Covalent bonds are formed by
• Overlap of hybrid orbitals with atomic orbitals
• Overlap of hybrid orbitals with other hybrid
  orbitals

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Formation of sp3 Hybrid Orbitals
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Formation of Covalent Bonds in CH4
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sp3-Hybridized N Atom in NH3
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Formation of sp Hybrid Orbitals
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Formation of sp2 Hybrid Orbitals
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How to predict the hybridization of the central
atom?

1. Draw the Lewis structure of the molecule.
2. Count the number of lone pairs AND the number of
   atoms bonded to the central atom

of Lone Pairs of Bonded Atoms
Hybridization
Examples
2
sp
BeCl2
3
sp2
BF3
4
sp3
CH4, NH3, H2O
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sp3d
PCl5
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sp3d2
SF6
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Bonding in Ethylene, C2H4
Sigma bond (s) covalent bonds formed by
orbitals overlapping end to-end , with the
electron density between the nuclei of the
bonding atoms
Pi bond (p) a covalent bond formed by sideways
overlapping orbitals with electron density
concentrated above and below plane of nuclei of
the bonding atoms
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Another View of p Bonding in Ethylene, C2H4
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Bonding in Acetylene, C2H2
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Describe the bonding in CH2O
C 3 bonded atoms, 0 lone pairs C sp2
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Sigma (s) and Pi Bonds (p)
Single bond
1 sigma bond
1 sigma bond and 1 pi bond
Double bond
Triple bond
1 sigma bond and 2 pi bonds
How many s and p bonds are in the acetic acid
(vinegar) molecule CH3COOH?
s bonds 6
1 7
p bonds 1
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Intermolecular Forces

• Intermolecular forces attractive forces between
  molecules.
• Van der Waals forces
• - the attractive or repulsive force between
  molecules due to covalent bonds or to the
  electrostatic interaction of ions with one
  another or with neutral molecules.
• The term includes
• - permanent dipolepermanent dipole forces
• - instantaneous induced dipole-induced
  dipole
• (London dispersion force). Examples
  interaction
• between H2, Cl2, F2, CH4

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Hydrogen Bond
The hydrogen bond is a special dipole-dipole
interaction between they hydrogen atom in a polar
N-H, O-H, or F-H bond and an electronegative O,
N, or F atom.
A B are N, O, or F
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Why is the hydrogen bond considered a special
dipole-dipole interaction?