Chemical Bonding
Ionic and Covalent Bonds
Chemical Bonding: Basic Concepts
Chemical bond
- a lasting attraction between atoms, ions or molecules
that enables the formation of chemical compounds.
- the bond may result from the electrostatic force of
attraction between oppositely charged ions as in ionic bonds; or
through the sharing of electrons as in covalent bonds
Chemical Bonding: Basic Concepts
Ionic Bonds
Chemical Bonding: Basic Concepts
Ionic Bonds
Chemical Bonding: Basic Concepts
Ionic Bonds
Chemical Bonding: Basic Concepts
Ionic Bonds
Chemical Bonding: Basic Concepts
Ionic Bonds
Octet Rule:
Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence e-s
Chemical Bonding: Basic Concepts
Ionic Bonds
Chemical Bonding: Basic Concepts
Ionic Bonds
Chemical Bonding: Basic Concepts
Lewis Electron-Dot Symbol (Gilbert Newton Lewis)
consists of the element’s chemical symbol plus a dot for each
valence electron
Ex. Sulfur
Ionic compounds: Lewis structure
Ex. Sodium chloride (formation of table salt)
Lewis dot symbol representation:
Chemical Bonding: Basic Concepts
More examples
Write the lewis structure or lewis dot symbol representation of
the following ionic compounds:
1. Magnesium bromide
2. Potassium sulfide
3. Calcium chloride
Chemical Bonding: Basic Concepts
More examples
4. Aluminum oxide?
Chemical Bonding: Basic Concepts
Properties of Ionic Compounds
1.
Chemical Bonding: Basic Concepts
Properties of Ionic Compounds
2. High melting and boiling points
Chemical Bonding: Basic Concepts
Lattice Energy
- the energy required to completely separate one mole of a
solid ionic compound into its gaseous ions.
Electrostatic Potential Energy:
Example: Arrange the ionic compounds NaF, CsI, and CaO in
order of increasing lattice energy.
Chemical Bonding: Basic Concepts
Properties of Ionic Compounds
3. Brittle
Chemical Bonding: Basic Concepts
Properties of Ionic Compounds
3. Brittle
Chemical Bonding: Basic Concepts
Properties of Ionic Compounds
3. Brittle
Chemical Bonding: Basic Concepts
Properties of Ionic Compounds
4. Does not conduct electricity in solid state , but does in liquid
and aqueous state.
Chemical
Bonding:
Basic
Concepts
Properties of Ionic Compounds
Chemical Bonding: Basic Concepts
Covalent Bonding
A chemical bond formed by sharing a pair of electrons is a
covalent bond.
Examples: The hydrogen molecule and chlorine molecules
Lewis Structures:
The more common convention:
Line – shared electron pair
Dots – unshared electron pairs (lone pairs or nonbonding pairs)
Chemical Bonding: Basic Concepts
Covalent Bonding
Lewis Structures:
More Examples:
Draw the Lewis Structure of the following.
1. Hydrofluoric acid
2. Water
3. Ammonia
4. Methane
Note: For nonmetals, the number of valence electrons in a neutral atom is the
same as the group number.
Chemical Bonding: Basic Concepts
Covalent Bonding
Lewis Structures:
Answers:
Cl Cl
H F
H O
H
H N H
H
H
H C H
H
Chemical Bonding: Basic Concepts
Multiple Bonds
In many molecules, atoms attain complete octets by sharing
more than one pair of electrons.
Double bond - two electron pairs are shared by two atoms.
Ex. Oxygen molecule:
Triple bond - sharing of three pairs of electrons, such as in the
Nitrogen molecule:
Chemical Bonding: Basic Concepts
Multiple Bonds
General Rule:
The length of the bond between two atoms decreases as
the number of shared electron pairs increases.
Chemical Bonding: Basic Concepts
Drawing Lewis Structure
1. Sum the valence electrons from all atoms, taking into
account overall charge.
- from periodic table determine number of valence
electrons
- anion (add one electron)
- cation (subtract one electron)
Chemical Bonding: Basic Concepts
Drawing Lewis Structure
2. Write the symbols for the atoms, show which atoms are
attached to which, and connect them with a single bond (a
line, representing two electrons).
- chemical formulas are often written in the order in
which the atoms are connected in the molecule or ion
example: HCN
- for many polyatomic molecules and ions, central atom is
usually written first .
example: CO32- and SF4
- central atom is least electronegative
Chemical Bonding: Basic Concepts
Drawing Lewis Structure
3. Complete the octets around all the atoms bonded to the
central atom.
- hydrogen atom has only a single pair of electrons
around it.
4. Place any leftover electrons on the central atom.
5. If there are not enough electrons to give the central atom an
octet, try multiple bonds.
Chemical Bonding: Basic Concepts
Drawing Lewis Structure
Example: Draw the Lewis Structure of the ff
(a) PCl3
(b) HCN
(c) BrO3- ion
Chemical Bonding: Basic Concepts
Formal Charge and Alternative Lewis Structure
•
•
•
It is possible to draw more than one Lewis structure with the
octet rule obeyed for all the atoms.
To determine which structure is most reasonable, we use
formal charge.
Formal charge is the charge on an atom that it would have if
all the atoms had the same electronegativity.
Chemical Bonding: Basic Concepts
Formal Charge and Alternative Lewis Structure
To calculate formal charge:
• All nonbonding electrons are assigned to the
atom on which they are found.
• Half the bonding electrons are assigned to each
atom in a bond.
Formal charge is:
•
valence electrons - number of bonds - lone pair
electrons
Chemical Bonding: Basic Concepts
Formal Charge
Consider:
C N
For C:
•
•
•
There are 4 valence electrons (from periodic table).
In the Lewis structure there are 2 nonbonding electrons and 3 from the triple
bond. There are 5 electrons from the Lewis structure.
Formal charge: 4 - 5 = -1.
Prentice Hall © 2003
Chapter 8
Chemical Bonding: Basic Concepts
Formal Charge
Consider:
C N
For N:
•
•
•
There are 5 valence electrons.
In the Lewis structure there are 2 nonbonding electrons and 3 from the triple
bond. There are 5 electrons from the Lewis structure.
Formal charge = 5 - 5 = 0.
• We write:
C N
Prentice Hall © 2003
Chapter 8
Chemical Bonding: Basic Concepts
Formal Charge and Alternative Lewis Structure
The most stable structure has:
• the lowest formal charge on each atom,
• the most negative formal charge on the most
electronegative atoms.
Chemical Bonding: Basic Concepts
Exceptions to octet rule
1. Molecules and polyatomic ions containing an odd number of
electrons
ex:
Chemical Bonding: Basic Concepts
Exceptions to octet rule
1. Molecules and polyatomic ions containing an odd number of
electrons
ex:
Chemical Bonding: Basic Concepts
Exceptions to octet rule
2. Molecules and polyatomic ions in which an atom has fewer
than an octet of valence electrons.
Example. BF3
Chemical Bonding: Basic Concepts
Exceptions to octet rule
2. Molecules and polyatomic ions in which an atom has fewer
than an octet of valence electrons.
Example. BF3
Chemical Bonding: Basic Concepts
Exceptions to octet rule
3. Molecules and polyatomic ions in which an atom has more
than an octet of valence electrons.
Ex. PF5 and SF4
- Hypervalent molecules
Chemical Bonding: Basic Concepts
Exceptions to octet rule
3. Molecules and polyatomic ions in which an atom has more
than an octet of valence electrons.
Ex. PF5 and SF4
- Hypervalent molecules
Chemical Bonding: Basic Concepts
Properties of Covalent Molecular Compounds
1. Liquid or gas at room temperature. They are soft and
squishy in solid form
2. Have low melting and boiling points
3. Do not conduct electricity
4. Don’t dissolve well in water
Chemical Bonding: Basic Concepts
Bond Polarity and Electronegativity
• In a covalent bond, electrons are shared.
• Sharing of electrons to form a covalent bond does not imply
equal sharing of those electrons.
• There are some covalent bonds in which the electrons are
located closer to one atom than the other.
• Unequal sharing of electrons results in polar bonds.
Chemical Bonding: Basic Concepts
Bond Polarity and Electronegativity
Bond polarity is a measure of how equally or unequally the
electrons in any covalent bond are shared.
Nonpolar covalent bond – when electrons are shared equally, as
in Cl2 and N2.
Polar covalent bond - one of the atoms exerts a greater
attraction for the bonding electrons than the other
We can use the difference in electronegativity between two
atoms to determine the polarity of the bond the atoms form.
(If the difference is very large then the bond is ionic).
Chemical Bonding: Basic Concepts
Bond Polarity and Electronegativity
Electronegativity: The ability of one atoms in a
molecule to attract electrons to itself.
• Pauling set electronegativities on a scale from 0.7
(Cs) to 4.0 (F).
• Electronegativity increases
• across a period and
• down a group.
Chemical Bonding: Basic Concepts
Bond Polarity and Electronegativity
Difference in electronegativity is a gauge of bond
polarity:
• electronegativity differences around 0 result in
non-polar covalent bonds (equal or almost equal
sharing of electrons);
• electronegativity differences around 2 result in
polar covalent bonds (unequal sharing of
electrons);
• electronegativity differences around 3 result in
ionic bonds (transfer of electrons).
Chemical Bonding: Basic Concepts
Bond Polarity and Electronegativity
Example:
The positive end (or pole) in a polar bond is represented + and
the negative pole -.
Chemical Bonding: Basic Concepts
Bond Polarity and Electronegativity
Example: In each case, which bond is more polar?
(a) B-Cl or C-Cl
(b) P-F or P-Cl.