CHEM 110 - Section 4
Guest Instructor:
Prof. Elizabeth Gaillard
Fall 2011
Lewis Dot Symbols for
Representative Elements
Principal Types of Chemical Bonds:
Ionic and Covalent
• Ionic bond - a transfer of one or more electrons
from one atom to another
• Forms attractions due to the opposite charges of the
atoms
• Covalent bond - attractive force due to the
sharing of electrons between atoms
• Some bonds have characteristics of both types
and not easily identified as one or the other
1
Ionic Bonding
• Representative elements form ions that obey the
octet rule
• Ions of opposite charge attract each other
creating the ionic bond
• When electrons are lost by a metal and electrons
are gained by a nonmetal
– Each atom achieves a “Noble Gas” configuration
– 2 ions are formed; a cation and anion, which are
attracted to each other
Ionic Bonding
Consider the formation of NaCl
Na + Cl  NaCl
Sodium has a low
ionization energy it
readily loses this
electron
Na  Na+ + e-
Chlorine has a high
electron affinity
When chlorine gains
an electron, it gains
the Ar configuration

..
 .. 
: Cl  e   : Cl :
..
 .. 
When sodium loses the
electron, it gains the
Ne configuration
Essential Features of Ionic Bonding
• Atoms with low I.E. and low E.A. tend to form
positive ions
• Atoms with high I.E. and high E.A. tend to form
negative ions
• Ion formation takes place by electron transfer
• The ions are held together by the electrostatic
force of the opposite charges
• Reactions between metals and nonmetals
(representative elements) tend to be ionic
2
Ion Arrangement in a Crystal
• As a sodium atom loses one electron, it becomes a
smaller sodium ion
• When a chlorine atom gains that electron, it
becomes a larger chloride ion
• Attraction of the Na cation with the Cl anion
forms NaCl ion pairs that aggregate into a crystal
Covalent Bonding
Let’s look at the formation of H2:
H + H  H2
• Each hydrogen has one electron in its valance
shell
• If it were an ionic bond it would look like this:
H   H   H   H :

• However, both hydrogen atoms have an equal
tendency to gain or lose electrons
• Electron transfer from one H to another usually
will not occur under normal conditions
• Instead, each atom attains a noble gas
configuration by sharing electrons
H  H  H :H
Each hydrogen
atom now has two
electrons around it
and attained a He
configuration
The shared
electron
pair is called a
Covalent Bond
3
Covalent Bonding in Hydrogen
Features of Covalent Bonds
• Covalent bonds form between atoms with
similar tendencies to gain or lose electrons
• Compounds containing covalent bonds are
called covalent compounds or molecules
• The diatomic elements have completely
covalent bonds (totally equal sharing)
– H2, N2, O2, F2, Cl2, Br2, I2
.. ..
.. ..
: F   F :  : F : F :
.. ..
.. ..
Each fluorine is
surrounded by 8
electrons – Ne
configuration
Examples of Covalent Bonding
..
..
2H    O  H : O : H
..
..
2e– from 2H
6e– from O
2e– for H
8e– for O
H
.
..
4H    C  H : C : H


H
4e– from 4H
4e– from C
2e– for H
8e– for C
4
Polar Covalent Bonding and
Electronegativity
• The Polar Covalent Bond
– Ionic bonding involves the transfer of
electrons
– Covalent bonding involves the sharing of
electrons
– Polar covalent bonding - bonds made up
of unequally shared electron pairs
somewhat positively charged
..
.. somewhat negatively charged
H  F: H : F:


• The electrons spend
These two
more time with fluorine electrons are
• This sets up a polar
not shared equally
covalent bond
• A truly covalent bond
can only occur when
both atoms are identical
Polar Covalent Bonding in Water
• Oxygen is electron rich = • Hydrogen is electron
deficient = +
• This results in unequal
sharing of electrons in the
pairs = polar covalent bonds
• Water has 2 covalent bonds
5
Electronegativity
• Electronegativity - a measure of the ability of
an atom to attract electrons in a chemical bond
• Elements with high electronegativity have a
greater ability to attract electrons than do
elements with low electronegativity
• Consider the covalent bond as competition for
electrons between 2 positive centers
– The difference in electronegativity determines the
extent of bond polarity
Electronegativities of Selected
Elements
electronegativity increases
electronegativity increases
• The most electronegative elements are found in the upper
right corner of the periodic table
• The least electronegative elements are found in the lower
left corner of the periodic table
Electronegativity Calculations
• The greater the difference in electronegativity
between two atoms, the greater the polarity of
their bond
• Which would be more polar, a H-F bond or H-Cl
bond?
• H-F … 4.0 - 2.1 = 1.9
• H-Cl … 3.0 - 2.1 = 0.9
• The HF bond is more polar than the HCl bond
6
Naming Compounds and Writing
Formulas of Compounds
• Nomenclature - the assignment of a correct
and unambiguous name to each and every
chemical compound
• Two naming systems:
– ionic compounds
– covalent compounds
Formulas of Compounds
• A formula is the representation of the
fundamental compound using chemical symbols
and numerical subscripts
– The formula identifies the number and type of the
various atoms that make up the compound unit
– The number of like atoms in the unit is shown by the
use of a subscript
– Presence of only one atom is understood when no
subscript is present
Ionic Compounds
• Metals and nonmetals usually react to form
ionic compounds
• The metals are cations and the nonmetals are
anions
• The cations and anions arrange themselves in a
regular three-dimensional repeating array called
a crystal lattice
• Formula of an ionic compound is the smallest
whole-number ratio of ions in the substance
7
Writing Formulas of Ionic Compounds
from the Identities of the Component Ions
• Determine the charge of each ion
– Metals have a charge equal to group number
– Nonmetals have a charge equal to the group number
minus eight
• Cations and anions must combine to give a
formula with a net charge of zero
• It must have the same number of positive charges as
negative charges
Predict Formulas
Predict the formula of the ionic compounds formed
from combining ions of the following pairs of
elements:
1. sodium and oxygen
2. lithium and bromine
3. aluminum and oxygen
4. barium and fluorine
Writing Names of Ionic Compounds
from the Formula of the Compound
• Name the cation followed by the name
of the anion
• A positive ion retains the name of the
element; add the anion suffix -ide
8
Writing Names of Ionic Compounds
from the Formula of the Compound
•
If the cation of an element has several ions of
different charges (as with transition metals) use a
Roman numeral following the metal name
• Roman numerals give the charge of the metal
•
Examples:
•
FeCl3 is iron(III) chloride
•
FeCl2 is iron(II) chloride
•
CuO is copper(II) oxide
Common Nomenclature System
• Use -ic to indicate the higher of the
charges that ion might have
• Use -ous to indicate the lower of the
charges that ion might have
• Examples:
• FeCl2 is ferrous chloride
• FeCl3 is ferric chloride
Stock and Common Names for
Iron and Copper Ions
9
Common Monatomic Cations and
Anions
• Monatomic ions - ions consisting of a single
charged atom
Polyatomic Ions
• Polyatomic ions - ions composed of 2 or
more atoms bonded together with an overall
positive or negative charge
– Within the ion itself, the atoms are bonded using
covalent bonds
– The positive and negative ions will be bonded to
each other with ionic bonds
• Examples:
• NH4+ ammonium ion
• SO42- sulfate ion
Common Polyatomic Cations and
Anions
10
Name These Compounds
1. NH4Cl
2. BaSO4
3. Fe(NO3)3
4. CuHCO3
5. Ca(OH)2
Writing Formulas of Ionic Compounds
from the Name of the Compound
• Determine the charge of each ion
• Write the formula so that the resulting
compound is neutral
• Example:
Barium chloride:
Barium is +2, Chloride is -1
Formula is BaCl2
Determine the Formulas from
Names
Write the formula for the following ionic
compounds:
1. sodium sulfate
2. ammonium sulfide
3. magnesium phosphate
4. chromium(II) sulfate
11
Covalent Compounds
• Covalent compounds are typically formed
from nonmetals
• Molecules - compounds characterized by
covalent bonding
• Not a part of a massive three-dimensional
crystal structure
• Exist as discrete molecules in the solid, liquid,
and gas states
Naming Covalent Compounds
1. The names of the elements are written
in the order in which they appear in
the formula
2. A prefix indicates the number of each
kind of atom
Naming Covalent Compounds
3. If only one atom of a particular element is
present in the molecule, the prefix mono- is
usually omitted from the first element
Example: CO is carbon monoxide
4. The stem of the name of the last element is
used with the suffix –ide
5. The final vowel in a prefix is often
dropped before a vowel in the stem name
12
Name These Covalent Compounds
1. SiO2
2. N2O5
3. CCl4
4. IF7
Writing Formulas of Covalent
Compounds
• Use the prefixes in the names to determine the
subscripts for the elements
• Examples:
• nitrogen trichloride
NCl3
• diphosphorus pentoxide
P2O5
• Some common names that are used:
–
–
–
–
H2O
NH3
C2H5OH
C6H12O6
water
ammonia
ethanol
glucose
Provide Formulas for These
Covalent Compounds
1. nitrogen monoxide
2. dinitrogen tetroxide
3. diphosphorus pentoxide
4. nitrogen trifluoride
13
3.3 Properties of Ionic and
Covalent Compounds
• Physical State
– Ionic compounds are usually solids at room
temperature
– Covalent compounds can be solids, liquids, and
gases
• Melting and Boiling Points
– Melting point - the temperature at which a
solid is converted to a liquid
– Boiling point - the temperature at which a
liquid is converted to a gas
Physical Properties
• Melting and Boiling Points
– Ionic compounds have much higher melting points
and boiling points than covalent compounds
– A large amount of energy is required to break the
electrostatic attractions between ions
– Ionic compounds typically melt at several hundred
degrees Celsius
• Structure of Compounds in the Solid State
– Ionic compounds are crystalline
– Covalent compounds are crystalline or amorphous –
having no regular structure
Physical Properties
• Solutions of Ionic and Covalent Compounds
– Ionic compounds often dissolve in water, where they
dissociate - form positive and negative ions in
solution
– Electrolytes - ions present in solution allowing the
solution to conduct electricity
– Covalent solids usually do not dissociate and do not
conduct electricity - nonelectrolytes
14
Comparison of Ionic vs. Covalent
Compounds
Ionic
Covalent
Composed of
Metal + nonmetal 2 nonmetals
Electrons
Transferred
Shared
Physical state
Solid / crystal
Dissociation
Yes, electrolytes
Any / crystal
OR amorphous
No,
nonelectrolytes
Low
Boiling/Melting High
Drawing Lewis Structures of
Molecules and Polyatomic Ions
Lewis Structure Guidelines
1. Use chemical symbols for the various
elements to write the skeletal structure of
the compound
–
–
–
The least electronegative atom will be placed in
the central position
Hydrogen and halogens occupy terminal
positions
Carbon often forms chains of carbon-carbon
covalent bonds
Lewis Structure Guidelines
2. Determine the number of valence electrons
associated with each atom in the compound
–
–
–
Combine these valence electrons to determine
the total number of valence electrons in the
compound
Polyatomic cations, subtract one electron for
every positive charge
Polyatomic anions, add one electron for every
negative charge
15
Lewis Structure Guidelines
3. Connect the central atom to each of the
surrounding atoms using electron pairs
• Next, complete octets of all the atoms
bonded to the central atom
• Hydrogen needs only two electrons
• Electrons not involved in bonding are
represented as lone pairs
• Total number of electrons in the structure
must equal the number of valence electrons
in step 2
Lewis Structure Guidelines
4. Count the number of electrons you have
and compare to the number you used
•
•
•
•
If they are the same, you are finished
If you used more electrons than you have,
add a bond for every two too many you used
Then, give every atom an octet
If you used less electrons than you have….see
later exceptions to the octet rule
5. Recheck that all atoms have the octet rule
satisfied and that the total number of
valance electrons are used
Drawing Lewis Structures of
Covalent Compounds
Draw the Lewis structure of carbon dioxide, CO2
Draw a skeletal structure of the molecule
1. Arrange the atoms in their most probable order
C-O-O
and/or
O-C-O
2. Find the electronegativity of O=3.5 & C=2.5
3. Place the least electronegative atom as the central
atom, here carbon is the central atom
4. Result is the O-C-O structure from above
16
Drawing Lewis Structures
5. Find the number of valence electrons for each atom
and the total for the compound
1 C atom x 4 valence electrons = 4 e2 O atoms x 6 valence electrons = 12 e16 e- total
: :
: :
6. Use electron pairs to connect the C to each O with a
single bond
O:C:O
7. Place electron pairs around the atoms
:O:C:O:
This satisfies the rule for the O atoms, but not for C
Drawing Lewis Structures of
Covalent Compounds
9.
In this structure, the octet rule is satisfied
: :
Redistribute the electrons moving 2 e- from each O,
placing them between C:O
: :
8.
C::O::C
• This is the most probable structure
• Four electrons are between C and O
• These electrons are share in covalent bonds
• Four electrons in this arrangement signify a double
bond
10. Recheck the electron distribution
• 8 electron pairs = 16 valence electrons, number
counted at start
• 8 electrons around each atom, octet rule satisfied
Lewis Structures of
Polyatomic Ions
• Prepare Lewis structures of polyatomic
ions as for neutral compounds, except:
• The charge on the ion must be
accounted for when computing the
total number of valence electrons
17
Lewis Structure of
Polyatomic Cations
Draw the Lewis structure of ammonium ion, NH4+
Draw a skeletal structure of the molecule
1.
2.
Ammonium has this structure and charge:
The total number of valence electrons is determined by
subtracting one electron for each unit of positive
charge
1 N atom x 5 valence electrons = 5 e4 H atoms x 1 valence electron = 4 e- 1 electron for +1 charge
= -1 e8 e- total
3.
Distribute these 8 e- around the skeletal structure
Lewis Structure of Polyatomic
Anions
Draw the Lewis structure of carbonate ion, CO32-
Draw a skeletal structure of the molecule
1.
Carbon is less electronegative than oxygen
•
•
2.
This makes carbon the central atom
Skeletal structure and charge:
The total number of valence electrons is determined by
adding one electron for each unit of negative charge
1 C atom x 4 valence electrons = 4 e3 O atoms x 6 valence electron = 18 e+ 2 negative charges
= 2 e24 e- total
3.
Distribute these e- around the skeletal structure
Lewis Structure of Polyatomic
Anions
Draw the Lewis structure of carbonate ion, CO324.
Distributing the electrons around the central carbon
atom (4 bonds) and around the surrounding O atoms
attempting to satisfy the octet rule results in:
5.
This satisfies the octet rule for the 3 oxygen, but not
for the carbon
Move a lone pair from one of the O atoms to form
another bond with C
6.
18
Lewis Structure, Stability, Multiple
Bonds, and Bond Energies
• Single bond - one pair of electrons are
shared between two atoms
• Double bond - two pairs of electrons are
shared between two atoms
• Triple bond - three pairs of electrons are
shared between two atoms
• Very stable
H : H or H - H
.. ..
O :: O or O  O
 
.. ..
N  N or N  N
Bond energy - the amount of energy
required to break a bond holding two
atoms together
triple bond > double bond > single bond
Bond length - the distance separating the
nuclei of two adjacent atoms
single bond > double bond > triple bond
Lewis Structures and Resonance
• Write the Lewis structure of CO32• If you look around you, you will probably see
the double bond put in different places
• In some cases it is possible to write more than
one Lewis structure that satisfies the octet rule
for a particular compound
19
..
:O:
:
:O:
::
..
:O:
:
..
..
..
..
..
: O : C :: O  : O : C : O :  : O :: C : O :




• Experimental evidence shows all bonds are
the same length, meaning there is not really
any double bond in this ion
• None of theses three Lewis structures exist,
but the actual structure is an average or
hybrid of these three Lewis structures
• Resonance - two or more Lewis structures
that contribute to the real structure
Lewis Structures and Exceptions
to the Octet Rule
1. Incomplete octet - less then eight
electrons around an atom other than H
– Let’s look at BeH2
1 Be atom x 2 valence electrons = 2 e2 H atoms x 1 valence electrons = 2 etotal 4 e– Resulting Lewis structure:
H : Be : H or H – Be – H
Odd Electron
2. Odd electron - if there is an odd number of
valence electrons, it is not possible to give
every atom eight electrons
•
Let’s look at NO, nitric oxide
•
It is impossible to pair all electrons as the
compound contains an ODD number of valence
electrons
N - O
20
Expanded Octet
3.
•
Expanded octet - an element in the 3rd period or
below may have 10 and 12 electrons around it
Expanded octet is the most common exception
•
Consider the Lewis structure of PF5
•
Phosphorus is a third period element
1 P atom x 5 valence electrons = 5 e5 F atoms x 7 valence electrons = 35 e40 e- total
•
Distributing the electrons results in this Lewis structure
Lewis Structures and Molecular
Geometry: VSEPR Theory
• Molecular shape plays a large part in
determining properties and shape
• VSEPR theory - Valance Shell Electron Pair
Repulsion theory
• Used to predict the shape of the molecules
• All electrons around the central atom arrange
themselves so they can be as far away from
each other as possible – to minimize electronic
repulsion
VSEPR Theory
• In the covalent bond, bonding electrons
are localized around the nucleus
• The covalent bond is directional, having
a specific orientation in space between
the bonded atoms
• Ionic bonds have electrostatic forces
which have no specific orientation in
space
21
Molecular Bonding
• Bonding pair = two electrons shared
by 2 atoms
– H:O
• Nonbonding pair = two electrons
belonging to 1 atom, pair not shared
– N:
• Maximal separation of bonding pairs
= 4 corners of a TETRAHEDRON
A Stable Exception to the Octet
Rule
• Consider BeH2
– Only 4 electrons surround the beryllium atom
– These 2 electron pairs have minimal repulsion
when located on opposite sides of the structure
– Linear structure having bond angles of 180°
Another Stable Exception to the
Octet Rule
• Consider BF3
– There are 3 shared electron pairs around the central
atom
– These electron pairs have minimal repulsion when
placed in a plane, forming a triangle
– Trigonal planar structure with bond angles of 120°
22
Basic Electron Pair Repulsion of
a Full Octet
• Consider CH4
– There are 4 shared electron pairs around the central
Carbon
– Minimal electron repulsion when electrons are placed at
the four corners of a tetrahedron
– Each H-C-H bond angle is 109.5°
• Tetrahedron is the primary structure of a full octet
Basic Electron Pair Repulsion of
a Full Octet with One Lone Pair
Consider NH3
• There are 4 electron pairs around the central
Nitrogen
• 3 pairs are shared electron pairs
• 1 pair is a lone pair
– A lone pair is more electronegative with a greater electron repulsion
– The lone pair takes one of the corners of the tetrahedron without being
visible, distorting the arrangement of electron pairs
• Ammonia has a trigonal pyramidal structure with 107° angles
Basic Electron Pair Repulsion of
a Full Octet with Two Lone Pairs
Consider H2O
• There are 4 electron pairs around the central Oxygen
• 2 pairs are shared electron pairs
• 2 pairs are lone pairs
– All 4 electron pairs are approximately tetrahedral to each other
– The lone pairs take two of the corners of the tetrahedron without being
visible, distorting the arrangement of electron pairs
• Water has a bent or angular structure with 104.5° bond angles
23
Predicting Geometric Shape Using
Electron Pairs
Basic Procedure to Determine
Molecular Shape
1. Write the Lewis structure
2. Count the number of shared electron pairs and
lone pairs around the central atom
3. If no lone pairs are present, shape is:
•
•
•
2 shared pairs - linear
3 shared pairs - trigonal planar
4 shared pairs - tetrahedral
4. Look at the arrangement and name the shape
•
•
•
•
•
Linear
Trigonal planar
Bent
Trigonal pyramid
Tetrahedral
More Complex Molecules
Consider dimethyl ether
• Has 2 different central atoms:
• oxygen
• carbon
– CH3 (methyl group) has tetrahedral geometry (like methane)
– Portion of the molecule linking the two methyl groups would bond
angles similar to water
24
Determine the Molecular
Geometry
• PCl3
• SO2
• PH3
• SiH4
Lewis Structures and Polarity
• A molecule is polar if its centers of positive and
negative charges do not coincide
• Polar molecules when placed in an electric field
will align themselves in the field
• Molecules that are polar behave as a dipole (having
two “poles” or ends)
• One end is positively charged the other is negatively
charged
• Nonpolar molecules will not align themselves in
an electric field
Determining Polarity
To determine if a molecule is polar:
• Write the Lewis structure
• Draw the geometry
• Use the following symbol to denote the polarity
of each bond
Positive end of the
bond, the less
electronegative atom
Negative end of the bond,
more electronegative atom
attracts the electrons more
strongly towards it
25
Practice Determining Polarity
Determine whether the following bonds and
molecules are polar:
1. Si – Cl
1. O2
2.
H–C
2. HF
3.
C–C
3. CH4
4.
S – Cl
4. H2O
Properties Based on Electronic
Structure and Molecular Geometry
• Intramolecular forces – attractive forces
within molecules – Chemical bonds
• Intermolecular forces – attractive forces
between molecules
• Intermolecular forces determine many
physical properties
– Intermolecular forces are a direct consequence
of the intramolecular forces in the molecules
Solubility and Intermolecular
Forces
Solubility - the maximum amount of solute
that dissolves in a given amount of
solvent at a specific temperature
• “Like dissolves like”
– Polar molecules are most soluble in polar
solvents
– Nonpolar molecules are most soluble in
nonpolar solvents
• Does ammonia, NH3, dissolve in water?
• Yes, both molecules are polar
26
Interaction of Water and
Ammonia
• The - end of ammonia, N, is attracted to the + end of
the water molecule, H
• The + end of ammonia, H, is attracted to the - end of
the water molecule, O
• The attractive forces, called hydrogen bonds, pull
ammonia into water, distributing the ammonia
molecules throughout the water, forming a
homogeneous solution
Interaction of Water and Oil
• What do you know about oil
and water?
– “They don’t mix”
• Why?
– Because water is polar and
oil is nonpolar
• Water molecules exert their
attractive forces on other water
molecules
• Oil remains insoluble and
floats on the surface of the
water as it is less dense
Boiling Points of Liquids
and Melting Points of Solids
• Energy is used to overcome the
intermolecular attractive forces in a
substance, driving the molecules into a
less associated phase
• The greater the intermolecular force, the
more energy is required leading to
– Higher melting point of a solid
– Higher boiling point of a liquid
27
Factors Influencing Boiling and
Melting Points
• Strength of the attractive force holding the
substance in its current physical state
• Molecular mass
• Larger molecules have higher m.p. and b.p. than
smaller molecules as it is more difficult to convert a
larger mass to another phase
• Polarity
• Polar molecules have higher m.p. and b.p. than
nonpolar molecules of similar molecular mass due to
their stronger attractive force
Melting and Boiling Points –
Selected Compounds by Bonding Type
28