Transcript Chemical Bonds

Chapter 7.
Chemical Bonds
Chemical bonds are the attractive
forces that hold atoms together in a
complex unit.
Types of Bonds
Ionic bonds are electrostatic forces
between ions.
Oppositely charged ions attract one
another.
Covalent bonds form when atoms share
electrons.
Bonds give shapes to molecules
Atoms are not charged
Valence Electrons
Valence electrons are the electrons in the
outermost s sub-shell and electrons in any
other unfilled subshell.
Valence electrons are the least tightly held
electrons in an atom, and they are the
ones that "do chemistry".
An element's position on the Periodic Table
shows how many valence electrons it has.
Valence Electrons
Representative elements (Groups 1A – 7A)
have 1 to 7 electrons.
We can represent them as dots around the
element's symbol.
These are called Lewis Symbols.
Lewis Symbols of 1st 20 Elements
Stable Electron Configurations
Atoms are most stable when they have filled
valence shells.
For representative elements, each atom
wants 8 electrons surrounding it.
Transition metals are most stable with filled
or half-filled outer s and d subshells.
Atoms will lose, gain or share electrons to
reach these stable configurations.
The Octet Rule
Representative elements want 8 electrons in
their valence shells.
Noble gases have filled valence shells.
Group 6A and 7A nonmetals gain electrons to fill their valence shells.
Become anions
Group 1A, 2A, and 3A metals lose electrons to fill their valence shells.
Become cations
Predict Charges
For ions of the following metals:
Na,
Ca,
Al
For ions of the following nonmetals:
F,
O,
P
Ionic Bonds
Ionic bonds are chemical bonds that result
from the attraction of positive and negative
ions for each other.
Metal cation(s) and nonmetal anion(s)
Higher charges, stronger bonds
Ionic Bonds
Ionic compounds are chemical compounds
characterized by ionic bonds between atoms.
An ionic compound will have a formula such
that charge neutrality is achieved.
Na1+ + Cl1-  NaCl
Mg2+ + 2 Cl1-  MgCl2
Al3+ + 3 Cl1-  AlCl3
Structures of Ionic Compounds
Ionic bonds are non-directional electrostatic
forces
Ions arrange themselves in an array (usually
a crystal lattice) that
Maximizes interactions between oppositely
charged ions
Minimizes interactions between ions of like
charge
Structure of NaCl
(2 dimensions)
Structure of NaCl
(3 dimensions)
Other Crystal Structures
Details about Ionization
Ionization Energy
Electron Affinity
Covalent Bonds
Covalent bonds are chemical bonds that
result when two nuclei attract and share
the same electrons.
Covalent bonds form between atoms of
nonmetals.
Covalent Bonds
Covalent compounds are characterized by
covalent bonds between atoms.
A covalent compound will have a formula
such that all the atoms share electrons in
such a way that the octet rule is satisfied.
Lewis Structures
Lewis Structures are groupings of Lewis
symbols that show transfer of electrons in
ionic compounds or sharing of electrons
in covalent compounds.
Lewis Structure of H2
Bond Distance
Lewis Structures of Diatomic
Molecules
Lewis Structure for Ammonia (NH3)
Multiple Covalent Bonds
Two atoms can share more than two electrons.
Drawing Lewis Structures
Determine the number of electrons
A. Needed to give each atom an octet
except hydrogen, which needs two
B. Available (all valence electrons for
all atoms in structure)
C. Shared = Needed  Available
Drawing Lewis Structures
Choose the central atom for the structure
A. NEVER hydrogen
B. CARBON if it's in the formula
C. Choose atom furthest left and/or down
on the Periodic Table
Drawing Lewis Structures
Draw structure
A. Write symbol of central atom
B. Arrange other atoms around it
C. Show shared electrons as dots
between atoms
D. Show other available electrons as
dots around atoms
Drawing Lewis Structures
Check structure
A. Is the right atom in the center?
B. Have the correct number of electrons
been shared?
C. Are all available electrons shown?
D. Is the octet rule satisfied?
Drawing Lewis Structures
Examples:
H 2O
CO2
HCN
SO3
Drawing Lewis Structures
Resonance Structures are two or more
Lewis Structures for the same species
that differ only in the position of the
electrons.
There must be at least one double bond
in the structure for there to be
resonance.
Drawing Lewis Structures
Polyatomic Ions are charged groups of
atoms, held together by covalent bonds.
They often appear in parentheses in
chemical formulas, e.g. Al2(SO4)3.
Draw their structures in the usual way,
but account for the charge in the
available electrons.
Examples:
PO43 NH41+ CO32
Drawing Lewis Structures
Acids of Polyatomic Anions:
Draw the structure of the anion, with
hydrogen atoms bonded to the
oxygen atoms.
Examples:
H3PO4
H2CO3
Molecular Geometry
Lewis Structures give numbers and types of
bonds in molecules and polyatomic ions.
Lewis Structures do not convey information
about the shapes of the molecules and
ions.
Molecular Geometry
VSEPR:
Valence Shell Electron Pair Repulsion Theory.
VSEPR theory is an explanation of the shapes
of simple molecules that uses Lewis Structures. It is based on the fact that electrons
repel each other, and groups of electrons will
get as far away from each other as possible in
a molecule or polyatomic ion.
Molecular Geometry
VSEPR electron groups are groups of
valence electrons that are present in a
localized region in a molecule.
Each bond is a group (single or multiple)
Each non-bonded pair is a group
If the central atom follows the octet rule,
there can be 2, 3, or 4 electron groups
around it.
Molecular Geometry
Electronic geometries are descriptions of
the arrangement of electrons about the
central atom in a molecule or ion.
# of Electron Regions
about Central Atom
2
3
4
Electronic
Geometry
Linear
Trigonal Planar
Tetrahedral
Molecular Geometry
The Three Possible Electronic Geometries
(where the electrons are)
Molecular Geometry
What? That's not where you told me the
atomic orbitals were!
Nope! Now we're looking at molecules, with
hybrid orbitals.
Hybrid orbitals are combinations of atomic
orbitals. These give rise to the shapes of
molecules and polyatomic ions.
A specific type of hybrid orbital is associated
with each electronic geometry.
Linear Electronic Geometry,
sp Hybridization
Trigonal Planar Electronic Geometry,
sp2 Hybridization
Tetrahedral Electronic Geometry,
sp3 Hybridization
Molecules with Linear Electronic
Geometry, sp Hybridization
Molecular Geometry is Linear
CO2 (carbon dioxide)
HCN (hydrogen cyanide)
C2H2 (acetylene)
Molecules with Trigonal Planar
Electronic Geometry, sp2
Hybridization
Molecular Geometry
SO3 (sulfur trioxide)
Trigonal Planar
SO2 (sulfur dioxide)
Angular/Bent
Molecules with Tetrahedral Electronic
Geometry, sp3 Hybridization
Molecular Geometry
CH4 (methane)
Tetrahedral
NH3 (ammonia)
Trigonal Pyramidal
H2O (water)
Angular/Bent
Molecular Geometry
Examples:
CCl4 (carbon tetrachloride)
H3O1+ (hydronium ion)
C2H4 (ethylene)
Types of Bonds
 (sigma) bonds result from end-to-end overlap
of orbitals, electron density is on bond axis.
Types of Bonds
 (pi) bonds result from parallel overlap of orbitals; there is no electron density on bond axis.
Types of Bonds
There can be more than one  bond
between two atoms.
Types of Bonds
Resonance is really a  bond distributed over
several  bonds, as in the carbonate anion
Bond Polarity
We've looked at ionic bonds and covalent
bonds. Actually, there's a continuum, and
most bonds fall in the middle.
Bond Polarity
Representing bond polarity:
+

Electronegativities
Bond Classification
Look up electronegativities of atoms,
calculate difference between them
Difference:
0.0 to 0.4
0.5 to 1.9
 2.0
Bond Type:
Nonpolar covalent
Polar covalent
Ionic
Polarity of Molecules
A nonpolar molecule or ion has a symmetrical distribution of electrical charge.
A polar molecule or ion has an unsymmetrical distribution of electrical charge.
Look at polarities of individual bonds. If the
polarities cancel out, the molecule is not
polar. If they don't, the molecule is polar.
Polarity of Molecules
Rule:
If all the electron regions around the central
atom in a simple structure are bonds,
and
all the substituent (surrounding) atoms are
the same
the molecule or ion is not polar.
In any other case, it is polar.
Polarity of Molecules
Polarity of Molecules
Polarity of Molecules
Examples:
SO3
SO2
NH3
H 2O
CH2O
NO31