صفحه 1:
Bonding
صفحه 2:
Objectives
= Define chemical bond
® Distinguish among the following bond
types based upon how electrons are
involved: pure covalent, polar covalent,
coordinate covalent, ionic, and metallic
= Draw Lewis Structures for simple
molecules.
صفحه 3:
The Chemical Bond
= Simply put, a chemical bond is the
attractive force that holds two atoms
together.
= Chemical bonds always describe the
interactions of valence electrons
between two adjacent atoms.
= Different types of interactions result
in different types of chemical bonds.
صفحه 4:
Types of Chemical Bonds:
- 00
= Metallic
= Covalent
صفحه 5:
The lonic Bond
= lonic Bonds occur between a
metal and a non-metal
= When two atoms combine by
transfer of electrons, ions are
produced. The opposite charges
of the ions hold them together.
= This electrostatic force, created
by electron transfer, is called an
fonic bond.
صفحه 6:
3
= 2Na + Cl, ---> 2Na* + 2CI
= If a chloride ion and a sodium ion are
brought together, there will be an
attractive force between them. If the ions
are brought almost into contact, the force
will be great enough to hold the two ions
together.
صفحه 7:
lonic Bonding
= lonic compounds are formed
during the transfer of electrons
between many atoms in a solid
state.
= A crystal lattice of regularly
organized, ی pattern of
cations and anions forms
= The effective charge of the
compound is zero
صفحه 8:
lonic Bonding
= Elements can be assigned
oxidation numbers for ionic
bonding.
Sulfur, with 6 valence electrons, 9 ی
will tend to gain 2 electrons to attain
the stable octet configuration. The
oxidation number of sulfur for ionic
bonding is 2-. The negative two is its
electric charge after gaining two
electrons.
صفحه 9:
lonic Bonding
۳11۶ compounds are characterized
= high melting points
® ability to conduct electricity in
the molten state
= high solubility in water
= ability to crystallize as sharply
defined particles
صفحه 10:
Bond Funnies
ANOTHER CASUALTY IN THE WAR OF THE SODIUM ATOMS
صفحه 11:
Bond Funnies
en would mind telling m.
what it sou w that you find so attractive
صفحه 12:
Metallic Bonding
= A metallic bond is defined as the
attraction of a metallic cation for
delocalized electrons.
صفحه 13:
Electron Sea Model
= All the metal atoms contribute
their valence (outer energy level)
electrons to form a “sea” of
electrons
= The valence electrons are not held
by any specific atom and can move
freely from one atom to the next
= Because they are free to move
they are called delocalized
electrons
صفحه 14:
Electron Sea Model
Alkali metal Alkaline earth meta
صفحه 15:
Metallic Bonding
= Because the electrons are no longer
held to one atom, metallic cations
are formed.
= Each metal aad is bonded to the
other metal cations in the lattice,
via the surrounding sea of electrons.
= A metallic bond is the attraction of a
metal cation for delocalized
electrons
صفحه 16:
Properties of Metals
= High electrical conductivity
= Metals are malleable -they can be
hammered into sheets
= Ductile- they can be drawn into a
wire
= Moderately high melting points
= Extremely high boiling points
صفحه 17:
Properties of Metal
Explained
= Electrical current occurs when
electrons enter a wire and push other
electrons down the wire.
= Metals have high electrical
conductivity because the delocalized
electrons are free to move.
صفحه 18:
Properties of Metal
Explained
= Ductility and malleability both
occur because an applied force
allows metal cations to move
through the delocalized electron
field to a new location, without
suffering a complete break in the
ee of the crystal lattice
15-02۳
صفحه 19:
Properties of Metal
Explained
= Moderately high melting points occur
because the electron sea in the solid
state already allows some movement
in the lattice without a great deal of
additional energy.
= But to completely separate atoms from
another, because they are associated
with many other atoms, not just one,
requires a great deal of energy, hence
their high boiling points.
صفحه 20:
Covalent Bonding
= When two atoms which both need to
gain electrons come together, sharing
valence electrons to complete the
octet rule, a covalent bond is formed
= Because electrons are not given up,
no ions are produced.
= Covalent bonds generally occur
between non-metals
صفحه 21:
Bond Energy
= The amount of energy needed to
break the attractive force holding 2
atoms together. -
= The higher the energy, the shorter
the bond, and therefore, the shorter
the distance between atoms.
= The higher the energy, the stronger
the bond and the more difficult it is
to break.
صفحه 22:
Lewis Structure
v Shows how valence electrons are
arranged among atoms in a molecule.
v Reflects central idea that stability of a
compound relates to noble gas
electron configuration. (octet-duet
rule)
صفحه 23:
Lewis Structures
= Share valence electrons to make
sure that every atom has a noble gas
electron configuration
= Two electrons shared forms a single
covalent bond
= Examples
و
Cl
صفحه 24:
Try these
5 0,
ل ۱۳
Ark)
Cees
صفحه 25:
Drawing Lewis Structures
S=N-A
S = number of shared electrons
N = number of electrons needed (8 x
number of nonhydrogen atoms + 2 x
number of hydrogen atoms)
A = number of electrons available (sum of
the valence electrons of all atoms in
molecule + 1 for each negative and - 1
for each positive charge in a polyatomic
ion)
صفحه 26:
Drawing Lewis Structures
_ The number of bonds = number of
shared electrons/2
Put the least electronegative atom
in the center
Arrange the other atoms
symmetrically around the central
atom
Draw in the bonds
Fill incomplete octets
صفحه 27:
3
Draw the Lewis structure for the
molecule
CH,O
صفحه 28:
3
Draw the Lewis structure for CCl,
صفحه 29:
3
Draw the Lewis structure for NH,
صفحه 30:
3
Draw the Lewis structure for NH,*
صفحه 31:
3
Draw the Lewis structure رفن 2
صفحه 32:
Resonance,
Electronegativies, Polarity
صفحه 33:
Objectives
= Distinguish among the following bond
types based upon how electrons are
involved: pure covalent, polar covalent,
coordinate covalent, ionic, and metallic.
= Use the electronegativity differences
between bonding atoms to predict the
type of bond.
صفحه 34:
3
Draw the Lewis Structure for SO,.
صفحه 35:
Resonance
= Formal Definition: Description of the
ground state of a molecule with
delocalized electrons as an average
of several Lewis structures.
ns seals eae state doesn't
switch rapidly between the separate
structures: it is an average.
= We call the true structure a
resonance hybrid
صفحه 36:
3
Draw the Lewis Structure for SO,.
صفحه 37:
3
Draw the Lewis Structure for
benzene, C,H,. This molecule is a
cyclic hydrocarbon.
صفحه 38:
Electronegativity and
Bonding
= Electrons are transferred
between atoms when the
difference in electronegativity
between them is high. This
results in an ionic bond.
= If the electronegativity difference
between two reacting atoms is
small, electron sharing
occurs.This results in a covalent
bond.
صفحه 39:
The Pauling Electronegativity
Values
صفحه 40:
Electronegativity and
Bonding
= At what point in electronegativity
difference does the changeover occur?
The answer is not simple.
= The electronegativity of an atom
varies اد depending upon the
atom with which it is combining.
= Another factor is the number of other
atoms with which the atom is
combining.
صفحه 41:
Three Possible
Types of Bonds
a) nonpolar
covalent
b) polar covalent
C) ionic
صفحه 42:
Electronegativity and
Bonding
= A scale showing the percent of
transfer of electrons (percent
ionic character) has been
constructed. The amount of
transfer depends on the
electronegativity difference
between two atoms.
صفحه 43:
a Covalent Bond and the Electronegativity
Difference of the Bonded Atoms
2
Electronegativity difference
صفحه 44:
How to decide?
Nonpolar بجاو
|
Covalent Ionic
0 17 353
A EN
50% ionic
صفحه 45:
Table 12-6
Fe [Co |Ni
1.74) 1.79| 1.83
Ru Rh ۵
4.42 | 1.87 | 1.78
۲ [Pt
1.52 | 1.88 | 1.06
۲۵ [Dy |Ho [er [Tm
110 | 116 | 1.16] 8
Bk ict Es |Fm |Md
estimated ————- 1.2
صفحه 46:
Halogen Bonding
(kJ/mol)
569
432
366
299
' Covalent bonds tend to have
۱ electron clouds
lonic bonds tend to have distinct
electron clouds
Are these bonds ionic or covalent?
صفحه 47:
The Polar Covalent Bond
= Since the electronegativity of each element
differs, we should consider that in a covalent
bond one of the atoms attracts the shared
pair more strongly than the other. The
resulting bond is said to be polar covalent.
= Since one atom in the bond will attract the
electrons more strongly, there will be a
partial negative charge (&) near that end of
the bond.
= The atom holding the electrons less strongly
will have a partial positive charge (5°) near its
end.
صفحه 48:
Polar Molecules
= Polar bonds, unless symmetrically arranged,
produce polar molecules.
= In CCI,, the four C—Cl bonds are each polar but their
symmetrical arrangement around the C produces a
nonpolar molecule.
= In H,O and HF, however, the polar bonds are not
symmetrically arranged and produce a polar
molecule.
Pol ar Molecules
صفحه 49:
Properties of Polar
Substances
= Polar molecules are generally
soluble in water (hydrophilic).
= Non-polar molecules are
generally insoluble in water
(hydrophobic).
صفحه 50:
Properties of Polar
Substances
= Substances composed of nonpolar molecules
are generally gases or low-boiling liquids.
= Substances composed of polar molecules
generally have higher boiling points
= Hence, many polar molecules are solids under
normal conditions.
= A polar molecule is sometimes called a dipole.
= A dipole has asymmetrical charge distribution.
= In the teacher demonstration, which covalent
molecules were polar? Non-polar?
صفحه 51:
So, just how many
Covalent Bonds are there?
= Pure Covalent
= Polar Covalent
= Coordinate Covalent
صفحه 52:
Pure Covalent
= Each atom contributes one
electron
= Electrons are shared equally
between atoms
= A nonpolar bond results
= If all bonds in the compound are
nonpolar, the molecule is
nonpolar
صفحه 53:
Polar Covalent
= Electrons are not shared equally
between atoms
= A dipole moment exists, so a polar
bond results
= If the polar bonds are arranged
symmetrically, the molecule is
nonpolar, but if not, the molecule is
polar
صفحه 54:
Coordinate Covalent
= Only one atom contributes
electrons to the bond
NH, + Ht > NH,*
H
[ +
ene evel
A H
صفحه 55:
Chemistry at Work
Microwave Cooking
Polar H,0 molec ‘e vibrated by the micro
lem, f= 101° cy
This friction causes heating.
The food must contain moisture (polar H,0 molecules)
صفحه 56:
More
Chemistry at
۱/۹
صفحه 57:
VSEPR Theory
« ۷۱6۴۸66 5۳۱6۱۱ 6166۲۵۱ ۲
repulsion theory (VSEPR) (1957) is
a model in chemistry that aims to
generally represent the shapes of
individual molecules.
صفحه 58:
Molecular Geometry
= VSEPR theory is based on the idea
that the geometry of a molecule or
polyatomic ion is determined
primarily by:
= Repulsion among the pairs of electrons
associated with a central atom.
= Electron pairs may be bonding or
nonbonding (also called lone pairs).
= Only valence electrons of the central
atom influence the molecular shape in a
meaningful way.
صفحه 59:
Electron Repulsion
= Lone pair electrons take up more
space than bonded pair electrons
= A molecule must avoid or minimize
these repulsions to remain/stay
stable (lower in energy).
صفحه 60:
Steric Electron Pairs
= The steric number is the sum of the
total number of neighbors of a central
atom in a chemical compound and the
number of lone pairs on it.
= E.g. In methane, the steric number for
the carbon atom is 4.
" N.B. For our purposes, single bonds,
double bonds, and triple bonds all
count as one steric pair.
صفحه 61:
Predicting Molecular
Geometry
= In order to achieve this:
= Draw a Lewis structure
= Include all bonds
= Include all lone pairs of electrons
= Find the steric number on the central
atom.
صفحه 62:
Determining the Geometry of
Methane
= Draw the Lewis Structure for
methane
= What is the steric number?
= How should the bonds be spaced to
minimize repulsion?
= What shape is this?
= What is the bond angle?
صفحه 63:
Solution: Methane
= The methane molecule (CH,) is
tetrahedral because there are four
pairs of electrons. The four hydrogen
atoms are positioned at the vertices
of a tetrahedron, and the bond angle
is 109.5°.
صفحه 64:
Determining the Geometry of
Ammonia
= Draw the Lewis Structure for
ammonia
= What is its steric number?
= How should the bonds be spaced to
minimize repulsion?
= What shape is this?
= What is the bond angle?
صفحه 65:
Solution: Ammonia
= The ammonia molecule (NH,) has three
pairs of electrons involved in bonding, but
there is a lone pair of electrons on the
nitrogen atom.
= |t is not bonded with another atom;
however, it influences the overall shape
through repulsions.
= As in methane, there are 4 regions of
electron density.
= Therefore, the overall orientation of the
regions of electron density is tetrahedral.
صفحه 66:
Solution: Ammonia
= But, there are only three outer atoms.
= The overall shape of the molecule is
trigonal pyramidal because the lone pair is
not "visible."
= This makes the angle <109°.
= The shape of a molecule is found from the
relationship of the atoms even though it
can be influenced by lone pairs of
electrons.
صفحه 67:
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صفحه 68:
Hybridization
صفحه 69:
Method 2: How to Draw a
Lewis Structure for a
Compound
صفحه 70:
I. Draw a skeleton
structure.
A skeleton structure is a rough map showing the
arrangement of atoms within the molecule. In
general, you need to determine the skeleton
experimentally, but here are a few guidelines for
redicting skeleton structures from molecular
ormulas.
Central atoms are usually
™ the atoms with highest valence, or
= the largest atoms, or
= the least electronegative atom.
= H and the halogens are usually outside atoms.
= Don't put more than four atoms around a central
atom unless the central atom is third period or lower.
صفحه 71:
Il. Count total valence
electrons.
= Add the number of electrons in the valence
shells of all atoms in the molecule.
"= If the molecule is charged, add an electron
for each negative charge and subtract an
electron for each positive charge.
= Noble gas compounds are very uncommon
(except on general chemistry tests!) Should
you encounter one, each noble gas atom
has 8 valence electrons.
صفحه 72:
۱۱۱, Connect the structure.
= Draw a bond between the central Flue} an]
and each outside atom.
= Each bond uses 2 valence electrons.
IV. Place electrons on
outside atoms.
= Use remaining electrons to satisfy the
octets for each of the outside atoms.
= If you run out of electrons at this point, the
skeleton structure was wrong. Go back to
step I.
صفحه 73:
V. Place all remaining
electrons on the central
atom.
If there are more than 8 electrons on the central atom, and
the central atom is not third period or lower, you counted
the number of valence electrons incorrectly. Go back to
step Il.
If the octet on the central atom is not complete, try sharing
lone pairs of outside atoms to form double or triple bonds.
Write one multiply bound structure for each outside atom
with a lone pair to share; these are resonance structures.
If you can't get an octet on the central atom, at this point,
check to see whether the total number of valence electrons
for this molecule is odd. /t's impossible to give octets to all
atoms in an odd electron molecules. Get as close to an
octet as possible by forming multiple bonds.
