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Section 9.1
Bonding of Atoms
Section 9.2
Molecular Shape and Parity
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Bonding of Atoms
• Predict the type of bond that forms between
atoms by calculating electronegativity
differences.
• Compare and contrast characteristics of
ionic, covalent, and polar covalent bonds.
• Interpret the electron sea model of metallic
bonding.
Bonding of Atoms
alkali metal: any element from Group 1:
lithium, sodium, potassium, rubidium, cesium,
or francium
Bonding of Atoms
electronegativity
ductile
shielding effect
conductivity
polar covalent bond
metallic bond
malleable
The difference between the
electronegativities of two atoms
determines the type of bond that forms.
A Model of Bonding
• Atoms form two different types of bonds—
ionic and covalent.
A Model of Bonding (cont.)
• The properties of any compound,
particularly its physical properties, are
related to how equally the electrons are
shared.
Electronegativity—An Attraction
for Electrons
• Electronegativity is a measure of the
ability of an atom in a bond to attract
electrons.
Electronegativity—An Attraction
for Electrons (cont.)
• Electronegativity is a periodic property—it
varies in a predictable pattern across a
period and down a group on the periodic
table.
• Electronegativity tends to increase from left
to right across a period.
• Electronegativity tends to decrease from
top to bottom down a group.
Electronegativity—An Attraction
for Electrons (cont.)
Electronegativity—An Attraction
for Electrons (cont.)
• Noble gases are considered to have
electronegativity values of zero and do not
follow periodic trends.
• Fluorine is the most electronegative element
on the periodic table, meaning it has the
greatest attraction for electrons of any
element.
Shielding Effect
• Shielding effect is the tendency for the
electrons in the inner energy levels to block
the attraction of the nucleus for the valence
electrons.
• Tends to increase top to bottom down a
group as more energy levels are added to
separate the valence electrons from the
nucleus
• Does not change across a period from left
to right since no new energy levels are
added within a period.
Shielding Effect
• Notice that calcium has more distance between
its nucleus and its valence electrons than
magnesium does.
• Therefore, the shielding effect is greater in
calcium than in magnesium.
Electronegativity—An Attraction
for Electrons
• The farther the bonding atoms are from
each other on the periodic table, the
greater their electronegativity difference.
Ionic Character
• Electronegativity difference (∆EN) is a
measure of the degree of difference of
ionic character in a bond.
• ∆EN is calculated by subtracting the smaller
electronegativity from the larger, so the ∆EN
is always positive.
• ∆ (delta) means difference.
Ionic Character (cont.)
• When the electronegativity difference in a
bond is 2.0 or greater, the sharing of
electrons is so unequal that you can
assume that the electron on the less
electronegative atom is transferred.
Ionic Character (cont.)
Ionic Character (cont.)
• The greater the difference in the
electronegatives of two atoms, the more
ionic the bond between the atoms.
• The greater ionic bond between two atoms,
the higher the melting and boiling points for
the substance formed by the bond.
• The greater the distance between the
bonding atoms on the periodic table, the
more ionic the bond between the atoms.
Covalent Character
• Two atoms of the same element form pure
covalent bonds because the difference in
electronegativity is zero and the valence
electrons are shared equally.
• All diatomic elements have pure covalent
bonds.
Covalent Character (cont.)
• A bond in which the electronegativity is less
than or equal to 0.5 is considered to be a
covalent bond with electrons that are
shared almost equally.
Covalent Character (cont.)
Polar Covalent Bonds
• Bonds in which the pair of electrons is
shared unequally have electronegativity
differences between 0.5 and 2.0.
• A bond that forms when electrons are shared
unequally is called a polar covalent bond.
Polar Covalent Bonds (cont.)
• Polar covalent bonds have a significant
degree of ionic character.
– Represented by delta plus (+) and delta
minus (–) to indicate a partial positive
charge and partial negative charge.
–  means “partial charge”
Polar Covalent Bonds (cont.)
• Compounds with pure covalent bonds have
different properties from compounds with
polar covalent bonds, such as lower
melting and boiling points.
Polar Covalent Bonds (cont.)
• Unequal sharing causes an imbalance in
the distribution of charge about the two
bonding atoms.
Polar Covalent Bonds (cont.)
• Water has polar bonds.
• A hydride is a compound formed between any
element and hydrogen.
Ionic, Nonpolar Covalent, & Covalent
Bonds
Ionic bonds
•
have a difference in electronegativity (∆EN) of
2.0 and greater.
Nonpolar covalent bonds
•
have a difference in electronegativity (∆EN) of
0.0 to 0.5 (including 0.5)
Polar covalent bonds
•
have a difference in electronegativity (∆EN)
between 0.5 and 2.0.
Bonding in Metals
• When a metal can be pounded or rolled
into thin sheets, it is malleable.
• Ductile metals can be drawn into wires.
Bonding in Metals (cont.)
• Electrical conductivity is a measure of
how easily electrons can flow through a
material to produce an electric current.
• These properties—malleability, ductility, and
electrical conductivity—are the result of the
way that metal atoms bond with each other.
Bonding in Metals (cont.)
Bonding in Metals (cont.)
• A metallic bond is the bond that results when
metal atoms release their valence electrons to
a pool of electrons shared by all the metal
atoms.
• Electrons flow freely around all metal atoms,
which allows the atoms to:
•Conduct electricity
•Deform without breaking
• This is referred to as the electron sea model.
Section Assessment
With few exceptions, electronegativity
values ___ as you move from left to right
in any period of the periodic table.
A. increase
B. decrease
C. stay the same
Section Assessment
Which element has the highest value of
electronegativity?
A. carbon
B. oxygen
C. fluorine
D. iron
Molecular Shape and Polarity
• Diagram Lewis dot diagrams for molecules.
• Formulate three-dimensional geometry of
molecules from Lewis dot diagrams.
• Predict molecular polarity from
three-dimensional geometry and bond polarity.
Molecular Shape and Polarity
electronegativity: a measure of the ability of
an atom in a bond to attract electrons
Molecular Shape and Polarity
double bond
triple bond
polar molecule
The shape of a molecule and the
polarity of its bonds determine whether
the molecule as a whole is polar.
The Shapes of Molecules
• Molecular shape and polarity are related to
each atom’s attraction for the electrons in a
bond.
• Any two atom compound will always be linear.
• Hydrogen gas is composed of 2 hydrogen
atoms bonded together in a linear fashion
• In NaCl, the sodium and chloride ions are
arranged in a linear fashion
• The shortest distance between 2 points is
a line.
The Shapes of Molecules (cont.)
• Electron pairs repel each other and cause
molecules to be in fixed positions relative
to each other.
• Unshared electron pairs also determine the
shape of a molecule.
• Unshared electrons pairs are also called
nonbonding pairs or lone pairs.
• Electron pairs are located in a molecule as far
apart as they can be (like charges repel).
The Shapes of Molecules (cont.)
• In a tetrahedral arrangement, such as
water, the repulsions between the electron
pairs are minimized.
• The nonbonding electrons require more room
so they distort the tetrahedral arrangement by
squeezing the bonding pairs
closer together and
decreasing the bond angle.
• The shape of a water molecule
is referred to as “bent” and is a
modified tetrahedron.
• The hydrogen bond angles are
105° instead of 109.5°.
The Shapes of Molecules (cont.)
• A bond formed by sharing two pairs of
electrons between two atoms is called a
double bond.
• Carbon dioxide—the oxygen atom shares two
pairs of electrons with the carbon atom.
• Its geometry is linear.
The Shapes of Molecules (cont.)
• Ammonia—the arrangement is tetrahedral
but the geometry of its four atoms is a
triangular pyramid.
The Shapes of Molecules (cont.)
• Methane is the simplest hydrocarbon and
forms a perfect tetrahedron with bond
angles of 109.5°.
•Methane’s geometry
is tetrahedral.
The Shapes of Molecules (cont.)
• Hydrocarbons are organic compounds
composed only of carbon and hydrogen.
• Ethane is second member of hydrocarbon
series known as the alkanes.
• Its geometry is double tetrahedral.
The Shapes of Molecules (cont.)
• Alkanes are hydrocarbons that contain only
carbon and hydrogen atoms with single
bonds between all the atoms.
– A tetrahedral arrangement of bonding
electron pairs around each carbon atom
provides the most space for the electrons.
The Shapes of Molecules (cont.)
• Ethene, also known as ethylene, is the
simplest of the alkenes, or hydrocarbons in
which one or more double bonds link
carbon atoms.
– In order for the carbon
atoms to acquire an octet
of electrons, a double
bond must exist between
the carbons.
– Ethene’s geometry is a
flat, triangular
arrangement of atoms.
– Trigonal planar.
The Shapes of Molecules (cont.)
• Ethene gas helps fruits to ripen more quickly
• Ethyne, more commonly known as acetylene,
is the first member of a hydrocarbon series
called the alkynes, which are unsaturated
hydrocarbons that contain a triple bond
between two carbon atoms.
• Ethyne is very unstable due to the triple bond
between carbon atoms.
• Ethyne is used as fuel for welding torches.
The Shapes of Molecules (cont.)
• A bond formed by sharing three pairs of
electrons between two atoms is called a
triple bond.
• A triple bond makes the linear molecule rigid.
Molecular Substitutions
• Different elements can be substituted into
any of the compounds discussed
previously in the following way:
• Hydrogen and halogens can be
substituted for each other due to the fact
that halogens and hydrogen need only
one additional electron to give them a
noble gas configuration.
• Elements in the same group can be
substituted for other elements due to
their similar number of valence
electrons.
Molecular Substitutions
• Examples:
• In water, any halogen can be substituted for
hydrogen atoms.
• Any group 16 element can be substituted for
oxygen atoms in water or other compounds.
• Any group 14 element can be substituted for
carbon atoms in methane or other
compounds.
• Any group 15 element can be substituted for
nitrogen atoms in ammonia or other
compounds.
Polar and Nonpolar Molecules
• A polar molecule, or dipole, is a molecule
that has a positive and a negative pole.
• Water molecules attract one another because
they have positive and negative ends.
• Ammonia is another molecule with polar
bonds.
Polar and Nonpolar Molecules
• How do you decide if a molecule is polar or
nonpolar?
• If a molecule is very symmetrical, it is
unlikely to be polar, for example methane.
• If a molecule is asymmetrical, it is more
likely to be polar, like water.
• To determine if polar or nonpolar, find the
∆EN for each side of the molecule.
• If the ∆EN is greater than 0.5 between one
side of the molecule and the other, it is
considered to be polar.
Polar and Nonpolar Molecules (cont.)
Polar and Nonpolar Molecules (cont.)
• Carbon dioxide is a nonpolar molecule
since the linear arrangement of the
molecules results in no separation of the
positive and negative charge.
Polar and Nonpolar Molecules (cont.)
• The force between dipole molecules is an
attraction of the positive end of one dipole
for the negative end of another dipole.
Polar and Nonpolar Molecules (cont.)
• Properties of polar molecules differ from
nonpolar molecules.
• Melting point and boiling points of polar
substances tend to be higher than those of
nonpolar molecules of the same size.
Polar and Nonpolar Molecules (cont.)
Ions, Polar Molecules, and
Physical Properties
• Ionic compounds exhibit a narrower range
of physical properties than covalent
compounds.
Ions, Polar Molecules, and
Physical Properties (cont.)
• Ionic compounds tend to be brittle, solid
substances with high melting points.
• Covalent substances may be solids, liquids,
or gases at room temperature.
Section Assessment
Which exhibit a narrower range of
physical properties?
A. ionic
B. covalent
Section Assessment
Carbon dioxide is a ___ molecule.
A. polar
B. nonpolar
C. dipole
D. triple bond
Key Concepts
• Bond character varies from ionic to covalent. There is no
clear-cut division between the types of bonds.
• Electronegativity — a measure of the attraction that an atom
has for shared electrons— can be estimated from the
periodic table.
• Electronegativity difference, ΔEN, is a measure of the
degree of ionic character in a bond.
• A ΔEN = 2.0 or greater occurs when elements form ionic
bonds; ΔEN = 0.5 – 2.0 reflects polar covalent bonds; and
ΔEN < 0.5 reflects covalent bonds.
• Metal atoms bond by sharing in a sea of
valence electrons.
Key Concepts
• Electron pairs about the central atom are either lone
(nonbonding) pairs or bonding pairs.
• The polarity of the bonds and the shape of the
molecule determine whether a molecule is polar or
nonpolar.
• Interparticle forces determine many of the physical
properties of substances.
The noble gases are considered to have
electronegativity values of zero.
A. true
B. false
___ is the tendency for the electrons in
the inner energy levels to block the
attraction of the nucleus for the valence
electrons.
A. Conductivity
B. Electronegativity
C. Shielding effect
D. Periodic property
When can you assume that the electron on a
less electronegative atom is transferred to a
more electronegative atom?
A. when the element is a noble gas
B. when the electronegativity difference in a
bond is 2.0 or greater
C. when the ∆EN is negative
D. when the electronegativity difference in a
bond is less then 2.0
Which of the following elements do not
form pure covalent bonds?
A. Br2
B. I2
C. NaCl
D. N2
A hydride is a compound formed between
any element and ___.
A. hydrogen
B. helium
C. oxygen
D. a noble gas
Which property is not the result of the
way that metal atoms bond with each
other?
A. malleability
B. ductility
C. electrical conductivity
D. solubility
What kind of bond occurs within a
molecule with unequal sharing of
electron pairs?
A. ionic bond
B. polar covalent bond
C. non-polar covalent bond
D. hydride bond
The two lone pairs of electrons of a water
molecule do what to the bond angle between the
hydrogen atoms and the oxygen atoms?
A. They attract the hydrogen atoms and
increase the angle greater than 109.5°.
B. They occupy more space and squeeze the
hydrogen atoms closer together.
C. They do no affect the bond angle.
D. They create structures with more than one
correct angle.
Which compound forms a perfect
tetrahedron?
A. water
B. ammonia
C. methane
D. carbon dioxide
___ are hydrocarbons that contain only
carbon and hydrogen atoms with single
bonds between all the atoms.
A. Alkanes
B. Alkenes
C. Tetrahedrals
D. Alkynes
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Table 9.1
Physical Properties of Ionic
and Covalent Compounds
Figure 9.2
Electronegativity
Figure 9.8
Bond types
Figure 9.18
Molecular shape
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