Transcript Chapter, 7 Ions

Chapter11 Chemical Bonding
Dr. Schuerch
Pyrite FeS2
Fluorite (CaF2)
Calcite CaCO3
Valence Electrons
• Valence electrons are the electrons in the highest
occupied energy level of an element’s atoms
– The valence electrons largely determine the chemical properties of
an element and are usually the only electrons used in chemical
bonds
– The group A number of an element in periodic table is related to
the number of valence electrons it has
– Example of valence electrons in Period 2:
Group 1A
Lithium
1s2 2s1
Group 2A
Beryllium
1s2 2s2
Group 3A
Boron
1s2 2s2 2p1
Group 4A
Carbon
1s2 2s2 2p2
Group 5A
Nitrogen
1s2 2s2 2p3
Group 6A
Oxygen
1s2 2s2 2p4
Group 7A
Fluorine
1s2 2s2 2p5
Group 8A
Neon
1s2 2s2 2p6
1 VE
2 VE
3 VE
4 VE
5 VE
6VE
7 VE
8 VE
Lewis Dot Structure
AKA Electron Dot Structure
• Electron dot structures are a diagram that shows the
valence electrons of an atom as dots around that atoms
chemical symbol
1. Atoms have the same amount of dots as they do valence
electrons
2. Do not pair dots until one dot occupies each quadrant of the
chemical symbol
3. It does not matter which quadrant you start adding dots to
Lithium
1 valence
electron
Beryllium
2 valence
electron
Boron
3 valence
electron
Carbon
4 valence
electron
Nitrogen
5 valence
electron
Oxygen
6 valence
electron
Fluorine
7 valence
electron
Neon
8 valence
electron
Li
Be
B
C
N
O
F
Ne
The Octet Rule
• When forming compounds, atoms tend to
lose or gain electrons to achieve the electron
configuration of a noble gas, which (except
for helium) has eight electrons in its highest
energy level
– Metallic atoms tend to lose their valence electrons
produce a(n) cation, or a positively charged ion.
– Most nonmetallic atoms achieve a complete octet
by gaining or sharing electrons
Noble Gas Electron Configurations
•
•
•
•
He
Ne
Ar
Kr
•
With exception of helium, Noble Gases have 8
electrons in their outermost energy level.
•
Their s and p orbitals are completely filled
1s2
1s2 2s2 2p6
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6
Formation of Cations
• Write the electron configurations for the
following metals and circle the electrons lost
when each metal forms a cation
Mg
Al
K
• After the above metals lose their electrons, what
noble gasses do their electron configurations
mimic?
• What charge do the ions formed have?
Formation of Cations
• Write the electron configurations for the
following metals and circle the electrons lost
when each metal forms a cation
Mg
Al
K
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2 3p64s1
• After the above metals lose their electrons, what
noble gasses do their electron configurations
mimic?
• What charge do the ions formed have?
Formation of Anions
•
Atoms of most nonmetallic elements achieve noble-gas
configurations by gaining electrons to become anions, or
negatively charged ions
What property of nonmetallic elements makes them more likely to
gain electrons than lose electrons?
•
–
•
•
They have relatively full valence shells
Elements of the halogen family gain one electron to become
halide ions
How many electrons will each element gain in forming an ion?
–
Nitrogen
(group 5)
3
–
Oxygen
(group 6)
2
–
Sulfur
(group 6)
2
–
Bromine
(group 7)
1
Write the symbol and electron configuration for the ions
formed from nitrogen, oxygen, sulfur, and bromine and
name the noble gas with the same configurations
•
•
•
•
Nitride
Oxide
Sulfide
Bromide
N3O2S2Br-
1s2 2s2 2p6; Ne
1s2 2s2 2p6; Ne
1s2 2s2 2p6 3s2 3p6; Ar
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6; Kr
Write the symbol and electron configuration for the ions
formed from nitrogen, oxygen, sulfur, and bromine and
name the noble gas with the same configurations
•
•
•
•
Nitride
Oxide
Sulfide
Bromide
N3O2S2Br-
1s2 2s2 2p6
1s2 2s2 2p6
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6
Ne
Ne
Ar
Kr
Equations showing the formation of ions
Ion equations (AKA half reactions)
Formation of Cations
Li → Li1+ + 1e-
Be → Be2+ + 2e-
What noble gas electron
configuration due the
ions formed mimic?
Formation of Anions
N + 3e- → N
3-
O + 2e- → O
2-
F + 1e- → F
1-
Ionic Bonds and Ionic Compounds
•
What is an ionic bond?
– It is the electrostatic force of attraction that
binds oppositely charged ions
2. In ionic compounds, the charges of the cations
and anions must balance to produce an
electrically neutral substance
3.
•
•
Be + 2F → BeF2
Why do beryllium and fluorine
combine in a 1:2 ratio?
– They combine in a 1 : 2 ratio because
each beryllium atom gives up two
electrons, but each fluorine atom only
accepts one electron. Therefore, two
fluorine atoms are needed to react with
each beryllium atom
Chemical Formulas
•
•
A chemical formula shows the types and
numbers of atoms in the smallest
representative unit of a substance
List the numbers and types of atoms
represented by these chemical formulas
4
Formula Unit
•
What is a formula unit?
–
A formula unit is the lowest whole-number ratio of
ions in a ionic compound
•
•
Sodium chloride (NaCl) has a formula unit of 1:1, one Na+
for one ClMagnesium chloride (MgCl2) has formula unit of 1:2, one
Mg2+ for two Cl-
– Why is the ratio of magnesium ions to chloride ions
in MgCl2 is 1:2
•
There must be twice as many chloride ions (Cl-) to balance
the magnesium ions (Mg2+) to form an electrically neutral
compound
Properties of Ionic Compounds
•
Ionic compounds exist as collections of
positively and negatively charged ions
arranged in repeating three-dimensional
patterns
•
Ionic compounds have coordination
number that tell us the number of ions of
opposite charge that surround an ion in a
crystal
•
Most ionic compounds are crystalline
solids at room temperature
•
Ionic compounds generally have a high
melting points
Sodium chloride (NaCl)
Cesium chloride (CsCl)
Rulite (TiO2)
Properties of Ionic Compounds
(Continued)
•
•
•
•
Ionic compounds are electrically neutral
Ionic compounds form very stable
structures
When dissolved in water, ionic
compounds can conduct electricity
When melted, ionic compounds can
conduct electricity
Molecular Compounds
•
What is a covalent bond?
– A covalent bond is a bond formed
when two atoms share electrons
H
– Covalent bonds form when the
difference in electronegativity
H C H
between the atoms involved is not
great enough to completely strip an
H
electron
•
•
If the difference in electronegativity
between the atoms involved is greater
than 2.0, the bond will be ionic
If the difference electronegativity
between the atoms involved is 2.0 or
less, the bond will be covalent
H
O
H
Molecular Compounds
• Most elements found in nature, with the
exception of the noble gases, exist as molecules
• What is a molecule?
– A molecule is a neutral group of atoms joined
together by covalent bonds
• Seven elements exist as diatomic molecules
– hydrogen gas (H2), nitrogen gas (N2), oxygen gas (O2), fluorine
gas (F2), chlorine gas (Cl2), liquid Bromine (Br2), and solid
Iodide (l2)
– Except for hydrogen, they form a “7” on the periodic table
– Compounds that are formed when two or more atoms
combine to form molecules are called molecular
compounds
Molecular Compounds
•
Molecular
compounds in
nature:
– Oxygen
– Water
– Ozone
• Non-molecular
compounds in nature:
– Neon
– Helium
• Properties of molecular compounds
• Low melting and boiling points
• Many exist as gases or liquids at room
temperature
• Usually consist of two or more nonmetals
Molecular Formulas
•
Molecular formulas show the types and
numbers of atoms present in a molecule
of a compound
•
•
•
•
Carbon dioxide
Ethane
Ammonia
CO2
C2H6
NH3
Molecular formulas do not show the
arrangement of the atoms in a molecule
Molecular Models
• Molecular models can be used to show the
arrangement of atoms in a molecule
– Notice, each line connecting the atoms means a pair
of electrons is shared between them
Molecular formula
Perspective drawing
Space-filling molecular model
Ball-and-stick model
Lewis structure
The Nature of Covalent Bonding
•
The Octet Rule in Covalent Bonding
–
How does electron sharing occur in forming covalent bonds?
•
•
Sharing of electrons usually occurs so that atoms acquire the
electron configurations of noble gases
Single Covalent Bonds
–
–
–
A shared pair of electrons is represented by a single dash
Structural formulas show the arrangement of atoms in
molecules
Below is an electron dot structure showing a water molecule.
Notice two pairs of electrons are unshared.
Water Molecule
H
OH
Double and Triple Covalent Bonds
•
A chemical bond formed when atoms have two
pairs of electrons is called a double covalent
bond or double bond for short
C
•
+
O
+
O
O C O
or
O C O
A chemical bond in which three pairs of
electrons is shared is called a triple covalent
bond or triple bond for short
•
The nitrogen molecule has a three covalent bonds
N
+
N
N
N
or
N
N
Drawing Lewis Structures
1. Sum the valence electrons from all the
atoms. Do not worry about keeping track
of which electrons come from which
atoms. It is the total amount of electrons
that is important
2. Use a pair of electrons to form a bond
between each pair of bound atoms
3. Arrange the remaining electrons to satisfy
the duet rule for hydrogen and the octet
rule for the second-row elements
Lewis Structures
• Give the Lewis structure for each of the
following:
a)
b)
c)
d)
e)
f)
HF
N2
NH3
CH4
CF4
NO+
Coordinate Covalent Bonds
•
A coordinate covalent bond is a covalent bond in which one
atom contributes both bonding electrons
H
H N:
H
+
H+
H
H N
H
+
H
Resonance
O
O
O
O
• Resonance occurs when more than one valid
Lewis structure can be written for a particular
molecule
• The resulting electron structure of the molecule is
given by the average of these resonance
structures and is usually represented by a doubleheaded arrow
N
N
N
O
O
O
O
O
Bond Dissociation Energies
•
Bond dissociation energy is the energy
required to break the bond between two
covalently bonded atom
–
Molecules with high dissociation energies are
relatively unreactive
•
The bond energy for the covalent bond C—O is
347 KJ/mol
–
The higher the bond disassociation energy the
more stable the bond is
Exceptions to the Octet Rule
•
The octet rule cannot be satisfied in
molecules whose total number of
valence electrons is an odd number.
There are also molecules in which an
atom has fewer, or more, than a
complete octet of valence electrons
•
–
Why does NO2 not follow the octet rule?
–
There is an unpaired electron present in both
possible resonance structures
Molecular Orbitals
•
A molecular orbital is formed when two
atoms combine and their atomic orbitals
overlap to produce orbitals that apply to
the entire molecule
•
•
When two s atomic orbitals combine, the
molecular orbital formed is called a(n) sigma
bond
When p atomic orbitals overlap, sigma or pi
bonds are formed
Hybrid Orbitals
•
Orbital hybridization theory can describe both
the shape and bonding of molecules
•
Orbital hybridization occurs when several
atomic orbitals mix to form the same total
number of equivalent hybrid orbitals
Methane
H
H C H
H
sp3
Ethene
H
H
C C
sp2
Ethyne
H
H C C
H
sp
H
Molecular Structure: The VSEPR
Model
• The valence shell electron-pair repulsion
(VSEPR) model is useful in predicting the
geometries of molecules formed from
nonmetals.
– Main postulate is the structure around a given
atom is determined principally by minimizing
electron-pair repulsions
– Bonding and nonbonding pairs of electrons
will position themselves as far apart as
possible
Step to Apply the VSEPR Model
• Draw the Lewis structure for the molecule
• Count the electron pairs and arrange them in
the way that minimizes repulsion (put them
as far apart as possible)
• Treat double and triple bonds as you would a
single bond
• Determine the positions of the atoms from the
way the electron pairs are shared
• Determine the name of the molecular
structure from the position of the atoms
Bond Angles and Geometry
Summary of VESPR Model
• Determine the Lewis structure(s) for the molecule
• For molecules with resonance structures, use any of
the structures to predict the molecular structure
• Sum the electron pairs around the central atom
• In counting pairs, count each multiple bond as a single
effective pair
• The arrangement of pairs is determined by minimizing
electron-pair repulsions
• Lone pairs require more space than bonding pairs do.
Chose an arrangement that gives the lone pairs as
much room as possible. Recognize that the lone pairs
may produce a slight distortion of the structure at
angles less than 120 degrees
Polar Covalent Bonds
– Occurs when one atom in a covalent
bond is more electronegative than it’s
partner
•
•
Results in uneven sharing of electrons
Causes a fractional positive charge on the less
electronegative atom and a fractional negative
charge on the more electronegative atom
δ+
δ-
Relative charges
H-Cl
Polarity of bond
Bond Polarity and Dipole Moments
•
Some regions of a polar molecule are slightly
negative and some are slightly positive
– However, a molecule containing a polar bond is not
always polar (next slide)
•
•
A molecule that has two poles is dipolar and
displays a dipole moment
When polar molecules are placed in an electric
field, they all line up with the same orientation in
relation to the charge plates
Dipole Moment of
H2O
Bond Polarity and Dipole Moments
•
Molecules with polar bonds are not always polar
–
Occurs when bond polarity in a molecule cancel each other out
•
Examples are CO2, BF3, and CCl4
Types of molecules with Polar Bonds but NO Resulting Dipole
Moment
Type
Linear molecules
with two identical
bonds
Planer molecules
with three identical
bonds 120 degrees
apart
Tetrahedral
molecules with four
identical bonds
109.5 degrees
Cancelation
of Polar
Bonds
Lewis
Structure
O C O
Example
𝐶𝑂2
F
B
F
F
𝐵𝐹3
Cl
Cl
C
Cl
Cl
𝐶𝐶𝑙4
Ball and Stick
Model
Polar Molecules
•
Are the following molecules polar or
nonpolar?
–
–
–
–
H2O
CO2
NH3
HCl
Intermolecular Attractions and
Molecular Properties
•
At room temperature, some compounds are
gases, some are liquids, and some are solids
–
The physical properties of a compound depend on
the type of bonding it displays—in particular, on
whether it is ionic or covalent
•
Some solids have very high melting points or will not melt
at all, but simply decompose without melting
–
Most of these very stable substances are network solids (or
network crystals), solids in which all of the atoms are
covalently bonded to each other
» Melting a network solid would require breaking covalent
bonds throughout the solid
Bonding
Characteristic
Ionic Compound
Covalent Compound
Representative unit
Formula unit
molecule
Physical state
Solid
Solid, liquid, or gas
Melting point
High
Low
Solubility in water
Usually high
High to low