Chapter 8
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Transcript of Chapter 8
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Chapter 8Covalent Bonding
VSEPR Theory
Molecular Shape
Polar or NonPolar
Properties of Molecular Substances
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Why Do Atoms Bond? The stability of an atom, ion or compound is related
to its energy– lower energy states are more stable.
Metals and nonmetals gain stability by transferring electrons (gaining or losing) to form ions that have stable noble-gas electron configurations.– Ionic Bonding
Another way atoms can gain stability is by sharing valence electrons with other atoms, which also results in noble-gas electron configurations.– Covalent Bonding
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The most stable arrangement of atoms exists at the point of maximum net attraction, where the atoms bond covalently and form a molecule.
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The Covalent Bond Atoms will share electrons in order to
form a stable octet. Covalent bond : the chemical bond
that results from the sharing of valence electrons
also called a molecular bond
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The Molecule formed when two or more atoms bond
covalently The smallest piece in a covalent compoundFormed when the proton of one atom is
attracted to the electron cloud of another atom.
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Models Molecules
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Single Covalent Bonds
In a single covalent bond a single pair of electrons is shared
This can be represented with a Lewis structure
A single line represents a single covalent bond
A single pair of electrons
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• Bonding pair: a pair of electrons shared by two atoms
• Lone pair: an unshared pair of electrons on an atom
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Formation of Water
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Group 17 elements will form one covalent bond.
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Group 16 elements will form two covalent bonds.
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Group 15 elements will form three covalent bonds.
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Group 14 elements will form four covalent bonds.
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Sigma Bonds • Single covalent bonds are also called
sigma bonds:• the electron pair is centered between
two atoms.
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Multiple Covalent Bonds
When more than one pair of electrons is shared, a multiple covalent bond is formed
Multiple bonds are made up of sigma bonds and pi bonds: formed when parallel orbitals share electrons.
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Double Covalent Bond Two pairs of electrons are shared Contains one sigma and one pi bond.
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Triple Covalent Bond Three pairs of electrons are shared Has one sigma and two pi bonds.
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Strength of Covalent Bonds
The strength of covalent bonds is determined by the bond length:– distance between the bond
nuclei Bond length is determined by:
– The size of the atoms involved—larger atoms have longer bond lengths
– How many pairs of electrons are shared—the more pairs of electrons shared, the shorter the bond length is.
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Bond Dissociation Energy the amount of energy
required to break a bond Indicates the strength of a
covalent bond When a bond forms, energy
is released; When a bond breaks, energy
must be added Each covalent bond has a
specific value for its bond
dissociation energy.
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Bond Energy and Bond Length A direct relationship exists between bond
energy and bond length– Shorter Bond– Stronger Bond – Higher Bond Dissociation Energy – Longer Bond– Weaker Bond – Lower Bond Dissociation Energy
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Energy Changes An endothermic reaction is one where
a greater amount of energy is required to break a bond in reactants than is released when the new bonds form in the products.
An exothermic reaction is one where more energy is released than is required to break the bonds in the initial reactants.
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Naming Molecules Molecular Formula
Shows what atoms and how many are in a molecule
Examples: Nonmetal-Nonmetal Combinations
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Naming (Binary Compounds) The first element is always named first using
the entire element name The second element is named using its root
and adding the suffix -ide Prefixes are used to indicate the number of
atoms of each element that are present in the compound
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Common Names Many compounds were discovered and
given common names long before the present naming system was developed (water, ammonia, hydrazine, nitric oxide).
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Binary Acids An acid that contains hydrogen and one other
element Ex. HCl ion ends –ide. Name the acid with hydro-root of the anion-ic HCl (hydrogen and chloride ) becomes
hydrochloric. HCl in a water solution is called hydrochloric
acid.
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Naming Acids Acids contain hydrogen as the first
element Binary Acids: H bonded to one other
element An ion that ends –ide Name the acid with hydro-root-ic Example: HCl
Hydrogen ion and chloride ion Hydrochloric acid
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Oxyacids An acid that contains both a hydrogen atom and an
oxyanion. Example: HNO3 Identify the oxyanion present. name ends with the suffix –ate, replace it with the
suffix –ic. If the name ends with suffix –ite, replace it with suffix
–ous, NO3 is the nitrate ion so the acid is nitric acid.
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Acid Naming Summary Anion name Acid name
--ide Hydro-root-ic
--ite Root---ous
--ate Root-ic
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Structural Formulas A structural formula uses letter symbols and bonds to
show relative positions of atoms.
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Lewis Structures
Used to predict the structural formula Show arrangement of the atoms and
un-bonded electrons
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Five steps to draw Lewis structures:1. Count the total number of valence electrons
in all atoms involved.
2. Decide how the elements are arranged in the structure and draw it out.
1. Hydrogen is always an end atom.
2. Central atom is usually written first in compound
3. Central atom has least attraction for the electrons
4. Usually closer to left on periodic table
3. Subtract the # of electrons used in the bonds.
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4. Satisfy the octets of the terminal atoms.
5. Place any remaining electrons around the central atom to satisfy its octet. If the central atom cannot be satisfied, make a multiple bond using a lone pair from the terminal atoms.
6. Check your work
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Examples
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Drawing Lewis structures for polyatomic ions is very similar to drawing Lewis structures for covalent
compounds EXCEPT in finding the number of electrons available for bonding
Count the total number of valence electrons in all atoms involved.
If the polyatomic ion is negatively charged, ADD the charge to the number of valence electrons.
If the ion is positively charged, SUBRACT the charge from the number of valence electrons.
Follow the rest of the steps to drawing Lewis structures.
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Examples
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Resonance Structures When a molecule or polyatomic ion has
both a double bond and a single bond, it is possible to have more than one correct Lewis structure:
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Resonance a condition that occurs when more than
one valid Lewis structure can be written for a molecule or ion.
The structures are called resonance structures.
A molecule that undergoes resonance behaves as if it has only one structure.
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Exceptions to the Octet Rule
Three Ways Molecules Might Violate the Octet Rule
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Odd Number of Valence Electrons Some molecules have an odd number of valence
electrons and cannot form an octet around each atom Example: NO2
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Sub Octet Some compounds form with fewer than 8 electrons present around an atom. Boron BF3
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Coordinate covalent bond when one atom donates an entire pair of electrons to be
shared with atoms or ions that need two more electrons. Boron compounds often do this
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Expanded Octet Some elements can have more than eight electrons in
their valence shell Because of d-level electrons PCl5
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How? The d orbital starts to hold electrons.This occurs in atoms in Period 3 or higher. When you draw Lewis structures for these
compounds, extra lone pairs are added to the central atom OR the central atom will form more than four bonds.
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Things to Remember Any exceptions to the Octet Rule are on
the central atom
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Molecular Shape How a molecule “looks” Determines properties The shape of a molecule determines
whether or not two molecules can get close enough to react
We describe shape using the VSEPR model
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VSEPR This model is based on the fact that
electrons pairs will stay as far away from each other as possible
Valence
Shell
Electron
Pair
Repulsion
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How to apply VSEPR 1. Draw the Lewis Structure for a
Molecule
2. Count the pairs of bonded electrons
3. Count the pairs of unbonded electrons
4. Match the information with the VSEPR chart to classify the shape of the molecule
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Atoms will assume certain bond angles: the angle formed by any two terminal atoms and the central atom
Lone pairs take up more space than bonded pairs do.
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http://www.chem.purdue.edu/gchelp/vsepr/structur.html
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Molecular Polarity Molecules are either polar or nonpolar
depending on the bonds in the molecule.
We must look at the shape (geometry) of a molecule to determine polarity.
Symmetric molecules are nonpolar. Asymmetric Molecules are Polar
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Polar or NonPolar Determine if a molecule is polar or
nonpolar by– Looking at a model of the molecule– Looking at a Lewis Structure of the
molecule
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Solubility of Polar Molecules Bond type and shape of the molecule
determine solubility Polar substances and ionic substances
will dissolve in polar solvents Nonpolar substances will only dissolve
in nonpolar substances
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Intermolecular Forces Nonpolar Molecules: Van der Waals intermolecular
Forces Very Weak forces between molecules
Polar Molecules: have dipole-dipole intermolecular bonding. Stronger intermolecular Forces
Polar Molecules with Hydrogen Bonding: hydrogen bonded to nitrogen, oxygen or fluorine, it will have hydrogen bonding between molecules. A very strong dipole-dipole interaction Very strong intermolecular Forces High boiling points, high melting points