Covalent Bonding Chapter 8. 8.1 The Covalent Bond In order for an atom to gain stability, it can...
Transcript of Covalent Bonding Chapter 8. 8.1 The Covalent Bond In order for an atom to gain stability, it can...
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Covalent BondingChapter 8
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8.1 The Covalent Bond
In order for an atom to gain stability, it can gain, lose, or share electrons.
Atoms that share electrons and form covalent bonds in order to complete their valence shells are called molecules. diatomic molecules
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Single Covalent Bonds
Lewis structures can be used to represent the arrangement of electrons in a molecule. A line or a pair of vertical dots is used to represent a single
covalent bond.
Single bonds (2 electrons shared) is also called a sigma bond (σ).
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Multiple Covalent Bonds
Double Bonds
Triple Bonds
A multiple covalent bond consists of one sigma and at least one pi (π)bond. How many sigma and pi bonds are in methane? How many
in a molecule of oxygen?
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The Strength of Covalent Bonds
Bond length the distance between two bonded nuclei
as the number of shared pairs increases, the bond length decreases
the shorter the bond length, the stronger the bond
Bonds and Energy Endothermic reaction: more energy required to break
existing bonds than is released when the new bonds form
Exothermic reaction: more energy is released during product bond formation than is required to break bonds in the reactants
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8.2 Naming Molecules
Binary Molecular Compounds The first element in the formula is always named first, using
the entire element name.
The second element in the formulat 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.
The first element never uses the mono- prefix.
If using a prefix results in two consecutive vowels, one is dropped.
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Practice
How would you write sulfur trioxide?
phosphorus pentafluoride?
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Naming Acids
Binary Acids The first word has the prefix hydro- to name the hydrogen
part of the compound. The rest of the first word consists of a form of the root of the second element plus the suffix –ic.
Oxyacids Identify the polyatomic ion present. The first word consists
of the root of the polyatomic ion. The first word has a suffix that depends on the polyatomic ion’s suffix. If the polyatomic ion ends with the suffix –ate, replace it with the suffix –ic. If the polyatomic ion ends in –ite, replace it with –ous.
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Practice
hydrochloric acid sulfuric acid H3PO3
acetic acid HBr carbonic acid
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8.3 Molecular Structure
Structural formulas use the symbols and bonds to show relative positions of atoms.
Lewis structures Predict the location of certain atoms. Least electronegative
is usually the center, and hydrogen is always terminal.
Determine the number of electrons available for bonding.
Determine the number of bonding pairs.
Place the bonding pairs.
Determine the number of electron pairs remaining.
Determine whether the central atom satisfies the octet rule.
You may have to convert bonds around the central atom to multiples to satisfy the octet.
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Resonance and Exceptions
When more than one valid Lewis structure is possible, the compound is considered to have resonance. nitrite, sulfur dioxide
Exceptions to the Octet Rule
Odd number of valence electrons (NO2)
Suboctets and coordinate covalent bonds (BH3 and NH3)
Expanded octets (PCl5)
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8.4 Molecular Shapes
VSEPR Model – Valence Shell Electron Pair Repulsion Bond angle
Electron pairs repulse each other in a molecule
Lone pairs take up more space than bonded pairs
Hybridization
Atomic orbitals can mix and form new, hybrid orbitals
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Practice
linear (BeCl2)
bent (H2O)
trigonal planar (AlCl3)
tetrahedral (CH4)
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8.5 Electronegativity & Polarity
Electronegativity is the tendancy of an atom to attract electrons.
Bond character Nonpolar (EN < 0.4)
Polar covalent (EN 0.4 – 1.7)
Ionic (EN > 1.7)
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Polar Covalent Bonds
Molecular polarity partial charges occur in molecules (electrons are unequally
shared) and are symbolized by the letter delta
Polarity and molecular shape The shape of a molecule can determine polarity.
Water and carbon tetrachloride
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Dipoles and Polarity
dipole moment: the direction of the polar bond within a molecule (arrow) arrow points at the more electronegative end
a molecule is polar if the dipole moments do NOT cancel out
a molecule is nonpolar if the dipole moments cancel
polarity of shapes: always polar: trigonal pyramidal and bent
for others, polarity depends upon atoms attached to central atoms
to predict polarity
write electron dot
determine shape
determine dipole moment
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Solubility
Bond type and shape of a molecule determines its polarity.
LIKE DISSOLVES LIKE!!
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Properties of Covalent Compounds
Intermolecular Forces Dispersion force, or induced dipole
Dipole-dipole force
Weak IMF result in highly volatile compounds (oxygen and carbon dioxide) and relatively soft solids (paraffin).
Covalent Network Solids
diamond and quartz
extremely hard solids and nonconductors