Chem 101 Exam 3 Study Guide:
o Know how to write Lewis Structures and poly atomic ions (6.3-6.4)
o Resonance Structures: Is one of two or more Lewis structures for the
same molecule. The atoms are in the same locations, but the different
electron arrangements.
o Resonance Hybrid: If the actual structure of the molecule is intermediate
between the two or more resonance structures.
o Resonance Stabilization: When the electrons are stable in a structure, they try to
be more spread out. Resonance stabilization makes and important contribution to
the stability of many molecules.
o Know how to write Lewis structures that include multiple equivalent
resonance structures:
o Formal charges: the charge an atom would have if all bonding electrons were
shared equally between bonding atoms.
o How to calculate: Formal charge = # of valence electrons – (number
of nonbinding electrons + ½ the number of bonding electrons)
o This can help distinguish between competing skeletal/resonance structures
▪ 1. The sum of all formal charges in a neutral molecule must be 0
▪ 2. The sum of all formal charges in an ion must equal the charge of the ion
▪ 3. Small (or 0) formal charges on individual atoms are better than
large ones
https://www.coursehero.com/file/79005424/Chem-101-Exam-3-Study-Guidedocx/
, ▪ 4. When formal charge cannot be avoided, negative formal charge
should reside on the most electronegative atom
o In this case structure A would be the one to choose because the formal charge
is zero meaning that the structure would be neutral.
o Exceptions to the Octet Rule:
o Incomplete Octets: Elements that tend to form incomplete octets. The most
important is Boron (B) which forms compounds with only 6 electrons around
B rather than 8.
o Expanded Octets: Elements in the third row of the periodic table and beyond
often have expanded octets, with up to 12 and sometimes 14 electrons.
o Free Radicals: Molecules with an odd number of electrons in their Lewis
structure. We cannot write good Lewis structures for free radicals. Ex.NO
and NO2
o Bond Properties:
o Polarity: the degree of polarity in a chemical bond depends on the
electronegativity difference between the two bonding atoms. The greater
the difference the more polar the bond.
o
o Electron Affinity: is a measure of how easily an atom accepts an additional
electron. It is crucial to chemical bonding because it involves the transfer or
sharing of electrons. Electron affinity is usually negative because an atom or
ion usually releases energy when it gains an electron.
▪ How to calculate EN values: For example, if we're looking at the
molecule HF, we would subtract the electronegativity of hydrogen
(2.1) from fluorine (4.0). 4.0 - 2.1 = 1.9
o Bond Energy: Bond energy of a chemical bond is the energy required to break
1 mole of the bond in the gas phase. Bond energies are positive because energy
must be put into a molecule to break a bond.
● Dipole moment: measure of the polarity of a particular bond or molecule.
https://www.coursehero.com/file/79005424/Chem-101-Exam-3-Study-Guidedocx/
o Know how to write Lewis Structures and poly atomic ions (6.3-6.4)
o Resonance Structures: Is one of two or more Lewis structures for the
same molecule. The atoms are in the same locations, but the different
electron arrangements.
o Resonance Hybrid: If the actual structure of the molecule is intermediate
between the two or more resonance structures.
o Resonance Stabilization: When the electrons are stable in a structure, they try to
be more spread out. Resonance stabilization makes and important contribution to
the stability of many molecules.
o Know how to write Lewis structures that include multiple equivalent
resonance structures:
o Formal charges: the charge an atom would have if all bonding electrons were
shared equally between bonding atoms.
o How to calculate: Formal charge = # of valence electrons – (number
of nonbinding electrons + ½ the number of bonding electrons)
o This can help distinguish between competing skeletal/resonance structures
▪ 1. The sum of all formal charges in a neutral molecule must be 0
▪ 2. The sum of all formal charges in an ion must equal the charge of the ion
▪ 3. Small (or 0) formal charges on individual atoms are better than
large ones
https://www.coursehero.com/file/79005424/Chem-101-Exam-3-Study-Guidedocx/
, ▪ 4. When formal charge cannot be avoided, negative formal charge
should reside on the most electronegative atom
o In this case structure A would be the one to choose because the formal charge
is zero meaning that the structure would be neutral.
o Exceptions to the Octet Rule:
o Incomplete Octets: Elements that tend to form incomplete octets. The most
important is Boron (B) which forms compounds with only 6 electrons around
B rather than 8.
o Expanded Octets: Elements in the third row of the periodic table and beyond
often have expanded octets, with up to 12 and sometimes 14 electrons.
o Free Radicals: Molecules with an odd number of electrons in their Lewis
structure. We cannot write good Lewis structures for free radicals. Ex.NO
and NO2
o Bond Properties:
o Polarity: the degree of polarity in a chemical bond depends on the
electronegativity difference between the two bonding atoms. The greater
the difference the more polar the bond.
o
o Electron Affinity: is a measure of how easily an atom accepts an additional
electron. It is crucial to chemical bonding because it involves the transfer or
sharing of electrons. Electron affinity is usually negative because an atom or
ion usually releases energy when it gains an electron.
▪ How to calculate EN values: For example, if we're looking at the
molecule HF, we would subtract the electronegativity of hydrogen
(2.1) from fluorine (4.0). 4.0 - 2.1 = 1.9
o Bond Energy: Bond energy of a chemical bond is the energy required to break
1 mole of the bond in the gas phase. Bond energies are positive because energy
must be put into a molecule to break a bond.
● Dipole moment: measure of the polarity of a particular bond or molecule.
https://www.coursehero.com/file/79005424/Chem-101-Exam-3-Study-Guidedocx/
