Drexel Chemistry 101- Exam 3
Comprehensive Study Guide with
Complete Solutions.
how to write Lewis structures for molecules - Answer Write the correct skeletal structure for
the molecule- least electronegative atom goes in middle
Calculate the total number of electrons for the Lewis structure by summing the valence
electrons (group number) of each atom in the molecule
hydrogen- only requires 2 electrons to complete its octet
Distribute the electrons among the atoms, giving octets (or duets in the case of hydrogen) to as
many atoms as possible
If any atoms lack an octet, form double or triple bonds as necessary to give them octets
double check number of electrons
how to write lewis structures for polyatomic ions - Answer same as molecules- but
Add one electron for each negative charge and subtract one electron for each positive charge
put lewis structure in brackets and write the charge on outside top right
resonance structure - Answer used when two or more valid Lewis structures can be drawn for
the same compound
one of two or more Lewis structures that have the same skeletal formula (the atoms are in the
same locations) but different electron arrangements. The actual structure of the molecule is
intermediate between the two (or more) resonance structures and is called a resonance hybrid
resonance hybrid - Answer actual structure of the molecule which is intermediate between 2
or more resonance structures
, resonance stabilization - Answer resulting stabilization of the electrons (that is, the lowering of
their potential energy due to delocalization)
delocalized electrons - Answer lowers their energy, it stabilizes them
formal charge - Answer helps us to distinguish among competing Lewis structures
the charge an atom would have if all bonding electrons were shared equally between the
bonded angles
calculated charge for an atom in a molecule if we completely ignore the effects of
electronegativity
calculate the formal charge on any atom as the difference between the number of valence
electrons in the atom and the number of electrons that it "owns" in a Lewis structure.
formal charge=number of valence electrons- (number of nonbonding electrons+1/2 number of
bonding electrons)
The sum of all formal charges in a neutral molecule must be zero.
The sum of all formal charges in an ion must equal the charge of the ion.
Small (or zero) formal charges on individual atoms are better than large ones.
When formal charge cannot be avoided, negative formal charge should reside on the most
electronegative atom.
when comparing- find the one that has the least amount of formal charge- that one has the best
resonance structure
exceptions to the octet rule - Answer Odd-Electron Species, Incomplete Octets, and Expanded
Octets
free radicals - Answer Molecules and ions with an odd number of electrons in their Lewis
structures
Comprehensive Study Guide with
Complete Solutions.
how to write Lewis structures for molecules - Answer Write the correct skeletal structure for
the molecule- least electronegative atom goes in middle
Calculate the total number of electrons for the Lewis structure by summing the valence
electrons (group number) of each atom in the molecule
hydrogen- only requires 2 electrons to complete its octet
Distribute the electrons among the atoms, giving octets (or duets in the case of hydrogen) to as
many atoms as possible
If any atoms lack an octet, form double or triple bonds as necessary to give them octets
double check number of electrons
how to write lewis structures for polyatomic ions - Answer same as molecules- but
Add one electron for each negative charge and subtract one electron for each positive charge
put lewis structure in brackets and write the charge on outside top right
resonance structure - Answer used when two or more valid Lewis structures can be drawn for
the same compound
one of two or more Lewis structures that have the same skeletal formula (the atoms are in the
same locations) but different electron arrangements. The actual structure of the molecule is
intermediate between the two (or more) resonance structures and is called a resonance hybrid
resonance hybrid - Answer actual structure of the molecule which is intermediate between 2
or more resonance structures
, resonance stabilization - Answer resulting stabilization of the electrons (that is, the lowering of
their potential energy due to delocalization)
delocalized electrons - Answer lowers their energy, it stabilizes them
formal charge - Answer helps us to distinguish among competing Lewis structures
the charge an atom would have if all bonding electrons were shared equally between the
bonded angles
calculated charge for an atom in a molecule if we completely ignore the effects of
electronegativity
calculate the formal charge on any atom as the difference between the number of valence
electrons in the atom and the number of electrons that it "owns" in a Lewis structure.
formal charge=number of valence electrons- (number of nonbonding electrons+1/2 number of
bonding electrons)
The sum of all formal charges in a neutral molecule must be zero.
The sum of all formal charges in an ion must equal the charge of the ion.
Small (or zero) formal charges on individual atoms are better than large ones.
When formal charge cannot be avoided, negative formal charge should reside on the most
electronegative atom.
when comparing- find the one that has the least amount of formal charge- that one has the best
resonance structure
exceptions to the octet rule - Answer Odd-Electron Species, Incomplete Octets, and Expanded
Octets
free radicals - Answer Molecules and ions with an odd number of electrons in their Lewis
structures
