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Chapter 13Lewis Structures
Lewis Structures
• Lewis structures are diagrams that show the bonding between atoms of a molecule, and the lone pairs of valence electrons in the molecule.
Steps for Writing Lewis Structures
1. Count the total number of outer level (valence) electrons. To do this use the Roman numeral from the group number above each element in the periodic table.
• PO4
• PO43-
• PO43+
Steps for Writing Lewis Structures2. Determine the layout of the molecule. The formula of the
molecule will often give you a hint as to its layout. For example in the molecule H3CCH3 there are two carbon atoms in the center with three hydrogen atoms bonded to each carbon.
• Carbon is always central• Hydrogen is never central• The element with the lowest electronegativity is central• The element which you have the least of is usually central• Group 17 elements are usually not central (unless you have
no other choice) because they can only form single bonds.– After determining the layout of the molecule. Arrange the elements
symmetrically around your central atom(s).
Steps for Writing Lewis Structures3. The valence electrons from step 1 are now used to
stabilize the atoms. This is done by using shared pairs (bonds) (see Table 1) to attach the atoms to the central atom. Use single bonds for all atoms first.
Valence electrons in Lewis Structures Appear as:
(1) Shared Pairs (form bonds)
(2) Unshared Pairs (Lone
Pairs) — Single bond (one shared pair) Weakest and longest covalent bond
xx, ○○, or ●●
═ Double bond (two shared pairs) Strength and length between single and triple bonds
≡ Triple bond (three shared pairs) Strongest and shortest covalent bond
Note the strengths and lengths of the different types of bonds.
Steps for Writing Lewis Structures3. After adding single bonds you may find that the atoms still need
more valence electrons to achieve their octets. If more valence electrons are needed use unshared pairs (lone pairs) (see Table 1) around each atom to give the atom an octet (8 valence electrons). There are some exceptions to the octet rule. The most common exception is hydrogen which only requires 2 electrons to fill its outer level and become stable.
Valence electrons in Lewis Structures Appear as:
(1) Shared Pairs (form bonds)
(2) Unshared Pairs (Lone
Pairs) — Single bond (one shared pair) Weakest and longest covalent bond
xx, ○○, or ●●
═ Double bond (two shared pairs) Strength and length between single and triple bonds
≡ Triple bond (three shared pairs) Strongest and shortest covalent bond
Steps for Writing Lewis Structures3. The valence electrons from step 1 are now used to
stabilize the atoms. This is done by using shared pairs (bonds) (see Table 1) to attach the atoms to the central atom. Use single bonds for all atoms first.
• After adding single bonds you may find that the atoms still need more valence electrons to achieve their octets. If more valence electrons are needed use unshared pairs (lone pairs) (see Table 1) around each atom to give the atom an octet (8 valence electrons). There are some exceptions to the octet rule. The most common exception is hydrogen which only requires 2 electrons to fill its outer level and become stable.
• If you still do not have a stable structure you may try double and triple bonds if C, N, or O is involved in the bond.
Steps for Writing Lewis Structures
4. If you cannot write a stable structure for the molecule using rules 1 – 3 add or remove unshared pairs to/from the central atom until you arrive at the desired number of valence electrons determined in step 1. This may give you a structure that appears to be unstable however some molecules can form which do not have stable octets.
• Common exceptions to the octet rule: H is stable with 2 valence electrons; B is stable with 6 valence electrons.
Writing Lewis Structures
CO2
Writing Lewis Structures
PO43-
Writing Lewis Structures
H2CO
Writing Lewis Structures
BrNO
Writing Lewis Structures
BrNO
Steps for Writing Lewis Structures2. Determine the layout of the molecule. The formula of the
molecule will often give you a hint as to its layout. For example in the molecule H3CCH3 there are two carbon atoms in the center with three hydrogen atoms bonded to each carbon.
• Carbon is always central• Hydrogen is never central• The element with the lowest electronegativity is central• The element which you have the least of is usually central• Group 17 elements are usually not central (unless you have
no other choice) because they can only form single bonds.– After determining the layout of the molecule. Arrange the elements
symmetrically around your central atom(s).
Writing Lewis Structures
HCCH
Steps for Writing Lewis Structures2. Determine the layout of the molecule. The formula of the
molecule will often give you a hint as to its layout. For example in the molecule H3CCH3 there are two carbon atoms in the center with three hydrogen atoms bonded to each carbon.
• Carbon is always central• Hydrogen is never central• The element with the lowest electronegativity is central• The element which you have the least of is usually central• Group 17 elements are usually not central (unless you have
no other choice) because they can only form single bonds.– After determining the layout of the molecule. Arrange the elements
symmetrically around your central atom(s).
Writing Lewis Structures
HCCH
Writing Lewis Structures
HCCH
Writing Lewis Structures
BeCl2
Steps for Writing Lewis Structures
4. If you cannot write a stable structure for the molecule using rules 1 – 3 add or remove unshared pairs to/from the central atom until you arrive at the desired number of valence electrons determined in step 1. This may give you a structure that appears to be unstable however some molecules can form which do not have stable octets.
Homework
Lewis Structures Worksheet
Homework
Lewis Structures Worksheet
