Todays goals

1
Todays goals

• Connectivity
• Lewis dot structures
• Formal charge

2
Admin stuff

• OWLs First Lewis Dot for Monday
• Many more for Thur.
• Reading Current Ch. 9
• (skip 9.3)
• Pick up handout
• Pick up test

3
Valence electrons
4
Lewis dot structures
A
A
(1)
(2)
(3)
A
A
(4)
(5)
(6)
A
A
How many electrons are on each of these atoms?
Lone pair vs. bond pair
Electrons associated with a single atom
Electrons associated with more than 1 atom
5
Connectivity

• 2nd period elements
• H Be B C N O F Ne

6
Connectivity rules are effective even for
complicated molecules like DNA
7
Connectivity

• 2nd period elements
• usual H Be B C N O F Ne
• bonds 1 2 3 4 3 2 1 0

High connectivity

• These numbers are true except when we have
• (1) Ions (N C in NH4 O- F in OH-)
• (2) More than an octet of electrons (SF6, XeF4)
• (3) Resonance structures, especially those
  including underbonding (HNO3)
• (4) Unequal electron contributions (2/0, not
  1/1)

8
Rules for Lewis dot structures

• Write the skeletal structure (the arrangement of
  atoms within the molecule)
• Count the total number of valence electrons
• Group number for each element valence
  electronsAdd electrons for negatively charged
  ions.Subtract electrons for positively charged
  ions.
• Join atoms by single bonds.
• Single bond 1 pair of electrons.
• Place lone pairs on terminal atoms (except H) to
  complete their octets
• Octet 4 electron pairs around an atom 8
  electronsHydrogen can have only 2 electrons.
• If there are more electrons left, place them as
  lone pairs on remaining atoms to complete their
  octets.
• If there are still atoms with incomplete octets,
  use the electrons from surrounding atoms to make
  double or triple bonds. Do not add electrons.
  "Borrow" them from surrounding atoms.Double bond
  2 pairs of electronsTriple bond 3 pairs of
  electrons

Cl2 O2 N2 CO
9
What is the Lewis dot structure of CO?
(1)
(2)
C
O
C
O
(4)
(3)
C
O
C
O
(6)
(5)
C
O
C
O
Isoelectronic with N2
Is there experimental confirmation?
10
Bond lengths
These numbers are typical values
Atomic sizes are reflected in these numbers, as
3rd period atoms have longer bonds than 2nd
period atoms.
Stronger bonds reduce interatomic distances
11
Rules for Lewis dot structures

• Write the skeletal structure (the arrangement of
  atoms within the molecule)
• Count the total number of valence electrons
• Group number for each element valence
  electronsAdd electrons for negatively charged
  ions.Subtract electrons for positively charged
  ions.
• Join atoms by single bonds.
• Single bond 1 pair of electrons.
• Place lone pairs on terminal atoms (except H) to
  complete their octets
• Octet 4 electron pairs around an atom 8
  electronsHydrogen can have only 2 electrons.
• If there are more electrons left, place them as
  lone pairs on remaining atoms to complete their
  octets.
• If there are still atoms with incomplete octets,
  use the electrons from surrounding atoms to make
  double or triple bonds. Do not add electrons.
  "Borrow" them from surrounding atoms.Double bond
  2 pairs of electronsTriple bond 3 pairs of
  electrons

Examples CCl4, OH, NH4 , HCO3
12
Handout
13

• Only available in class

14
Overhead slides
15
Rules for Lewis dot structures

• Write the skeletal structure (the arrangement of
  atoms within the molecule)
• Count the total number of valence electrons
• Group number for each element valence
  electronsAdd electrons for negatively charged
  ions.Subtract electrons for positively charged
  ions.
• Join atoms by single bonds.
• Single bond 1 pair of electrons.
• Place lone pairs on terminal atoms (except H) to
  complete their octets
• Octet 4 electron pairs around an atom 8
  electronsHydrogen can have only 2 electrons.
• If there are more electrons left, place them as
  lone pairs on remaining atoms to complete their
  octets.
• If there are still atoms with incomplete octets,
  use the electrons from surrounding atoms to make
  double or triple bonds. Do not add electrons.
  "Borrow" them from surrounding atoms.Double bond
  2 pairs of electronsTriple bond 3 pairs of
  electrons