The Microscopic Structure of Matter 2

1
The Microscopic Structure of Matter - 2

• Arrangements of atoms and orbitals in space
• Molecular Geometry
• Electronegativity and bond polarity
• Molecular Polarity

2
VSEPR

• Atomic vs molecular orbitals
• Atoms of each element is born with atomic
  orbitals
• Just fine if you are born perfect and stay single
  !!!
• Orbitals need to adjust when orbitals of one atom
  interact with another
• Hybridization (making friends, marriage)
• Formation of molecular orbitals
• Still only 2 electrons per orbital
• Valence shell electron pair repulsion
• Orbitals used in bonding as well as leftover
  orbitals stay as far away from each other as
  possible in order to avoid electron repulsions
• Need to know geometry and angles in order to know
  how to spread out orbitals

3
Molecular Geometry

• The shape of a molecule about a given atom
  depends on the number of groups surrounding that
  atom in the Lewis structure.
• Group An atom or a unshared pair of electrons
• Groups are arranged about a central atom in such
  a way as to keep the groups as far apart as
  possible.
• Since electron repel each other and since there
  are electrons around every atom and in unshared
  pairs, these have to be kept as far away from
  each other as possible

4
Arrangements of orbitals around a central atom

• Arrangements dictated by VSEPR
• Number of electrons around central atom 8 (obey
  octet rule) or 10 or 12
• Electrons around ALL other atoms in Lewis
  structure 8 (2 for H)
• Therefore for linear planar there has to be
  extra bond(s) along one or two orbitals
• Double triple bonds
• Different kind of bond

• Animation of geometries

http//science.widener.edu/svb/molecule/geometry/
5
Specific Geometries
LINEAR
BIPYRAMIDAL
PLANAR
TETRAHEDRAL
OCTAHEDRAL
ARRANGEMENTS OF GROUPS (ATOMS AND ELECTRON PAIRS)
6
Electronegativity - Again

• Electronegativity - A measure of the ability of
  an atom to attract electrons in a bond toward
  itself.
• Typical Values
• F(4.0) O(3.5) N(3.0) C(2.5)
• Cl(3.0) S(2.5) P (2.1) Si(1.8)
• H 2.1 (same as for P)
• Shared electrons in bonds will be pulled more
  toward the element with higher electronegativity

7
Bond Polarity

• Nonpolar Bond - A covalent bond in which the
  electrons are shared equally between the bonded
  atoms.
• Bonded atoms have the same electronegativity
• Polar Bond - A covalent bond in which there is
  unequal sharing of electrons between the bonded
  atoms.
• Bonded atoms have unequal electronegativities
• The atom having the higher electronegativity will
  have a slight negative charge (d-)
• The atom having the lower electronegativity will
  have a slight positive charge. (d)

8
Molecular Polarity

• Nonpolar Molecule - A molecule that does not have
  a net positive end and a net negative end.
• All bonds are either nonpolar or polarities in
  bonds cancel out because of symmetry
• Polar Molecule - A molecule that has a slight
  positive charge on one end and a slight negative
  charge on the other end.
• Bonds are polar and do not cancel each other out
  molecules are asymmetric

9
Determining Arrangements Bond Angles around
central atom(s) Step by Step

• Count the number of groups
• GROUPS ELECTRONS PAIRS AND BONDING GROUPS
• ONE ELECTRON PAIR ONE GROUP
• ONE BONDING GROUP ONE GROUP
• A bonding group a single or a double or a
  triple bond
• Arrangement Bond Angle Chart
• 2 groups linear 180
• 3 groups (trigonal) planar 120
• 4 groups tetrahedral 109.5
• 5 groups (trigonal) bipyramidal 90, 120, 180
• 6 groups octahedral 90, 180

10
Determining Geometries around central atom(s)
Step by Step

• IN DETERMINING GEOMETRY DO NOT LEAVE ARRANGEMENT
• Linear ? all geometries linear
• Planar
• 3 atoms ? planar
• 2 atoms and an electron pair ? bent
• Tetrahedral
• 4 atoms ? tetrahedral
• 3 atoms and an electron pair ? pyramidal
• 2 atoms and two electron pairs ? bent
• Bipyramidal
• 5 atoms ? bipyramidal
• 4 atoms and an electron pair ? seesaw
• 3 atoms and two electron pairs ? T-shaped
• 2 atoms and three electron pairs ? linear
• Octahedral
• 6 atoms ? octahedral
• 5 atoms and an electron pair ? square pyramidal
• 4 atoms and two electron pairs ? square planar

11
Specific Geometries
LINEAR
BIPYRAMIDAL
PLANAR
TETRAHEDRAL
OCTAHEDRAL
ARRANGEMENTS OF GROUPS (ATOMS AND ELECTRON PAIRS)
12
Rules for Overall Polarity of Neutral Species
ALWAYS USE THE ARRANGEMENT TO DETERMINE POLARITY

• Arrangement of Groups
• Linear
• Trigonal Planar
• Tetrahedral
• Trigonal Bipyramidal
• Octahedral

• Polarity
• if atoms on either side of the central atom are
  different, the species is polar
• if any group around central atom is different,
  the species is polar
• be careful with resonance
• if any group is different, the species is polar
• the species is polar unless all the groups are
  the same or if all the groups in the plane are
  the same and the groups in the axial positions
  are the same
• the species is polar unless all groups opposite
  each other are the same

13
Study Resources

• A quiz on structure and bonding
• http//lrc-srvr.mps.ohio-state.edu/under/chemed/qb
  ank/quiz/bank6.htm
• Do not do the following quizzes
• Lewis Structures Involving Odd Electron Species
• Bond Order
• Lattice Energy
• Type of Hybrid Orbital in a Single Species
• Alans Tutorial page lots of Lewis structures
• http//chemistry.alanearhart.org/Tutorials/Lewis/l
  ewis-part9.html
• Scroll to bottom of page
• Four quizzes pick any or all
• Formula given and answer revealed below
• Make the Lewis structure first then check to see
  if you got it right

14
A Formula for Success for Structure, Geometry
and Polarity

• Formula --------gtLewis structure
• Tool follow the rules for making a Lewis
  structure
• Lewis Structure -------gt Arrangement
• Tool Count the number of groups around the
  central atom and assign the arrangement name
• groups atoms (however connected, single, double
  or triple) and electron pairs
• Arrangement -------gt Geometry
• Tool Count the number of atoms attached to the
  central atom (NOT the number of bonds)
• staying in the same arrangement family, assign
  the geometry subclass name
• Geometry -------gt Bond Angles
• Tool assign all bond angles that connect atoms
  to each other
• do NOT leave the arrangement family
• Geometry -----gt Polarity (applies only to neutral
  species)
• Tool inspect the molecule in 3D and determine
  whether one end of the molecule is different than
  another
• if there is a difference, the molecule is polar,
  otherwise it is nonpolar