Lewis Structures Neutral Species 1: Obey Octet Rule

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Lewis Structures Neutral Species 1 Obey Octet
Rule

• of electrons in bonds plus of electrons in
  unshared pairs 8
• Exception H has 1 bond (2 electrons)
• "Normal" Number of Covalent Bonds for Neutral
  Species of Common Elements VALENCE
• F, Cl, Br, I 1
• O, S, Se, Te 2
• N, P, As 3
• C, Si 4
• A bond 2 electrons shared between atoms
• An unshared pair of electrons two electrons
  owned by only one atom

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Lewis Structures Neutral Species 2

• Sometimes we find non normal number of bonds
• Most commonly for N-O, P-O, As-O, S-O, Se-O,
  Cl-O, Br-O, or I-O bonds (also some others)
• O will have only 1 bond and the number of bonds
  to the other element will be greater than the
  normal number for that element
• HOWEVER, the total number of bonds will sum to
  the normal numbers for the elements involved.
• There will be formal charge on the two elements
  involved

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Lewis Structures Charged Species

• A species that has a - charge has a shortage of
  bonds over the normal number
• If a species has received electrons from
  elsewhere, it does not have to share as many
  electrons
• Therefore less bonds have to be made
• A species that has a charge has an excess of
  bonds over the normal number.
• if a species has given up some electrons, it has
  to involve more of the electrons it has kept
• Therefore more bonds have to be made
• The size of the - or charge tells you the
  shortage or excess of bonds
• 2 2 extra bonds -3 3 bond shortage)

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Making Lewis Structures - 1

• Count the number of bonds needed to get a neutral
  species for the atoms given in the formula
• Account for any charges
• subtract 1 for every negative in the formula
  (less bonds needed)
• add 1 for every positive in the formula (more
  bonds needed)
• The adding and subtracting here is not of
  electrons but of bonds needed
• Divide the value you get by 2 since the number of
  bonds you actually have is half of the value for
  individual atoms
• since a bond from one atom goes to another atom
  you have actually determined the bonding
  requirements of each atom NOT the number of bonds
  needed to make the species
• If you cannot connect the number of atoms with
  the number of bonds you have calculated you have
  a species that breaks the octet rule (you cannot
  connect 4 atoms with 2 bonds!!!)
• Other technique

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Making Lewis Structures - 2

• Start making the species using the number of
  bonds specified to connect the atoms in the
  formula
• Various different techniques apply for different
  conditions
• Flow diagram
• After adding bonds
• Count the number of bonds around each atom and
  multiply by 2 to get the number of electrons (1
  bond 2 electrons)
• if an atom in the structure is short of an octet
  (less than 8 electrons in bonds) add pairs of
  electrons to complete octets
• H atoms only gets a bond and no extra electrons
• Assign formal charge to each element in the
  structure if necessary
• Helps determine best structure (if alternate
  possibilities)
• Add brackets for charged species with charge
  indicated outside the bracket
• Check your final structure to make sure you used
  all of the bonding electrons available and no
  more or no less

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Formal Charge

• What is formal charge?
• An indication of whether an atom OWNS more or
  less electrons after bonding than it OWNED before
  bonding
• Ownership all electrons pairs and half the
  electrons in bonds with other atoms
• How is formal charge determined?
• Count the number of electrons in pairs owned by
  the atom plus half the bonding electrons
• Subtract this number from the number of valence
  electrons
• Examples
• 6 valence electrons 4 electrons in bonded atoms
  2
• 6 valence electrons 7 electrons in bonded atoms
  -1
• SENSE The more electronegative atom will have
  the negative formal charge and the less
  electronegative element will have the positive
  formal charge

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Lewis Structures 3 Discovering Species not
obeying the Octet Rule

• Successfully drawing Lewis structures for species
  that break the octet rule starts with recognizing
  that the octet rule does not apply.
• The octet rule is broken by having to few
  electrons (electron deficient species) or too
  many electrons (electron "excess" species)
• We will not deal with electron deficient species
  (B)
• Step 1 Follow steps assuming the octet rule is
  obeyed
• Step 2 Recognize the octet rule is broken
• Step 3 - Follow alternate procedure

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Making non octet Lewis Structures

• Place the least electronegative element in the
  middle (usually first element in formula)
• Connect this "central atom" by single bonds to
  the other atoms in the formula
• Put pairs of electrons around each of the
  peripheral atoms
• always 6 electrons in order to get the peripheral
  atoms to obey the octet rule
• Count the number of electrons used
• 2 e per bond plus electrons in electron pairs
  electrons used
• Determine the number of electrons available for
  bonding
• given by the column of the element in the
  Periodic Table (2nd digit for columns 10 through
  18)
• If the electrons available is greater than the
  electrons used, add extra electrons in pairs to
  the central atom - the structure is now
  complete.

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Lewis Structures with 3 different elements

• If H is the first element and there is O in the
  formula, then the second element in the formula
  is always the central atom in the Lewis structure
  and H (or Hs) is (are) attached to the O (or
  Os) attached to the central atom.
• EXAMPLES H2SO4, HCO3-, H2PO4-, HClO4, HBrO
• If H is not the first element, the elements that
  follow the first element in the formula are ALL
  attached to the first element in the formula
  which is the central atom.
• EXAMPLES SOF4, IOF5, CH2Cl2, CH3Br, CHI3, CH2O
  
• EXCEPTIONS there are certain formulas in which
  the elements are attached linearly

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Linear Lewis Structures

• These are either specifically marked as linear
  or are binary formulas with 2 atoms of the first
  element.
• Specifically marked the order of the elements
  in the formula often breaks the primary formula
  rule and has been written that way to tell you
  how to connect the atoms make the Lewis
  structure that has the lowest and most sensible
  formal charge
• EXAMPLE SCN-, OCS
• Binary formulas both first atoms are central
  atoms connect them to each other, add the atoms
  of the other element to these two making sure you
  use all the bonds calculated for and place in all
  the unpaired electrons. As always minimize
  formal charge.
• EXAMPLES N2H4, N2O4, C2H6
• Rules for organic compounds are different since
  these are usually written as a string of groups
  that each have to obey the rules for Lewis
  structures.

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• Count number of bonds for making neutral species
• Account for any charges
• - 1 for every negative in the formula
• 1 for every positive in the formula
• Divide number obtained by 2 NUMBER OF BONDS
• Can the number of atoms in the formula be
  connected by the number of bonds?

All Formulas
Formula breaks the octet rule
NO
YES
Formula obeys the octet rule
Does the formula have only one type of atom Axy
x number y charge

• Place least electronegative element in the middle
  (usually first element in formula)
• Connect this "central atom" by single bonds to
  the other atoms in the formula
• Put 3 pairs of electrons (6 electrons) around
  each of these peripheral atoms to get the
  peripheral atoms to obey the octet rule
• Count the number of electrons used in bonds and
  electron pairs electrons used
• Determine the number of electrons available for
  bonding (given by 2nd digit of column number in
  PT) subtract electrons if formula charged add
  electrons if formula - charged electrons
  available
• If the electrons available is greater than the
  electrons used, add extra electrons in pairs to
  the central atom - the structure is now
  complete.

YES
NO

• Connect all atoms with only single bonds first
• Add additional bonds between atoms until NUMBER
  OF BONDS obtained
• Add pairs of electrons to each atom in structure
  until there are 8 electrons around each atom in
  structure (except H no electrons, only one
  bond)
• Each bond 2 electrons
• If species is charged, bracket and place charge
  outside bracket

Does the formula have two types of atoms ABxy
or is formula HxAy x number y charge
OTHER FORMULAS
NO
YES

• Connect all H or B atoms to central A atom with
  single bonds first
• Add additional bonds between atoms until NUMBER
  OF BONDS obtained
• Add pairs of electrons to each atom in structure
  until there are 8 electrons around each atom in
  structure (except H no electrons, only one
  bond)
• Each bond 2 electrons
• If species is charged, bracket and place charge
  outside bracket

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Is the formula an acid or acid ion HxABzy
OTHER FORMULAS
NO
YES
Formula contains 3 or more different elements. Is
the formula labeled as LINEAR
YES
NO

• Connect all B atoms to central A atom with only
  single bonds first
• Connect H atoms ONLY to B atoms
• Add any remaining bonds between A-B atoms in
  which B is NOT connected to H
• Add pairs of electrons to each atom in structure
  until there are 8 electrons around each atom in
  structure (except H no electrons, only one
  bond)
• Each bond 2 electrons
• If species is charged, bracket and place charge
  outside bracket

• Connect the big atoms (non H) in the formula
  in order by single bonds (formulas often break
  the primary formula rule).
• If there is more than one atom of the first
  element in the formula, all these atoms are in a
  chain.
• There may be several ways to make the chain
• Add all H atoms in formula to big atoms on chain
• Add any unused bonds between big atoms (you may
  have to move some H atoms to complete your
  structure)
• Add pairs of electrons to each atom in structure
  until there are 8 electrons around each atom in
  structure (except H no electrons, only one
  bond)
• Each bond 2 electrons
• If species is charged, bracket and place charge
  outside bracket

• Connect all atoms following the first (central)
  atom to the first atom with single bonds first
• Add any remaining bonds. If this causes greater
  than an octet of electrons around this atom,
  remove an atom (always an H) and attach it one of
  the atoms that can have another bond
• Now add the extra bond(s) needed between the
  central atom and atom that can have another bond
• Add pairs of electrons to each atom in structure
  until there are 8 electrons around each atom in
  structure (except H no electrons, only one
  bond)
• Each bond 2 electrons
• If species is charged, bracket and place charge
  outside bracket

1. FOR ALTERATE STRUCTURES PICK THE ONE WITH THE
LEAST FORMAL CHARGE 2. FOR OCTET OBEYING
STRUCTURES CHECK TO SEE IF OCTET RULE IS BEING
VIOLATED REARRANGE ATOMS IF NECESSARY TO OBEY
RULE
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Making Lewis Structures

• H2, F2, Cl2, Br2, O2, N2
• HCl, H2O, NH3, NCl3,CCl4, SBr2, PF3
• CO2, SO2, SO3, CO
• NH4,NO3-, NO2-, CO32-, PO43-, SO42-, SO32-
• NO, ClO4-, BrO2-
• H2SO4, HCO3-, HPO42-, HCN, CHCl3, CBr2Cl2,SOCl2
  CH2O, CH2O2
• SCN-, CNS-,N2H4, C2H4O, C2H6O, OCS
• SF6, PCl5, ICl4-, I3-, SF2Cl2

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Making More Lewis Structures Mixed Up!

• COF2 SeCl2, SiF4, C2F4, SbF4-
• SbH3, CH4S, NO2, NO2F
• ClNO (linear), AsF4-, PF6-, ClO2-, BrO3-

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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

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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

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Specific Geometries
LINEAR
BIPYRAMIDAL
PLANAR
TETRAHEDRAL
OCTAHEDRAL
ARRANGEMENTS OF GROUPS (ATOMS AND ELECTRON PAIRS)
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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

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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