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Published on January 16, 2008

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Lewis Structures – Neutral Species 1: Obey Octet Rule:  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 Lewis Structures – Neutral Species 2:  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 Lewis Structures – Charged Species:  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) Making Lewis Structures - 1:  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 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 Making Lewis Structures - 2 Formal Charge:  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 Lewis Structures 3 – Discovering Species not obeying the Octet Rule:  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 Slide8:  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. Making non octet Lewis Structures Lewis Structures with 3 different elements:  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 H’s) is (are) attached to the O (or O’s) 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 Linear Lewis Structures:  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. Slide11:  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. All Formulas 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? NO YES Formula breaks the octet rule Formula obeys the octet rule Does the formula have only one type of atom = Axy x = number; y = charge 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 YES NO OTHER FORMULAS 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 Slide12:  OTHER FORMULAS Is the formula an acid or acid ion = HxABzy NO YES 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 YES NO Formula contains 3 or more different elements. Is the formula labeled as LINEAR 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 Making Lewis Structures:  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 Making More Lewis Structures – Mixed Up!:  Making More Lewis Structures – Mixed Up! COF2 SeCl2, SiF4, C2F4, SbF4- SbH3, CH4S, NO2+, NO2F ClNO (linear), AsF4-, PF6-, ClO2-, BrO3- Determining Arrangements & Bond Angles around central atom(s) – Step by Step:  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 Slide16:  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 Determining Geometries around central atom(s) – Step by Step Specific Geometries:  Specific Geometries BIPYRAMIDAL LINEAR TETRAHEDRAL PLANAR OCTAHEDRAL ARRANGEMENTS OF GROUPS (ATOMS AND ELECTRON PAIRS) Rules for Overall Polarity of Neutral Species:  Rules for Overall Polarity of Neutral Species 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 ALWAYS USE THE ARRANGEMENT TO DETERMINE POLARITY Study Resources:  Study Resources A quiz on structure and bonding http://lrc-srvr.mps.ohio-state.edu/under/chemed/qbank/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 Alan’s Tutorial page – lots of Lewis structures http://chemistry.alanearhart.org/Tutorials/Lewis/lewis-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

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