Lewis structures and VSEPR

Lewis structures CO2 NH3 CH4 H2O CH4 Please draw the valence shell picture for NH3 and CO2. CH4 CH4

NH3 N H N needs three covalent bonds to fill its valence shell.

C O C needs four covalent bonds to fill its valence shell. O requires two covalent bonds to fill its valence shell CO2 C O maximum number of pairs of electrons two main group atoms can share is 3 pairs.

Lewis structures show the bonding. Lewis structures do not show the molecular shape.

Valence Shell Electron Pair Repulsion or VSEPR VSEPR model gives molecular shape.

Bonds formation, not electron pair repulsion, is responsible for molecular geometries. L. S. Bartell J. Chem. Educ., 1968, 45 (12), p 754

VSEPR: Determine steric number (SN) Use SN to predict molecular shape

SN SN 2 5 3 6 4 axial site equitorial site more electronegatve atom always chooses axial site.

two methods to determine SN for VSEPR 1) central atom method 2) Lewis structure method

Central atom method of determining SN (easiest method when there is one atom in the center of the molecule/ion bonded to all the other atoms)

Z = Zentralatom Central atom method is based on the periodic table. H contributes 1 e- to central atom electron count. Br contributes 1 e-. O contributes 0 e-. N takes 1 e- away from central atom count. Z = Zentralatom

Use central atom method to determine SN and shape of:

Determine SN and shape of:

Determine SN and shape of: XeF5+ XeO3 IOF4- SOCl2 General rule: (99+% true in stable molecules/ ions) The least electronegative main group atom (excluding H) is at the center. Determine SN and shape of: XeF5+ XeO3 IOF4- SOCl2 c(F) > c(O) > c(N) = c(Cl)

The remarkable molecule OFCl

two methods to determine SN for VSEPR 1) central atom method 2) Lewis structure method

As the molecules get less central-atom-like it is easiest to use the Lewis-structure method for determining the steric number, SN

Use the Lewis-structure method to determine a plausible molecular shape for pyro-sulfuric acid.* sulfuric acid *method requires one to determine Lewis structure before determining molecular shape pyro- sulfuric acid

compound XnYm Have you seen XnYm before? Use metallic bonding yes Have you seen XnYm before? no atoms have 10-16 bonds each, often in closest packing Use metallic bonding yes Locate X and Y on the periodic table. Are X and Y both metals no Are X and Y from opposite sides of the periodic table? Are X and Y both p-block elements? Is Dc small or are X or Y 1st and 2nd row elements? Use covalent bonding model no yes yes or yes no no Use ionic bonding model central atom method determine Lewis structure Ionic metal atoms often 6 coordinate determine Lewis structure apply VSEPR Determine a plausible molecular/extended solid shape. If a portion of the compound might be covalent use covalent model for that portion. Is XnYm a molecule or extended solid? Is XnYm a molecule or extended solid? the shape of XnYm

Determine shapes of: I3- CO2 NH3 TeF6

formal charge

Two standard methods for electron counting a) formal charge method: Divide equally the shared electrons in bonds between the bonded atoms. Lone pairs ( ) are assigned to host atom. b) oxidation state method oxygen starts with six outer shell electrons O carbon starts with four outer shell electrons C

Two standard methods for electron counting a) formal charge method: Divide equally the shared electrons in bonds between the bonded atoms. Lone pairs ( ) are assigned to host atom. oxygen starts and ends with six outer shell electrons. O formal charge is zero. O carbon starts and ends with four outer shell electrons. C formal charge is zero. C

Please draw a Lewis structure for each of the the above three species. Where ever an atom has non-zero formal charge please note the non-zero charge next to the atom.

Please draw a Lewis structure for each of the the above three species. Where ever an atom has non-zero formal charge please note the non-zero charge next to the atom.

Correct Lewis structures require placement of all non-zero formal charges on the Lewis diagram.

octet rule

If formal charge is zero :

If formal charge is +1 then atom behaves as if it is an atom one column to the left. If formal charge is -1 then atom behaves as if it is an atom one column to the right. and so forth.

assuming atom has eight valence shell electrons.

assuming atom has eight outer shell electrons.

modern and traditional Lewis structures

deducing modern Lewis structures if at all possible, all main group atoms (besides H) have an outer (valence) shell with eight electrons. Hydrogen just needs two. This is the octet rule. Outer shell electrons are called valence electrons. 2. having done step 1, make formal charges as small as possible. 3. no more than 3 bonds (multiple bonds) allowed between two main group atoms. One of the two atoms in this multiple bond is typically a second row element. it's a puzzle and needs to be solved like a puzzle

draw Lewis structures for all the ions below:

carbonate has 3 resonance structures how many resonance structures does bicarbonate have?

draw Lewis structures for all the ions to the left:

deducing traditional Lewis structures make formal charges as small as possible. 2. second row main group elements almost always obey the octet rule. 3. other rows obey octet rule if it doesn't interfere with rules 1 and 2. 4. multiple bonds: allowed to make up to 3 bonds between a pair of main group atoms. One of the two atoms in multiple bond is typically a second row element. it's a puzzle and needs to be solved like a puzzle

please deduce both modern and traditional Lewis structures for the molecules and ions below Please also deduce SN and geometry: SO42-, the sulfate ion XeO4 PF3Cl2 the modern Lewis structure of PF3Cl2 is a resonant ''detached'' diagram where each resonant structure looks like the molecule is separated in two. But the two resonant structures taken together show a single cohesive whole molecule. Detached diagrams like this always require the three atoms to be in a straight row, a syzygy. The bonding is called the 3-center 4-electron bond.

comparative studies of modern and traditional Lewis structures show the modern method is more correct.

comparative studies of modern and traditional Lewis structures show the modern method is more correct.