MolecularLSRules/Resonance/FC/VESPR/Hybridization/Polarity/IMFs

Rules for Molecular Lewis Structures

1. Halogens can never have double or triple bonds (doesn’t mean it can’t have more than one single bond if central atom)

2. Fluorine + Hydrogen must always be at the END of the molecule. only one bond is permitted
   - F: most electronegative, H: least electronegativity

3. Carbon must have four bonds unless Carbon Monoxide (3 bonds)

4. Cl, Br, I can have more than one single bond around them if they are central atoms are to have an expanded octet (Z>=14)

5. oxygen never bonded to oxygen unless peroxide

6. oxygen bonded to hydrogen as many times as permissible for acids

If theoretical < available, what to do first

First, always check the central atom to see if the Z>=14.

End of Lewis Structures, what to do?

1. Count up total # of electrons

2. Check Bond # for each atom.

Polyatomic Lewis Structures

• if negative charge, add electrons to total

• if positive charge, subtract electrons from total

• NEVER FORGET BRACKETS AND CHARGE AT THE END

Addendum to maximum number of bonds rule

Discrepancy regarding the maximum number of bonds for the central atom (8 - group number), if you must place a multiple bond and it goes over the bond max, O P S and N can handle this situation.

Resonance

phenomenon that occurs when two or more Lewis Structures, that differ only in the placement of the multiple bond, can arise from the same skeletal structure

• each contributing structure is separated by double headed arrow

• Only occurs when you can be place the bond equally and identically to the other locations.

Occurs as a result of Delocalized Bonding (a pair of electrons is spread over a number of atoms rather being confined between two specific atoms.

Resonance Hybrid

a blend of all contributing structures

• contributing structures are all the possible Lewis structures that can be made through Resonance

the more contributing structures a molecule has, (ions too), the more stable the molecule.

Formal Charge

Hypothetical charge the atom in a molecule would have if we could redistribute the electrons in the bond evenly between the atoms.

• FC = valence e - #ofnonbondinge - (1/2)(bonding electrons)

To check formal charge calculations

• determine the sum of the formal charges for whole strcture

• sum must equal zero if a molecule, sum of FC in an ion must equal charge of Ion

Not actual charge of the atoms, just a bookkeeping procedure

What formal charges are preferable?

1. all formal charges of the atoms in the molecule are zeros is more perferable than one where some formal charges are not zero

2. if the lewis structure must have nonzero formal charges, the arrangement with the smallest nonzero formal charges is perferable

3. lewis structures are preferable when adjacent formal charges are zero or of the opposite sign

4. IMPORTANT: if lewis structure contains a negative formal charge (not all zero), the dominant lewis structure is the one in whpich the negative formal charge is assigned to the more electronegative atom.

VSEPR

Valence Shell Electron Pair Repulsion Theory

• a model used to predict the shapes of molecules and polyatomic ions from Lewis Structures

• Valence shell electrons repel each other (Coulombic Repulsive Interactions) so they spread out to minimize the repulsion and attain best possible shape/arrangement around central atom and surrounding atoms

• ‘AXE’ arrangement, A stands for central atom, X stands for # of bonds (where double and triple count as 1), E stands for pairs of non-bonding electrons around central atom

AX₂

Linear, Bond Angle = 180 degrees

AX₂E₁

Bent, Bond Angle = less than 120 degrees

AX₂E₂

Bent, Bond Angle = 104.5 degrees

AX₂E₃

Linear, Bond Angle = 180 degrees

AX₃

Trigonal Planar, Bond Angle = 120 degrees

AX₃E₁

Trigonal Pyramidal, Bond Angle = 107.5 degrees

AX₃E₂

T-Shape, Bond Angle = 90 degrees

AX₄

Tetrahedral, Bond Angle 109.5 degrees

AX₄E₁

Sea-Saw, Bond Angle = Complex

AX₄E₂

Square Planar, Bond Angle = 90 degrees

AX₅

Trigonal Bipyramidal, Bond Angle = 90 degrees and 120 degrees

AX₅E₁

Square Pyramidal, 90 degrees

AX₆

Octahedral, Bond Angle = 90 degrees

Hybridization (Valence Bond Theory)

a theory that explains the mixing of atomic orbitals when atoms form bonds, leading to the formation of hybrids

• five possible hybrids that can be made, sp, sp2, sp3, dsp3, d2sp3, (2-6)

• hybrid depends on the shape of molecule (molecular geometry)

To figure out what hybridization was used to form the particular molecule

1. create lewis structure

2. Apply VESPR

3. look at central atom; pairs of non-bonding electrons around the atom + how many bonds (triple + double, count as 1)

Vectors and how they are used

line segments have both magnitude and direction, (in chem - used to represent a dipole between two atoms)

• magnitude, using a value, just the intensity of the pull between the atoms.

• direction comes from the pull between the two atoms involved in a polar bond

How are vectors used

1. going EQUAL (in magnitude) but opposite in direction, the two dipoles (vectors) cancel out
   - they cancel only If the pull is the same in each direction (meaning same atom attached to central atom 

2. going in same general direction  additive (angle between vectors < 120 degrees) (as if one pull in that general direction)

Dipole Moment

the measurement of the polarity of a molecule.

• symbol: u

• essentially, a polar molecule has a dipole moment which measures the overall polarity

Polar Molecules vs Nonpolar Molecules

Polar molecules have at least one dipole that is not get opposed (cancelled out)

Nonpolar molecules are made entirely of nonpolar bonds or made up of several polar bonds which are equal but in opposite direction, cancelling each other out.

Consequence of being polar vs nonpolar?

• Polar molecules feel attraction toward external magnetic/electrical fields; this is the opposite reaction that occurs for nonpolar molecules

• additionally, like (only polar molecules or only nonpolar molecules) dissolves eachother.