Question

MCAT Tutorial: Chemical Bonding, Lewis Structures, VSEPR Theory

Answer 1

*sigh*

Answer 2

\({\bf{Bonding:}}\)
- ionic bond: two ions (cation positive + anion negative) attract electrostatically
- covalent bond: sharing of electrons between atoms, usu. nonmetals or metalloids
- octet rule: atoms "seek" to have 8 valence electrons in their most stable form
- coloumb's law: F = kq1q1/r^2, describes the electric force between two particles
- lattice energy: deltaE for forming solid from gaseous form
- polar: uneven sharing of electrons due to EN difference
- nonpolar: even distribution of electrons
> C-H bonds are nonpolar
- dipole moment: EN difference between atom, signified by delta + and delta - regions
- molecular dipole moment: overall dipole moment of the whole molecule based on bond type and distribution

boundary between ionic vs covalent (these are just estimations and vary from source to source): calculate the electronegativity difference and see where it belongs

0 - 0.4: nonpolar covalent
0.4 - 1.7: polar covalent
> 1.7: ionic

Answer 3

\({\bf{Lewis~Structures:}}\)
1. count the # of valence electrons (group # generally gives valence electron count) making sure to account for ions
2. put least EN atom in the center
3. draw basic bond structure, giving every atom 1 bond and subtracting 2 electrons from the total electron count for each bond
4. distribute the remaining electrons on the outer atoms (hydrogen only takes 2)
5. place any additional electrons on the central atom
6. change those additional electrons to double bonds to complete the octet as needed (most atoms need 4, sometimes B and Be can only take 2 or 3 bonds, many third period elements take 5+ bonds)

this is probably easier to see by example, so let's do CO2

Answer 4

2 carbon atoms, so 2(4)
1 oxygen atom, so 6

2(4)+6 = 14 valence electrons, now let's draw the main two bonds for the skeletal structure

Answer 5

that takes away 2 pairs of electrons, so 14 - 4 = 10 bonds left

now let's distribute electrons on the oxygens until the octet rule is satisfied

Answer 6

that's 3 pairs of e's left, so 10 - 3(2) = 4 e's left

our central carbon has not had its octet rule satisfied, so let's change each bond to a double bond

Answer 7

and that's our complete lewis structure!

Answer 8

Resonance Structures: when multiple lewis structures are valid the molecule will typically take an intermediate form between the structures

Formal Charge: # of valence electrons - (1/2)(bonding e's) - nonbonding/free e's
using the lewis diagram, it's # of valence electrons minus the number of bonds minus the number of unbound electrons
- the more probable lewis structures have formal charges closer to 0

Answer 9

\({\bf{VSEPR~Theory:}}\) Valence Shell Electron Pair Repulsion
describes the shape of a molecule based on the idea that electrons will arrange themselves to minimize repulsion

based on # of bonds around the central atoms and # of free electrons

Answer 10

https://d2gne97vdumgn3.cloudfront.net/api/file/XAxgCNnYQC6OjiXpnc66

just have to memorize this unfortunately, being able to visualize the structures will help though

Answer 11

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT