lewis structures: 5 steps 1.count valence e- available if an anion, add charge to # valence e- if a...
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Lewis Structures: 5 steps1. Count valence e- available
If an ANION, add charge to # valence e- If a CATION, subtract charge from # valence e-
2. Draw skeletonLeast electronegative atom is the central
atom(usually first element in formula, never H)Place surrounding atoms around the central
atomadd dashes to show bonds from central
atom to surrounding atoms
Lewis Structures-cont’d
3. Satisfy the octet rule for each surrounding atom using valence electrons. Note that H can only have 2 e-
4. Any remaining valence e- go on the central atom. 5. Check final structure
If the octet rule is not satisfied for the central atom, borrow e- pairs to make double or triple bonds.
It is possible for the central atom to have an expanded octet, (eg. SF6) or a contracted octet, (eg. BF3). Theses are exceptions to the octet rule.
Lewis Structures– H2O
1. # val e- available = 2 H = 2(1) = 2
1 O = 1(6) = 6= 8 e- available
2. Skeleton (O is central…)H O H
Lewis Structures– H2O
3. Add dots so that each has 8 e-Each dash is 2 e-
4. Count e- shown : 2 dashes = 4 e-
4 dots = 4 e-
= 8 e- total
5. Check:
8 e- shown = 8 e- available
So OK!!
Lewis Structure, C2H4
1. # e- available = 2 C = 2(4) = 8
4 H = 4(1) = 4= 12 e- available
2. Skeleton (C---C is central…)
Lewis Structure, C2H4, cont’d
3. Add dots so that each has 8 e-
Remember each dash = 2 e-
4. Count e- shown: 4 C-H bonds = 4 (2) = 8 e-
1 C-C bond = 1 (2) = 2 e-
4 dots = 4 e-
total = 14 e-
Lewis Structure, C2H4, cont’d
5. Compare # e- shown to # e- available14 e- shown >12 e- available
So must make a double bond:
• Remove 1 electron from each C
•Combine remaining 2 e- in a covalent bond, forming a double bond
Lewis Structure, C2H4, cont’d
Check: # e-shown = # e- available
12 = 12 OK!!
4 C-H bonds = 4 (2) = 8 e-
1 C=C bond = 1 (4) = 4 e-
total = 12 e-
(or count # lines = 6
6 lines *2e- each = 12 e-)
Lewis Structures Exercise
1. Get in pairs2. Molecules assigned3. Give “model kits”4. Each group use model to make
Lewis Structure5. Show model, draw LS on board
Lewis Structures Exercise- compounds
CH4 N2O NO3 –
NO2 PCl5 NH4 +
SO2 SF6
CO2 SO4 2-
NH3 SO3 2-
XeF4 NO2 –
Molecular Shape – VSEPR Theory
• Can relate Lewis Structure to 3-D shape of a molecule
ValenceShellElectronPairRepulsion Theory
e- pairs arrange themselves around a nucleus to minimize -/- repulsions
e- pairs get as far away from each other as possible and as close to the nucleus as possible
VSEPR Theory
• Use “AXnEm” designation
A = central atomX = atoms bonded to An = # atoms bonded to E = unshared e- PAIRS on A (lone pairs)m = # unshared e- pairs on A• Get from Lewis Structure!!
Use AXE to give shapen+m AXE example shape Bond angle
2 AX2E0 CO2 linear 180o
3 AX3E0 BF3Triangular
planar120o
3 AX2E1 SO2 bent <120o
Use AXE to give shapen+m AXE example shape Bond angle
4 AX4E0 CH4 Tetrahedral 109.5o
4 AX3E1 NH3Trigonal
pyramidal< 109.5o
4AX2E2
H2O Bent < 109.5o
Use AXE to give shape
n+m AXE example shape Bond angle
5 AX5E0 PF5
Triangular bipyramida
l
180o, 90o, 120o
5 AX4E1 SF4 See-saw90o, 180o,
<120o
5 AX3E2 ClF3 T-shape 90o, 180o
5 AX2E3 XeF2 linear 180o
Use AXE to give shape
n+m AXE example shapeBond angle
6 AX6E0 SF6 Octahedral 180o, 90o
6 AX5E1 BrF5Square
pyramidal180o, 90o
6 AX4E2 XeF4Square planar
180o, 90o
Be able to…
1. Draw a Lewis structure for any assigned molecule
2. Based on the Lewis Structure, give the AXE designation.
3. Determine molecule shape, bond angles, and polarity.
Molecule Polarity
• Tell if molecule has one “side” that the electrons like to congregate…
• Based on molecule shape and bond polarity
Bond Polarity
• Even though electrons are shared between two nuclei in covalent bonds, often the sharing is NOT – One atom often has greater affinity for e-
than other
• Look at differences in electronegativity (EN)
• The more EN the atom, the more it “hogs” the shared e-
Bond PolarityWhich is more EN? F or H?
FMake the “bond” an arrow
pointing toward the more EN atom
Put a “+” across the tail (other end) of the arrow
So e- spend more time around F
So F has a “partial negative” charge d-
H has a “partial positive” charge, d+
F H
F H
F H
F H- +
Is Water a polar molecule?
4. Rotate to look end on / smash into page
-
++
-
+
5. If different (+/-), then POLAR
(if same +/+ or -/-, then NONPOLAR)
Intermolecular Forces(IMF)
• Attractive forces between 2 or more molecules
• Need to consider molecule shape and polarity
3 types of IMF
1. Dispersion forces: between NONPOLAR molecules
(weakest)
2. Dipole: between POLAR molecules(intermediate)
3. Hydrogen bonds: special case of dipole forces, between H in one molecule and O, N, or F in another
(strongest)
Dipole Forces
• +/- attraction between POLAR molecules
Partial + on one molecule attracted to the partial – on a neighboring molecule
Hydrogen Bonds
• Special case of dipole attractions• In each molecule, must have H
bonded to O, N, or FPartial + (H) on one molecule attracted to the partial – (O, N, or F) on a neighboring molecule
Hydrogen bonds
• Individually, each H-bond is weak (compared to a covalent bond)
• Collectively, H-bonds are VERY strong, especially in large molecules like proteins or DNA
Dispersion Forces• Between 2 non-polar molecules• “temporary dipole – induced
dipole”“Temporary Dipole”
Dispersion Forces
The “temporary dipole” now “induces” a neighboring molecule to become a dipole
(- pushes e- away from it in the neighbor, making that end of the neighbor +)
Dispersion force = +/- attraction between temporary dipole and induced dipole
IMFs and States of Matter
• Think of the ability of a material to change phases as a measure of the strength of IMFs
Melting (s l)
• As Temperature increases, molecules vibrate/move/bounce more and more
• Gain enough Kinetic energy (KE) to overcome some IMFs
• Molecules can now slide around one another
Boiling (l g)
• As temperature increases, molecules gain more and more energy
• Soon overcome all IMF• Molecules no longer “attached” to
each other• Escape to the gas phase
Solutions
• Homogenous mixture of 2 or more substances
• Solute = material being dissolved, – Usually a solid– Present in least amount
• Solvent = material in which the solute is being dissolved– Usually a liquid– Present in greatest amounts
Before mixing…
Interact w/each other
Strong IMFs
Close packed materials (solid)
Interact w/each other
medium IMFs
Liquid- so molecules still slide around
During Mixing…
Replace IMFs from like molecules with IMFs from others
Now each solute has an IMF interaction with a solvent molecule
“Like Dissolves Like”
• If the IMFs between a solute and a solvent are similar, then the solute will dissolve in the solvent!
• Ex- NaCl in H2O
– NaCl is polar (ionic) H2O is polar (H-bonds)
– Similar IMFs, so NaCl will dissolve in water