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2.2 Structure-property relationships
A deeper look into the molecule

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Review: Different elements can combine either with ionic or covalent bonds
Na is a metal
Si is a semimetal
Cl is a nonmetal

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Let’s review how to draw Lewis structures (line-bond structures)

1. List how many valence electrons each element has

2. Put the CENTRAL ATOM in the middle, put all other atoms around it
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3. Draw all the valence electrons around each atom as dots
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4. connect 2 dots between each atoms as a single bond
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5. Count to see if hydrogens have 2 electrons, and everything else has 8 electrons (octet)
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6. Add multiple bonds if needed.

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Let’s try some structures.
Underlined are the central atoms

CHCl
3
(chloroform)
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H
2
O
2
(hydrogen peroxide)
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HCN (hydrogen cyanide)
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CH
3
COCH
3
(acetone)

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We also realize certain patterns…

Hydrogen atoms form one bond
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Carbon atoms form four bonds
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Nitrogen atoms form three bonds
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Oxygen atoms form two bonds
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Halogens form one bond when they are surrounding atoms; Fluorine is always a surrounding atom

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Let’s recall that some ions are molecules…
OH
NH
CO
CN -
-
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4
3
2to be able to deal with charges, we need to add or subtract the number of electrons accordingly
We need to add these electrons to the more electronegative (electronloving) atoms.

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Formal charges helps us keep track of charges on our molecule

FORMAL charges can be calculated as:
FC = valence electrons –
(#of surrounding electrons + #of bonds)
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The sum of all formal charges in a molecule will be the overall charge.
Go back to check the OH- ion.

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Let’s try the carbonate ion CO
3
2found in CaCO
3
(lime/corals/shells/toothpaste… putting the extra electrons on it.

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Let’s now try: H
3
O + the hydronium ion, which is the acidified form of water. (vs. OH )
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Since it’s a positive ion, we need to
REMOVE electrons.

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Do these:
OH -
NH
CN -
4
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Let’s review again how to draw
Lewis structures
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1. List how many valence electrons each element has. Check if the structure is an ion. We’ll need to add/subtract electrons if ever.

2. Put the CENTRAL ATOM in the middle, put all other atoms around it

3. Draw all the valence electrons around each atom as dots, adding or removing necessary electrons for ions

4. Connect 2 dots between each atoms as a single bond
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5. Count to see if hydrogens have 2 electrons, and everything else has 8 electrons (octet)
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6. Add multiple bonds if needed.
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7. Check formal charges. Add them up for the total ion charge.

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In reality we can find bonds that aren’t pure single or double bonds…
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RESONANCE STRUCTURES are sort of the different versions of the same thing. In fact the true structure is somewhat of an average of the different resonance structures.

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In reality we can find bonds that aren’t pure single or double bonds…
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OZONE is an allotrope of oxygen O
3 instead of normal O
2
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What is a possible Lewis structure?
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It absorbs UV light from the sun, protecting us, and it’s just a thin layer 3mm thick on average.
There’s a thinning/depleted area naturally occurring but also possible human interferences!

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QUIZ #5 For 1-4 a. Draw the Lewis structures of the ff. b. Show formal charges for each atom
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1. H
2
S a very poisonous gas (2 pts)
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2. C
2
H
2 acetylene, a gas used as fuel in welding (3 pts)
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3. pts)
NO
3
nitrate anion the biologically used form of nitrogen, Oxygen is more electron-loving than Nitrogen. (4
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4. Draw a possible resonance structure of the nitrate ion (1 pt)
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BONUS: name the ion: ______everyday waiting for you/ Darling don't be afraid I have loved you/ For a
thousand years/I'll love you for a thousand more (1pt)

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VSEPR = valence shell electron pair repulsion
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Lewis structures have either atoms or
electron pairs surrounding our central atom
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We treat these as groups that repel each other (same negative charges will repel each other)

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Depending on the number of electron groups, we’ll have different main geometries

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Linear Molecules
AX
2

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Trigonal planar molecules: AX
3
AX
2
E

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Tetrahedral molecules: AX
4
, AX
3
E,
AX
2
E
2

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Trigonal Bipyramidal AX
5
, AX
4
E,
AX
3
E e
, AX
2
E
3

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Octahedral, AX
6
, AX
5
E, AX
4
E
2

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Let’s try more structures now, and then determine their: 1. AXE notation 2. Electron domain geometry 3. Molecular geometry

CHCl
3
(chloroform)
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HCN (hydrogen cyanide)
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CH
3
COCH
3
(acetone)

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More!
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ClO
3
(chlorate ion)
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CO
3
2(carbonate ion AGAIN!)
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I
3
(triiodide ion)
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BrF
5
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XeF
4
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ClF
3

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Molecular structures bring about so much of a molecule’s functionality.

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Molecular structures bring about so much of a molecule’s functionality.
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