iiiiii molecular structure part i: lewis diagrams ch. 6 – molecular structure lpchem:wz (after...
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I II III
Molecular StructurePart I: Lewis DiagramsCH. 6 – MOLECULAR STRUCTURE
LP
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I. Lewis Diagrams(p. 170 – 175)
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Molecular Compounds:
Are made of nonmetalsNonmetals have high
electronegativity, so they do NOT release their electrons.
Two nonmetals share some of their valence electrons (in bonds) to achieve full octets.
The atoms are CO-valent-ly bonded!
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Shared Valence
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I. Lewis Dot Structures
Are designed to show the placement of valence electrons in covalently bonded compounds.Covalent compounds
= Molecules
= Compounds with shared electrons
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I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.This is the total number of
electrons available to bond the molecule.
(Remember: inner electrons do not participate in bonding.)
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I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.NO2
If the formula carries a CHARGE, add or subtract electrons accordingly.
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(e- tally)
1 N × 5e- = 5e-
2 O × 6e- = + 12e-
17e-
NO2- = 17e-
+1e -
= 18e-
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I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.If the formula carries a CHARGE,
add or subtract electrons accordingly:Negative charge: ADD electronsPositive charge: SUBTRACT
electrons
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I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.(This is the “electron tally.”)
CF4
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1 C × 4e- = 4e-
4 F × 7e- = +28e-
32e-1 2 3 4 5 6 7 8
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I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
NH3
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1 N × 5e- = 5e-
3 H × 1e- = + 3e-
8e-1 2 3 4 5 6 7 8
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I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
SO22-
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1 S × 6e- = 6e-
2 O × 6e- = 12e-
Charge = +2e-
20e-1 2 3 4 5 6 7 8
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I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
SO22-
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NOTE: Charged dot structures (like SO2
2- or PO43-) will
always be drawn inside square brackets!
1 2 3 4 5 6 7 8
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I. Lewis Dot Structures
Step 2: Draw a skeleton structure.A)The first element in the
formula goes in the center.*B)The second element goes
around the first element.Left, right, top, bottom(Don’t choose weird angles.)
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C
F
F
F
F
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I. Lewis Dot Structures
A) The first element in the formula goes in the center.*
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S HH
*BUT NOT HYDROGENH cannot be the center atom.If H is first in the formula, skip
it.Put the second element in the
center.Treat H as the second element.
H2S
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I. Lewis Dot Structures
Step 2: Draw a skeleton structure.C) Draw a bond (line)
connecting each secondary atom to the center.
(Do not connect secondary atoms to each other.)
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CF
F
F
F
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I. Lewis Dot Structures
Step 3: Calculate remaining electrons.A) Each bond (line) represents
two electrons that are shared between two atoms.Number of bonds x 2e- = # e- in bonds
B) Subtract the bonded e- from the total.
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I. Lewis Dot Structures
Step 3: Calculate remaining electrons.CF4
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CF
F
F
F
= 32e-
4 bonds × 2e- = - 8e-
24e-
These electrons will appear as dots.
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.A) Octet rule: every atom needs
8 electrons in its valence.*A bond is two valence electrons
that count as valence for both elements involved (at the same time).
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CO-valence!
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I. Lewis Dot StructuresA) Octet rule: every
atom needs 8 electrons in its valence.*
*BUT NOT HYDROGEN.H can only have two
electrons (because 1s is its only orbital)
When it has a bond, H is “full.”
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S HH
H2S
Both H’s are full.No dots for H!
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I. Lewis Dot StructuresA) Octet rule: every atom needs
8 electrons in its valence.**But not hydrogen.
B) Add electron dots to atoms as needed:You must use up all the e-’s
available.You may NOT use more e-’s than
that!
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.CH4
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CF
F
F
F
= 32e-
4 bonds × 2e- = 8e-
24e-
These electrons will appear as dots.
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.
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CF
F
F
F
24e- dots:C: 4 bonds x 2e- = 8 v.e-.
No dots needed on C
F: 1 bond x 2e- = 2 v.e-.
6 dots needed on each F
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.
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24e- dots:C: 4 bonds x 2e- = 8 v.e-.
No dots needed on C
F: 1 bond x 2e- = 2 v.e-.
6 dots needed on each F
CF
F
F
F
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.
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CF
F
F
F
24e- dots:We used exactly 24e-! Octet check:
Do all atoms* have 8 v.e-.?
YES! We win.
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.NH3
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N
H
HH
= 8e-
3 bonds × 2e- = -6e-
2e-
These electrons will appear as dots.
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N
H
HH
I. Lewis Dot Structures
Step 4: Distribute remaining electrons.
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We used exactly 2e-! Octet check:
Do all atoms* have 8 v.e-.? N has 8 H has 2 We win.
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.C) If you run out of electrons:
Share more!2 e- short = 1 double bond4 e- short = 2 double bonds
or 1 triple bond
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I. Lewis Dot Structures
Let’s try CO2
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(e- tally)
1 C × 4e- = 4e-
2 O × 6e- = + 12e-
16e-
16e- (e- tally)- 4e- (in 2 bonds)12e- (as dots)
C OO
But 12 dots won’t be enough.
We’re 2 e- pairs short.
That means we need 2 more bonds. (4 bonds total)
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I. Lewis Dot Structures
Let’s try CO2
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(e- tally)
1 C × 4e- = 4e-
2 O × 6e- = + 12e-
16e-
Here we go again:16e- (e- tally)- 8e- (in 4 bonds) 8e- (as dots)
C OO
We’re 2 e- pairs short.
That means we need 2 more bonds. (4 bonds total)
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I. Lewis Dot Structures
Let’s try CO2
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(e- tally)
1 C × 4e- = 4e-
2 O × 6e- = + 12e-
16e-
Here we go again:16e- (e- tally)- 8e- (in 4 bonds) 8e- (as dots)
C OO
Octet check! Remember,
each bond counts as 2e-
4 dots, 2 bonds 4 bonds 4 dots, 2 bonds
WIN
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I. Lewis Dot Structures
Step 4: Distribute remaining electrons.D) If you have left-over electrons:
Make an Expanded Octet This is a fancy name for “more than 8
electrons on the central atom.”
Central atom must be at an energy level ≥ 3. Expanded octets cannot exist in the 1st or 2nd E.L.
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4. Distributing Electrons
Make an Expanded Octet1) If there are simply left-over
electrons, put them on the central atom.
No multiple bonds allowed!The limit for expanded octets is 12
electrons total.ONLY the central atom gets more
than 8 electrons.
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I. Lewis Dot Structures
Let’s try SeCl4
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(e- tally)
1 Se × 6e- = 6e-
4 Cl × 7e- = + 28e-
34e-
34e- (e- tally)- 8e- (in 4 bonds)26e- (as dots)
But 26 dots won’t fit!
We’ve got 1 extra e- pair.
That pair goes on Se. (Se gets 10 e- total)Each Cl gets an octet.
Se
Cl
Cl
Cl
Cl
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4. Distributing Electrons
Make an Expanded OctetIf there are more than 4 atoms of
the secondary element, they still bond to the central atom.
Still no multiple bonds allowed!The limit is 6 secondary atoms bonded
to the central atom (Still 12 e- total)Still ONLY the central atom gets more
than 8 electrons.
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I. Lewis Dot Structures
Let’s try PF5
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What’s your e- tally?
1 P × 5e- = 5e-
5 F × 7e- = + 35e-
40e-
How many e- in bonds and how many in dots?
40e- (e- tally)- 10e- (in 5 bonds) 30e- (as dots)
Even before doing a tally we can tell that P must expand its octet in order to bond 5 F atoms.
The 5 Fs are distributed in a pentagon.(If there were six, it would be a hexagon.)
Octet check: P gets 10 e- total Each F gets an
octet.
P
F
FF
F F
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5. Finishing:
A) If your dot structure was for an ion:Place square brackets around
the ion and write the charge outside the brackets:
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ClO4-
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1 Cl × 7e- = 7e-
4 O × 6e- = 24e-
OO Cl O
O
Charge + 1e-
32e-
32e-
- 8e-
24e-
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5. Finishing
B) Resonance Structures Some molecules with double bonds
can’t be correctly represented by a single Lewis diagram.
The actual structure is an average of all the possibilities.
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B) Resonance Structures
Truth: the electrons are evenly distributed, but there isn’t a way to draw that in a Lewis dot structure. (Half a bond? Half a dot?)
Show all possible structures separated by a double-headed arrow.
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D. Resonance Structures LP
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OO S O
OO S O
OO S O
SO3 has 1 double bond which could be
in any of these three places:
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HOMEWORK:
Molecular Geometry WS1st two columns ONLY!(We’ll learn the others next time.)
Skyward!Do the worksheet first– it will give
you the answers to the Skyward problems.
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