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Lewis Dot Structures
Gateway to Understanding Molecular Structure
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Molecular Structure & Bonding
A molecular structure, unlike a simple molecular formula, indicates the exact 3-D nature of the molecule. It indicates which atoms are bonded to which atoms, and the 3-D orientation of those atoms relative to each other.
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Molecular Formula vs. Molecular Structure
Molecular formula – H2O
Molecular structure: .. ..
O
H H
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Molecular Structure
Two issues:• What is stuck to what?• How are they oriented?
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What is stuck to what?
The first thing you need to do in drawing a molecular structure is to figure out which atom sticks to which other atoms to generate a skeletal model of the molecule.
The skeletal model is called a Lewis Dot Structure.
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Lewis Dot Structures
The first step towards establishing the full 3-D geometry of a molecule is determining what is stuck to what and how each atom is connected.
Lewis Dot Structures provide this information.
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Two Rules
1. Total # of valence electrons – the total number of valence electrons must be accounted for, no extras, none missing.
2. Octet Rule – every atom should have an octet (8) electrons associated with it. Hydrogen should only have 2 (a duet).
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Total Number of Valence Electrons
The total number of available valence electrons is just the sum of the number of valence electrons that each atom possesses (ignoring d-orbital electrons)
So, for H2O, the total number of valence electrons = 2 x 1 (each H is 1s1) + 6 (O is 2s22p4) = 8
CO2 has a total number of valence electrons = 4 (C is 2s22p2) + 2 * 6 (O is 2s22p4) = 16
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Determining the number of valence electrons:
Full d-orbitals do not count as valence electrons. They belong to the inner shell.
For example:
As is [Ar]4s23d104p3
This is FIVE (5) valence electrons. The 3d is part of the inner shell (n=3) which is full.
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How many valence electrons does Ge have?
A. 12B. 14C. 3D. 4E. 5
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12
!
1s1
2s1
3s1
4s1
5s1
6s1
7s1
2s2
3s2
4s2
5s2
6s2
7s2
3d1
4d1
5d1
6d1
3d6
4d6
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3d7
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3d8
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3d9
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3d10
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3d2
4d2
5d2
6d2
3d3
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5d3
6d3
3d4
4d4
5d4
6d4
3d5
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1s2
2p6
3p6
4p6
5p6
6p6
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2p5
3p5
4p5
5p5
6p5
7p5
2p4
3p4
4p4
5p4
6p4
7p4
2p3
3p3
4p3
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6p3
7p3
2p2
3p2
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6p2
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2p1
3p1
4p1
5p1
6p1
7p1
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Take a look at Ge electron structure
[Ar]4s23d104p2
Full d-orbitals don’t count. So there are 4 valence electrons.
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How many valence electrons does Ti have?
A. 1B. 2C. 3D. 4E. 5
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How many valence electrons does Te have?
A. 15B. 16C. 3D. 5E. 6
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Two Rules
1. Total # of valence electrons – the total number of valence electrons must be accounted for, no extras, none missing.
2. Octet Rule – every atom should have an octet (8) electrons associated with it. Hydrogen should only have 2 (a duet).
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Central Atom
In a molecule, there are only 2 types of atoms:
1. “central” – bonded to more than one other atom.2. “terminal” – bonded to only one other atom.
You can have more than one central atom in a molecule.
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BondsBonds are pairs of shared electrons.
Each bond has 2 electrons in it.
You can have multiple bonds between the same 2 atoms. For example:
C-OC=OC OEach of the lines represents 1 bond with 2 electrons in it.
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Lewis Dot Structure
Each electron is represented by a dot in the structure
.
:Cl: ¨
That symbol with the dots indicate a chlorine atom with 7 valence electrons.
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Drawing Lewis Dot Structures
1. Determine the total number of valence electrons.
2. Determine which atom is the “central” atom.3. Stick everything to the central atom using a
single bond.
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Dot structure for H2O
1. Total number of valence electrons: 6 + (2 x 1) =8
2. Central Atom – typically, the central atom will be leftmost and/or bottommost in the periodic table. It is the atom that wants more than one thing stuck to it. H is NEVER the central atom.
3. Stick all terminal atoms to the central atom using a single bond.
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Dot structure for H2O
H – O – H
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Drawing Lewis Dot Structures
1. Determine the total number of valence electrons.2. Determine which atom is the “central” atom.3. Stick everything to the central atom using a single
bond.4. Fill the octet of every atom by adding dots.5. Verify the total number of valence electrons in the
structure.
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Dot structure for H2O
..
H – O – H ¨
That is a total of 8 valence electrons used: each bond is 2, and there are 2 non-bonding pairs.
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Do these two structures look the same?
A. YesB. No
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Are they the same?
A. YesB. NoC. What do you mean by “the same?
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Drawing Lewis Dot Structures1. Determine the total number of valence electrons.2. Determine which atom is the “central” atom.3. Stick everything to the central atom using a single bond.4. Fill the octet of every atom by adding dots.5. Verify the total number of valence electrons in the
structure.6. Add or subtract electrons to the structure by
making/breaking bonds to get the correct # of valence electrons.
7. Check the “formal charge” of each atom.
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Formal Charge of an atom“Formal charge” isn’t a real charge. It’s a pseudo-charge on a
single atom.
Formal charge = number of valence electrons – number of bonds – number of non-bonding electrons.
= number of valence - # of lines - # of dots
Formal charge (FC) is ideally 0, acceptably +/-1, on occasion +/- 2. The more 0s in a structure, the better.
The total of all the formal charges of each atom will always equal the charge on the entire structure (0 for neutral molecules).
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Dot structure for H2O
.. H – O – H ¨
FC (H) = 1-1-0 = 0FC (O) = 6 – 2 – 4 = 0
This is excellent, all the FCs are 0!
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DON’T EVER STOP AND THINK ABOUT WHERE THE ELECTRONS CAME FROM!!!
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Clicker
Choose the best Lewis Dot Structure for: SCl2
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N2S
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Another example
Let’s try CO2
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Drawing Lewis Dot Structures1. Determine the total number of valence electrons.2. Determine which atom is the “central” atom.3. Stick everything to the central atom using a single bond.4. Fill the octet of every atom by adding dots.5. Verify the total number of valence electrons in the
structure.6. Add or subtract electrons to the structure by
making/breaking bonds to get the correct # of valence electrons.
7. Check the “formal charge” of each atom.
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CO2
CO2
Total number of valence electrons = 4 from carbon + 2x6 from oxygen = 16
Central Atom?
Either C or O could be a central atom. C is more likely (to the left, to the left, to the left…)
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CO2
CO2
16 total valence electrons
O – C – O
Fill out the octets.. .. ..
:O – C - O: ¨ ¨ ¨
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Drawing Lewis Dot Structures1. Determine the total number of valence electrons.2. Determine which atom is the “central” atom.3. Stick everything to the central atom using a single bond.4. Fill the octet of every atom by adding dots.5. Verify the total number of valence electrons in the
structure.6. Add or subtract electrons to the structure by
making/breaking bonds to get the correct # of valence electrons.
7. Check the “formal charge” of each atom.
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CO2
16 total valence electrons
.. .. ..
:O – C - O: ¨ ¨ ¨Structure has 20 electrons in it. Too many!
I need to lose 4 electrons. What’s the best way to do that?
Make 2 bonds – each new bond costs 2 electrons
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CO2
:O = C = O: ¨ ¨Structure has 16 electrons in it. Just right!Notice, this works because there are 2 ways to count
the electrons:1. When I count the total # of electrons, I count each
electron once.2. When I count the electrons for each atom, I count
the bond twice (once for each atom in the bond)
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CO2
:O = C = O: ¨ ¨Is this the only structure I could have drawn?
I only needed two new bonds, I didn’t specify where they needed to go!
..:O C - O: ¨ .. :O - C O: ¨ Which is correct?
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Choosing between different structures?
The first test is formal charge::O = C = O: ¨ ¨FC (O) = 6 – 2 – 4 = 0FC (C) = 4 – 4 – 0 = 0 ..:O C - O: ¨ FC (left O) = 6 – 3 – 2 = 1FC (C) = 4 – 4 – 0 = 0FC (right O) = 6 – 1 – 6 = -1Based on formal charge the upper structure is the better one.
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I drew a happy Lewis structure (0,0,0). Is it the CORRECT structure
a. Yesb. Noc. Stop with the nonsensed. It’s another effing trick
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Are these even different?
..:O C - O: ¨ .. :O - C O: ¨ Depends on what I mean by different!
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Are they different? ..:O1 C – O2 : ¨ .. :O1 - C O2 : ¨ If I label them, I can see a difference. (Isotopic
labeling).If I don’t label them, they are interchangeable, just
rotate the top one to get the bottom one.
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Resonance ..:O1 C – O2 : ¨ .. :O1 - C O2 : ¨
O=C=OStructures that are identical, but differ only in the arrangement of
bonds are called resonance structures.
Resonance is always GOOD!
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Resonance
When you have resonance, the real structure is not any one of the individual structures but the combination of all of them.
You can always recognize resonance – there are double or triple bonds involved.
If you take the 3 different CO2 structures, the “average” is the original one we drew with 2 double bonds.
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ResonanceResonance is indicated by drawing all resonance
structures, separated by “ ”
.. .. :O C - O: :O - C O: :O = C = O: ¨ ¨ ¨ ¨
But this is not necessary in this case, as the last structure is also the combination of the 3 structures
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Nitrite ion
Draw the Lewis Dot structure for NO2-
How many valence electrons?N has 5, O has 6, but there’s one extra (it’s an
ion!)
5 + 2 (6) = 17 valence electrons + 1 extra = 18 valence electrons
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Nitrite LDS
What’s the central atom?
NitrogenO – N – O .. .. .. :O – N - O: ¨ ¨ ¨Total number of electrons?20 electrons – too many
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Nitrite LDS.. .. .. :O – N - O: ¨ ¨ ¨How do you fix the problem?Make a bond
.. .. .. :O = N - O: ¨What do you think?RESONANCE
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Nitrite LDS
.. .. .. .. .. ..:O = N - O: :O - N = O: ¨ ¨What’s the real structure look like?It’s an average of those 2. Kind of 1-1/2 bonds between
each N and O! In fact, if you measure the bond angles in nitrite, you find that they are equal (a double bond would be shorter than a single bond)
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Let’s try another…
CO32-
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N2H2
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PO43-
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Exceptions to the Octet Rule
There are exceptions to the octet rule:
1. Incomplete octets – less than 8 electrons.2. Expanded octets – more than 8 electrons
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Incomplete OctetsThe most common elements that show incomplete octets are B,
Be besides H.
So, for example, BCl3 has the Lewis structure: .. ..
: Cl – B – Cl: ¨ | ¨
: Cl : ¨Total valence electrons is correct at 24. FC (B) = 3 - 3 – 0 = 0FC (Cl) = 7- 1 - 6 = 0
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H, B, Be, Li
ALMOST NEVER HAVE COMPLETE OCTETS.
They are just too electron poor.
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Expanded OctetsThe most common atoms to show expanded octets are P and S.
It is also possible for some transition metals.
An example of an expanded octet would be PCl5: .. .. :Cl: :Cl: Total valence e- = 40 .. .. :Cl – P - Cl : FC(P) = 5 – 5 – 0 =0 ¨ | ¨ : Cl: FC (Cl) = 7 – 1 – 6 = 0 ¨
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How many resonance structures does SO42-
have
A. 2B. 3C. 4D. 5E. 6F. 7G. 8H. 9I. 10J. 11
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Those pesky d-orbitals
Once you get to n=3, you have d-orbitals available.
The octet rule really arises from the sum of the s-orbitals (2 electrons) and p-orbitals (6 electrons).
Anything with d-orbitals (10 electrons) COULD expand its octet when necessary.
So anything beyond P in the periodic table COULD. S & P USUALLY DO.
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Let’s talk bonds!
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What holds molecules together?
Bonds
Bonds are made up of?
Electrons
How do the electrons hold atoms together?
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Two ways:
• Ionic Bonds – attraction between ions of opposite charges
Na+ Cl-
• Covalent Bonds – sharing of electrons between adjacent atoms
PF3
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Are they really different?
Let’s share a pie!
Which pie are we actually sharing?
Mine YoursYours Mine
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Sharing doesn’t have to be equal!
Mine
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Ionic and covalent are part of a continuum
Ionic Covalent
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Two extremes
Mine Yours
Ours
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Something in the middle
Mine YoursYours Mine
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Ionic and covalent are part of a continuum
Ionic Uneven sharing Equal sharing
Non-polarPolar
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The truth about bonds
Covalent – bonding by sharing of electrons
Ionic – bonding by attraction between oppositely charged ions
Really, they are exactly the same thing!
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So, consider a bond, any bond:
Cl – Cl
Which case is this?
Equal sharing!
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So, consider a bond, any bond:
H-Cl
Which case is this?
Unequal sharing! How do you know?
They are on opposite sides of the Periodic table!
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A metal + a non-metal =
An ionic compound!
Non-metals love electrons, metals don’t!
There is a periodic trend for “electron love”: electronegativity or electron affinity.
Electronegativity increases to the right and going up (F is most electronegative, Fr is least)
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Electronegativity
Electronegativity is the ability of an atom to attract electrons to itself.
Electronegativity is important in predicting whether a bond is ionic or covalent.
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Loving electrons
I love pie.
I have a pie sitting in front of me.
You sort of like pie (or maybe you’re smaller than me!).
You get no pie!
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Loving electrons
I love pie.
I have a pie sitting in front of me.
You really, really, really love pie (or maybe you’re bigger than me!).
I get no pie.
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Loving electrons
I like pie.
I have a pie sitting in front of me.
You like pie.
We each get ½ the pie.
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Electrons are like pie!
The “sharing” of electrons is really a sliding scale from completely equal (non-polar bond) to completely unequal (ionic).
The electronegativity helps me decide.
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Suppose I’m oxygen…
…you need me to live!
I’m oxygen. How much do I like pie…er, electrons?
Check my electronegativity…
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I’m oxygen, I need a friend…
ONLY O has an electronegativity of 3.5. The only completely equal sharing of electrons is with O.
O2 – completely equal covalent bond. Non-polar.
Suppose, I make a new friend that is not myself (that would be NICE!) like N.
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O (EN = 3.5)N (EN = 3.0)
Close, but not the same. The difference is 0.5. What kind of bond is this?
POLAR covalent.
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Arbitrarily:
The polarity of a bond is determined by the difference in electronegativity between the atoms at either end of the bond.
E.N. = Larger E.N. – smaller E.N.
E.N. = 0 to 0.4 - NON-polar covalent bond
E.N. = 0.401 to 1.999 – POLAR covalent bond
E.N. = 2.0+ IONIC bond89Text 187752 and message to 37607
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Cl – Cl
E.N. = 3.0 – 3.0 = 0Non-polar
H-Cl
E.N. = 3.0 – 2.1 = 0.9Polar
NaCl E.N. = 3.0 – 0.9 = 2.1Ionic
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Polarity is represented as an arrow…
…pointing toward the more negative atom.
Cl – Cl
ᵟ+ ᵟ-
H-Cl
Na+ Cl-
NaCl92Text 187752 and message to 37607
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Bond polarity is local…
The polarity of a bond refers only to the bond itself: the two atoms that are bonded together.
For molecules as a whole, there is still “polarity” but it is a more complicated thing that depends on 3-D geometry.
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H2O
Polarity is a “vector”, it has size and direction. You can’t separate the two. Think of it as travel directions.
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If I leave my house and go 1 mile North and then 1 mile South, where am I?
1 mile North1 mile South
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If I leave my house and go 1 mile North, and then 1 mile West, where am I?
1 mile North
1 mile West
1.414 mi NW
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H2O
A polarity vector is just the direction that a proton would go (toward the negative), and the length of the vector is its magnitude.
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H2O
The polarity of the molecule is distinct from the polarity of the bonds in the molecule.
Non-polar bonds = Non-polar molecule
Polar bonds…depends on the geometry!
NetDipole
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Vector Addition
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Molecule Polarity
101
The O-C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.
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Molecule Polarity
102
The H-O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.
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