Download Covalent Bonding

Covalent Bonding: Part 1 (Chapter 7)
Lewis Structures
Dr. Harris
Lecture 9
9/18/12
HW: 7,9, 13, 14, 17,27
Recap
 Ionic compounds are formed by the transfer of electrons from a metal to a
nonmetal
 The result is a cation-anion pair. Each ion has reached its nearest noble gas
configuration
Na ([Ne] 3s1) + Cl ([Ne] 3s2 3p5) ---> Na+ Cl[Ne]
• The Coulombic attraction between the oppositely
charged ions is what holds the molecule together.
This is why ionic compounds are solids at room
temperature.
[Ar]
Na+ Cl-
Covalent Bonding
 Covalent bonding is electron sharing (not transfer) between nonmetals
 Consider Cl2(g)
 Each Cl has 7 valence electrons
[Ne]3s23p5
Cl
Cl
[Ne]3s23p5
 Chlorine is 1 electron short of a full octet ([Ar] configuration)
 Since both atoms are the same, they have the same electron affinities and
ionization energies, so 1 chlorine will not donate an electron to the other.
Covalent Bonding
 For both atoms to achieve an [Ar] configuration (full octet), they share a
pair of electrons between them. An element in a covalent bond will react
so that eight electrons occupy its valence shell. This is the OCTET RULE.
[Ar]
Cl Cl
[Ar]
Covalent bond
 To indicate the covalent bond, we use a solid line. This is a single bond
(2 electrons). Bonding represents an overlap of the valence orbitals.
Cl
Cl
 The electrons not involved in bonding are called lone pairs.
Covalent Bonding
 All elements in a covalent bond will achieve an octet
 Ex. OF2
 Oxygen has 6 valence electrons (needs 2)
 Fluorine has 7 (each F needs 1)
F
[He]2s22p5
F
O
[He]2s22p4
[He]2s22p5
 To achieve octets, each F will share its electron with O
F O F
F
O
F
[Ne]
[Ne]
[Ne]
Electronegativity
 Before we learn about how to draw a Lewis structure, it is very
important to consider what makes ionic and covalent bonds so
different: electronegativity
 Electronegativity is the ability of an atom to attract electrons to itself.
 Therefore, electrons are drawn more towards the more
electronegative atom in a molecule.
 Thus, when there is a difference in electronegativities between atoms
in a molecule, the electrons are NOT equally shared.
Table of Electronegativities
Electronegativity of
atoms increases up
and to the right.
Electronegativity
 Although we have discussed ionic and covalent bonds, most chemical
bonds are neither purely ionic or purely covalent
 Most compounds are an intermediate between the two.
Bond Type
Covalent
Polar Covalent
(Partially polar)
Ionic (Totally Polar)
Difference In
Electronegativity
< 0.4
0.4 – 2.0
> 2.0
Electronegativity
 Let’s consider NaCl. The difference in electronegativity between Na
and Cl is:
3.16 – 0.93 = 2.23
ionic
 Because the difference in electronegativity is so big, the Na electron is
completely pulled away by the Cl atom. So, the molecule is totally
polar (ionic)
Na+
Cl
 This is why there is no actual bond in ionic compounds, only coulombic
attraction.
Electronegativity
• Let’s look at another molecule, like HCl:
3.16 – 2.1 = 1.06
polar covalent
• The HCl molecule is not purely ionic or covalent, but BOTH.
• The electron density is unevenly distributed, such that more of the
electron density is on the Cl than on the H.
• Therefore, the H and Cl have partial charges. The arrow depicts the
direction of electron “pull”, or dipole. Any molecule with a net dipole has
polarity. We will discuss dipoles later.
+
δ
Partial positive character
H
Cl
δ
Partial negative character
General Rules of Drawing Covalent Lewis Structures
1.
Arrange atoms together. If possible, put the least electronegative
atom in the center. Hydrogen is an exception to this rule. Hydrogen
atoms are always terminal.
2.
Compute the total number of valence electrons. Account for
charges.
3.
Represent bonds with solid lines. Ensure octets around all atoms,
except hydrogen. Hydrogens can only accommodate 2 electrons.
4.
Add in remaining lone pairs
5.
If there are not enough electrons to complete an octet, consider
multiple bonds or formal charges.
Example
 Draw the Lewis structure of CCl4
 Carbon is the least electronegative atom (also, there is only one C), so C
is the central atom
 Compute the valence electrons
 C: [He] 2s2 2p2
Cl: [Ne] 3s2 3p5
• Each Cl needs 1, the C needs 4. Therefore, the C
will share each of its 4 valence electrons with Cl
Cl
Cl
C
Cl
Cl
Cl
Cl C Cl
Cl
Cl
Cl
C
Cl
Cl
Hydrogen
 Hydrogen CAN ONLY ACCOMMODATE 2 ELECTRONS
 Ex. H2(g) [He]-like core
H H
 HX where X = F, Cl, Br, I (halogens)
H X
 In chemical structures, hydrogens are always terminal atoms, meaning
that they are at the ends of a molecule
Organic Molecules
 Ex. Methanol, CH3OH
H
H H
H
O
C
Hydrogens are terminal, which means there is a C—O bond in the
center
C O
Now, C needs 3 more electrons around it to achieve an octet. O needs 1.
H
H C O H
H
H
H
C
H
O
H
Double and Triple Bonds
 There are instances where single bonds (2 e- bonds) are not enough to
satisfy the octet rule.
 Ex. C2H4 (ethane)
C
C H
H H
H C C H
H H
H
• As you can see, C only has 7 electrons around it, including a lone electron on
each C. This is highly unfavorable.
• Lone electrons migrate into the C-C bond to form a 4e- double bond.
H C C H
H H
H C
H
C H
H
H
H
C
C
H
H
Double and Triple Bonds
 6e- triple bonds are also possible, as in the case of N2 (g)
N
N
N N
Two unpaired electrons each
N
N
N
N
6 Triple Bond
• The N atoms are sharing 6 electrons, so each N now has a full octet.
Group Examples
 Draw the following Lewis Structures
 NH3
 H2O
 CH3Cl
 CO2
 HCN
Formal Charges
 So far, all of the atoms we’ve seen in covalent compounds have had a zero
charge.
 However, nonmetals can assume positive or negative charges in order to
facilitate the formation of a covalent bond
 Thus, to satisfy the octet rule in certain cases, formal charges have to be
applied
Formal Charges
 Consider ammonium, NH4+
-Lets start by looking at the valence electrons of the neutral atoms
H H H H
N
- As is, N can only make 3 bonds.
X
N
H H H H
- N loses an electron to accommodate the 4th H, attains a charge of 1+.
N = 1+
H=0
H+
H N H
H
+
H
H
N
H
H
Formal Charge Allows Us to Rule Out Structures
 Formal charges can help us determine if we’ve arranged a molecule
incorrectly
The preferred arrangement of atoms in a molecule is always the
arrangement that requires the least amount of formal charge
 If formal charges must exist, the more electronegative elements prefer
negative formal charges, and less electronegative elements prefer
positive ones.
Ambiguity in Atomic Arrangement
 Ex. Hydrogen cyanide, HCN
 We know that C is the central atom in HCN. But how can we rule out HNC?
Lets consider both possibilities
2
valence
H
8
valence
C
8
valence
N
• As we determined earlier, we have a CN
triple bond and an HC bond. All octets
are satisfied.
• There are no formal charges required
Ambiguity in Atomic Arrangement
2
valence
H
2
valence
H
8
valence
7
valence
N
C
8
valence
8
valence
N
C
+
+
-
 If we rearrange N and C, we have the following
structure. As shown, C is 1 electron short of an
octet.
 N can not make anymore bonds because it
already has an octet. N has a 1+ formal
charge because it loses a valence electron.
 C must take on a 1- formal charge to reach
an octet by accepting the N electron. Now,
all the atoms are satisfied, and the molecule
is neutral.
Ambiguity in Atomic Arrangement
 Now, our two possibilities are:
2
valence
H
8
valence
C
8
valence
N
2
valence
H
8
valence
8
valence
N
C
+
-
 Both structures satisfy the octet rule, and both are charge neutral, but
the HNC structure has formal charges on both N and C. Thus, the HCN
structure is preferred.
Group Example
 The correct structure of the NOCl is a central N with a double bond to
O, a single bond to Cl, and a lone pair on the N.
 Use formal charges to show why O is not the central atom.
Resonance
 When a covalent molecule has an overall charge, the charge is said to be
delocalized, meaning that the charge is spread over the whole molecule, as
opposed to being localized on a single atom
 Ex. Nitrite NO2 To draw this molecule, we put N as the central atom. We satisfy an
octet on one O by making a N=O double bond. We can apply the 1charge to the other O to complete its octet (O is more electronegative
than N, so it will not go on N)
O
N
O
The blue electron represents the
electron added by applying the
formal charge
Resonance
• You can’t distinguish between one O and the other. So, you could write
either of the following:
O
N
-
O
O
N
O
• These structure are called resonance structures. We can account for both
structures by writing the resonance form, as shown below. All resonance
structures have a double bond.
O
N
O
Expansion into the d-orbital
 Nonmetals with available d-orbitals (n> 3) can use these orbitals to hold
more than 8 electrons. Unless Carbon is present, assume that these atoms
are the central atoms.
 When drawing Lewis structures of molecules with expanded octets, put
octets around the outer atoms first, then account for any remaining
electrons on the central atom.
F
 Ex. SF4
• After completing octets
on the F atoms, there is a
lone pair remaining on S
F
S
F
F
10 electrons
around Sulfur
Group Examples
 Draw the Lewis structures for the following “expanded octet”
molecules
 PCl5
 SF6
 IF5
• Draw the resonance structures of:
• ClO4• PO43-
Always be mindful when
dealing with atoms of n>3 of
the possibility of surpassing
8 valence electrons.
Revisiting Dipoles: Polar vs. Nonpolar Molecules
 Dipole moments describe the coulombic attraction that is formed between
the partial positive and negative charges on nonmetals when electrons are
unevenly shared.
 Dipole moments are vectors, quantities with both magnitude and
direction. Imagine pulling on a rope. The force is in the direction of the
pull.
 If two people pull on opposing ends of a rope with equal force, the net
force is zero.
 Polar molecules have an overall dipole moment, nonpolar molecules
don’t.
Table of Electronegativities
Electronegativity of
atoms increases up
and to the right.
Here, electronegativity is labeled as β
δ
-
O
δ
+
C
O
δ
-
Equal force of attraction in opposite
directions: Non polar
+
Δβ = 0.89
Δβ = 0.89
δ
-
O
Δβ = 0.89
=0
δ
+
C
S
Δβ = 0.03
δ
-
Unequal dipole moments. Thus, there is
an overal dipole: Polar
+
=
Group Example
 Draw a water molecule. Then draw the dipoles and determine the net
dipole. Is the molecule polar?
H
net dipole =
O
+
H
=
polar