Download The Ties That Bind

The Ties That Bind
Chemical Bonding and
Interactions
Chemical Bonding and Interactions
1.
2.
Stable Electron Configurations
Electron-Dot (Lewis) Structures
1.
2.
3.
3.
Ionic Bonding
1.
2.
4.
Naming ionic compounds
Drawing
Covalent Bonding
1.
2.
3.
5.
6.
Drawing, Rules for Drawing
The Octet Rule
Some Exceptions to the Rule
Naming covalent compounds
Drawing
Electronegativity and Polar Covalent Compounds
Molecular Shapes and the VSEPR Theory
Intermolecular Forces of Attraction
1.
H-bonds, Dipole-Dipole, Ion-Dipole, London Dispersion
Forces
It’s all about stability…
1.
2.
3.
Fact: Noble gases are inert (undergo
few, if any, chemical reactions)
Theory: The inertness of these gases is a
result of their electronic structure
If elements could alter their electron
structures like those of the noble gases,
they would be less reactive.
Electron Configurations of Cations and Anions
Of Representative Elements
Na [Ne]3s1
Na+ [Ne]
Ca [Ar]4s2
Ca2+ [Ar]
Al [Ne]3s23p1
Al3+ [Ne]
Atoms gain electrons
so that anion has a
noble-gas outer
electron configuration.
Atoms lose electrons so that
cation has a noble-gas outer
electron configuration.
H 1s1
H- 1s2 or [He]
F 1s22s22p5
F- 1s22s22p6 or [Ne]
O 1s22s22p4
O2- 1s22s22p6 or [Ne]
N 1s22s22p3
N3- 1s22s22p6 or [Ne]
-1
-2
-3
+3
+2
+1
Cations and Anions Of Representative Elements
Na+: [Ne]
Al3+: [Ne]
O2-: 1s22s22p6 or [Ne]
F-: 1s22s22p6 or [Ne]
N3-: 1s22s22p6 or [Ne]
Na+, Al3+, F-, O2-, and N3- are all isoelectronic with Ne
What neutral atom is isoelectronic with H- ?
H-: 1s2
same electron configuration as He
Valence electrons are the outer shell electrons of an
atom. The valence electrons are the electrons that
particpate in chemical bonding.
Group
e- configuration
# of valence e-
1A
ns1
1
2A
ns2
2
3A
ns2np1
3
4A
ns2np2
4
5A
ns2np3
5
6A
ns2np4
6
7A
ns2np5
7
9.1
(Electron Dot Structures)
*We generally place the electrons one four sides of a square around the
element symbol.
*Octet rule: we know that s2p6 is a noble gas configuration. We assume that
an atom is stable when surrounded by 8 electrons (4 electron pairs).
A violent bond: Sodium and
Chlorine
Sodium is a soft, reactive metal.
 Chlorine is a greenish-yellow gas which is
toxic (WWI poison)
 Sodium chloride is used for our food.

Ionic Bonding
Consider the reaction between sodium and chlorine:
Na(s) + ½Cl2(g)  NaCl(s)
Chemistry In Action:
Sodium Chloride
Mining Salt
Solar Evaporation for Salt
The Ionic Bond
Li + F
1 22s22p5
1s22s1s
e- +
Li+ +
Li+ F [He]
1s
1s2[2Ne]
2s22p6
Li
Li+ + e-
F
F -
F -
Li+ F -
IONIC BONDING
-bonding due to
electrostatic attraction arising
from an exchange of electrons.
9.2
ionic compounds consist of a combination of cations
and an anions
• the formula is always the same as the empirical formula
• the sum of the charges on the cation(s) and anion(s) in each
formula unit must equal zero
The ionic compound NaCl
2.6
Ionic Bonding
*NaCl forms a very regular structure in which each Na+ ion is surrounded by
6 Cl- ions.
Similarly, each Cl- ion is surrounded by six Na+ ions.
There is a regular arrangement of Na+ and Cl- in 3D.
Note that the ions are packed as closely as possible.
Note that it is not easy to find a molecular formula to describe the ionic lattice.
SIDEBAR: Electrostatic (Lattice) Energy
Lattice energy (E) is the energy required to completely separate
one mole of a solid ionic compound into gaseous ions.
Q+Q E=k
r
Q+ is the charge on the cation
Q- is the charge on the anion
r is the distance between the ions
Lattice energy (E) increases
as Q increases and/or
as r decreases.
cmpd
MgF2
MgO
LiF
LiCl
lattice energy
2957 Q= +2,-1
3938 Q= +2,-2
1036
853
r F < r Cl
9.3
9.3
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
2.5
A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-
2.5
Test Yourself:
1.
2.
3.
MgO
MgBr2
The combination of the ion of
magnesium and the ion of nitrogen
Formula of Ionic Compounds
2 x +3 = +6
3 x -2 = -6
Al2O3
Al3+
1 x +2 = +2
Ca2+
1 x +2 = +2
Na+
O22 x -1 = -2
CaBr2
Br1 x -2 = -2
Na2CO3
CO322.6
2.6
2.7
Chemical Nomenclature

Ionic Compounds
 often
a metal + nonmetal
 anion (nonmetal), add “ide” to element name
BaCl2
barium chloride
K2O
potassium oxide
Mg(OH)2
magnesium hydroxide
KNO3
potassium nitrate
2.7

Transition metal ionic compounds
 indicate
charge on metal with Roman numerals
FeCl2
2 Cl- -2 so Fe is +2
iron(II) chloride
FeCl3
3 Cl- -3 so Fe is +3
iron(III) chloride
Cr2S3
3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide
2.7
Test Yourself
1.
Write the formula and draw the ionic structure
1.
2.
3.
4.
5.
6.
2.
Magnesium (II) bromide
Iron (III) Oxide
Sodium Azide
Ammonium Chloride
Magnesium Nitrate
Ammonium Phosphate
Write the name
1.
2.
3.
4.
5.
FeO
Fe2O3
LiBr
KCrO4
K2Cr2O7
2.7
Covalent Bonding
A covalent bond is a chemical bond in which two or more
electrons are shared by two atoms.
Why should two atoms share electrons?
F
+
7e-
F
F F
7e-
8e- 8e-
Lewis structure of F2
single covalent bond
lone pairs
F
F
lone pairs
single covalent bond
lone pairs
F F
lone pairs
9.4
Lewis structure of water
H
+
O +
H
single covalent bonds
H O H
or
H
O
H
2e-8e-2eDouble bond – two atoms share two pairs of electrons
O C O
or
O
O
C
double bonds
- 8e8e- 8ebonds
double
Triple bond – two atoms share three pairs of electrons
N N
triple
bond
8e-8e
or
N
N
triple bond
9.4
Lengths of Covalent Bonds
Bond
Type
Bond
Length
(pm)
C-C
154
CC
133
CC
120
C-N
143
CN
138
CN
116
Bond Lengths
Triple bond < Double Bond < Single Bond
9.4
9.4
Polar covalent bond or polar bond is a covalent
bond with greater electron density around one of the
two atoms
electron poor
region
H
electron rich
region
F
e- poor
H
d+
e- rich
F
d-
9.5
Electronegativity is the ability of an atom to attract
toward itself the electrons in a chemical bond.
Electron Affinity - measurable, Cl is highest
X (g) + e-
X-(g)
Electronegativity - relative, F is highest
9.5
9.5
Bond Polarity and Electronegativity
Electronegativity
Drawing Lewis Structures
1. Add the valence electrons.
2. Identify the central atom (usually the one with the
highest molecular mass, lowest electronegativity, or
closest to the center of the periodic table).
3. Place the central atom in the center of the molecule
and add all other atoms around it.
4.Place one bond (two electrons) between each pair of
atoms.
5.Complete the octet for the central atom.
6.Complete the octets for all other atoms. Use double
bonds if necessary.
7. Place remaining electrons on the central atom.
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – N is less electronegative than F, put N in center
Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5)
5 + (3 x 7) = 26 valence electrons
Step 3 – Draw single bonds between N and F atoms and complete
octets on N and F atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons
F
N
F
F
9.6
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is less electronegative than O, put C in center
Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4)
-2 charge – 2e4 + (3 x 6) + 2 = 24 valence electrons
Step 3 – Draw single bonds between C and O atoms and complete
octet on C and O atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons
Step 5 - Too many electrons, form double bond and re-check # of e-
O
C
O
2 single bonds (2x2) = 4
1 double bond = 4
8 lone pairs (8x2) = 16
Total = 24
O
9.6
Trial
1.
Write the Lewis stucture for
1.
2.
3.
4.
5.
6.
H2O
CO2
NH3
CH4
N2
BH3
Exceptions to the Octet Rule
The Incomplete Octet
BeH2
BF3
B – 3e3F – 3x7e24e-
Be – 2e2H – 2x1e4e-
F
B
H
F
Be
H
3 single bonds (3x2) = 6
9 lone pairs (9x2) = 18
Total = 24
F
9.9
Exceptions to the Octet Rule
Odd-Electron Molecules
NO
N – 5eO – 6e11e-
N
O
The Expanded Octet (central atom with principal quantum number n > 2)
SF6
S – 6e6F – 42e48e-
F
F
F
S
F
F
F
6 single bonds (6x2) = 12
18 lone pairs (18x2) = 36
Total = 48
9.9
Strengths of Covalent Bonds
Bond Enthalpies and Bond Length
Classification of bonds by difference in electronegativity
Difference
Bond Type
0
Covalent
2
0 < and <2
Ionic
Polar Covalent
Increasing difference in electronegativity
Covalent
Polar Covalent
share e-
partial transfer of e-
Ionic
transfer e-
9.5
Classify the following bonds as ionic, polar covalent,
or covalent: The bond in CsCl; the bond in H2S; and
the NN bond in H2NNH2.
Cs – 0.7
Cl – 3.0
3.0 – 0.7 = 2.3
Ionic
H – 2.1
S – 2.5
2.5 – 2.1 = 0.4
Polar Covalent
N – 3.0
N – 3.0
3.0 – 3.0 = 0
Covalent
9.5