Chemical Bonding Chapter 6 Sections 1, 2, and 5
Dec 24, 2015
Chemical Bonds
A chemical bond is the mutual electrical attraction between the nuclei and valence electrons of different atoms that bind the atoms together
Noble gases tend not to do this because of their filled s and p orbitals.
They have a stable octet: outer s and p orbitals are completely filled with e-’s ( 8 total)
Chemical Bonds
Atoms that don’t have a stable octet are more reactive because their potential energy is higher. They become more stable by decreasing their potential energy.
Octet Rule: chemical compounds tend to form so that each atom has an octet of e-’s in its highest occupied energy level
How to do this? Gain , lose or share electrons between atoms
Chemical Bonds
By forming a chemical bonds, atoms gain stability!
Chemical changes always involve energy
What type of bonds can be formed?Ionic bondCovalent bond
Nonpolar covalentPolar covalent
Chemical Bonds
Ionic bonding: bonds that result from electrical attractions between cations and anions
Covalent bonding: sharing of electron pairs between 2 or more atoms
*** In reality, bonding is often somewhere between the two extremes***
Two types of Covalent Bonds
Nonpolar –covalent: equal sharing of electron pairs
Polar-covalent: unequal attraction for the shared electrons
How can we determine the type of bond?
Knowing how strong an atom’s ability is to attract electrons (aka electronegativity), helps us determine if it will form a ionic or covalent bond with another atom.
A large difference in E.N. between atom’s will result in an ionic bond
A small difference between atom’s will result in a form of covalent bonding
NonpolarCovalentshare e-
Polar Covalent
partial transfer of e-
Ionic
transfer e-
Increasing difference in electronegativity
What type of Bond is it?
Electronegativity Difference Bond Type
0 to 0.3 Nonpolar Covalent
0.4 to 1.7 Polar Covalent
1.7 Ionic
Do you see any trends?
A metal and nonmetal tend to form ionic compounds
Nonmetal and nonmetal tend to form polar-covalent or nonpolar- covalent compounds
H F FH
Polar covalent bond or polar bond :covalent bond with greater electron density around one of the two atoms
electron richregion
electron poorregion e- riche- poor
d+ d-
Classify the following bonds as ionic, polar covalent,or covalent:
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 NonpolarCovalent
CsCl
H2S
N2
Properties of Molecular Covalent
CompoundsNot very soluble in water
Do not conduct electricity
Low melting points
Low boiling points
Can be solids, liquids and gases at room temperature
Review: What are valence electrons?
Lewis Dot Diagrams− an electron-configuration notation with
only the valence electrons of an element are shown, indicated by dots placed around the element’s symbol.
− the inner core electrons are not shown.
• Eight electrons in the valence shell (filling s and p orbitals) make an atom STABLE
s2p6
This is called the octet rule
The Octet Rule
• Bond formation follows the octet rule: Chemical compounds tend to form so that each atom:
by gaining, losing, or sharing electrons, has an octet of electrons in its valence energy level.
Lewis Structures for Compounds • The pair of dots between two symbols
represents the shared pair of a covalent bond.
F F
• Each fluorine atom is surrounded by three pairs of electrons that are not shared in bonds.
• An unshared pair, also called a lone pair, is a pair of electrons that is not involved in bonding and that belongs exclusively to one atom.
Lewis Structures
• The pair of dots representing a shared pair of electrons in a covalent bond is often replaced by a long dash.
H H
F F
covalent bond : is a chemical bond in which two or more electrons are shared by two atoms.
Why should two atoms share electrons?
F F+
7e- 7e-
F F
8e- 8e-
F F
F F
Lewis structure of F2
lone pairslone pairs
lone pairslone pairs
single covalent bond
single covalent bond
Multiple Covalent Bonds
• double covalent bond or double bond :covalent bond in which two pairs of electrons are shared between two atoms
• shown by two side-by-side pairs of dots or by two parallel dashes
HC
H HC
Hor
HC
HC
H
H
Multiple Covalent Bonds
• triple covalent bond or triple bond :covalent bond in which three pairs of electrons are shared between two atoms.
N N or N N
C C or C CH H H H
8e-
H HO+ + OH H O HHor
2e- 2e-
Single Bond – two atoms share one pair of electrons
Double Bond – two atoms share two pairs of electrons
single covalent bonds
O C O or O C O
8e- 8e-8e-double bonds double bonds
Triple Bond – two atoms share three pairs of electrons
N N8e-8e-
N N
triple bondtriple bond
or
Bond Type
Bond Length(pm)
C-C 154
CC 133
CC 120
C-N 143
CN 138
CN 116
Lengths of Covalent Bonds
Bond Lengths
Triple bond < Double Bond < Single Bond
1. Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center.
2. Count total number of valence e-. Add 1 for each negative charge. Subtract 1 for each positive charge.
3. Complete an octet for all atoms except hydrogen
4. If structure contains too many electrons, form double and triple bonds on central atom as needed.
Writing Lewis Structures
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – N is less electronegative than F, put N in center
F N F
F
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
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is less electronegative than O, put C in center
O C O
O
Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4) -2 charge – 2e-
4 + (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-
2 single bonds (2x2) = 41 double bond = 4
8 lone pairs (8x2) = 16Total = 24
resonance structure: one of two or more Lewis structures for a single molecule can be drawn to represent a molecule
O O O+ -
OOO+-
O C O
O
- -O C O
O
-
-
OCO
O
-
-
What are the resonance structures of the carbonate (CO3
2-) ion?
Exceptions to the Octet Rule
The Incomplete Octet
H HBeBe – 2e-
2H – 2x1e-
4e-
BeH2
BF3
B – 3e-
3F – 3x7e-
24e-
F B F
F
3 single bonds (3x2) = 69 lone pairs (9x2) = 18
Total = 24
Exceptions to the Octet Rule
Odd-Electron Molecules
N – 5e-
O – 6e-
11e-
NO N O
The Expanded Octet (central atom with principal quantum number n > 2)
SF6
S – 6e-
6F – 42e-
48e-
S
F
F
F
FF
F
6 single bonds (6x2) = 1218 lone pairs (18x2) = 36
Total = 48
VSEPR THEORYLewis Dot Diagrams are 2D but we live in a 3D world.
How are molecules actually arranged??Follows the Valance Shell Electron Pair Repulsion Theory or VSEPR
Predicting Molecular Geometry
1. Draw Lewis structure for molecule.
2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom.
3. Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4?
SO O
AB2E
bent
S
F
F
F F
AB4E
distortedtetrahedron
H F
electron richregion
electron poorregion
+d -d
The electronegativity of an atom will create a dipole,
or polar molecule.
Which of the following molecules have a dipole moment?H2O, CO2, SO2, and CH4
O HH
dipole momentpolar molecule
SO
O
CO O
no dipole momentnonpolar molecule
dipole momentpolar molecule
C
H
H
HH
no dipole momentnonpolar molecule
Intermolecular forces: attractive forces between molecules.
Intramolecular forces: hold atoms together, attractive forces within a molecule.
Generally, intermolecular forces are much weaker than intramolecular forces.
Properties of Ionic Compounds
Combination of ions (cation/anion)
Hard and Brittle
Tightly packed solids in a crystal lattice
Usually soluble in water
Conducts electricity when dissolved
High melting points