Chemical Bonds
Formation of Compounds
from atoms
Preparation for College Chemistry
Columbia University
Department of Chemistry
Trends in the Periodic Table
Trends in the Periodic Table
Lewis Structures
VSEPR Model
Atomic and Ionic Radii
Atomic Radii decrease going across a period
from left to right, increases going down group
Ionization Energy
Minimum energy necessary to remove an
electron from a neutral gaseous atom in its
ground state (IE > 0, ground state stable system)
X(g)
X+(g) + e- ∆E = IE1
X+(g
X2+(g) + e- ∆E = IE2
)
First Ionization Energies
Electron Affinity EA
Electron attachment energy. Energy released
when an atom in ground state gains a single
electron.
X(g) + e-
X-(g)
>0
EA
<0
Lewis Structures of Atoms
Gilbert Lewis.
American Chemist
F
2
5
2s p
F
P
2
3
2s p
P
H
F
+ H
H
+
F
F
H
F
H
H
F
F
The Octect Rule
+
F
O
H
+2 H
F
H
H
O
F
H
H
H
O
H
In H2O and HF, as in most molecules and polyatomic
ions, nonmetal atoms except H are surrounded by 8
electrons (an octet). Each atom has a noble gas
electronic configuration (ns2p6 )
Ionic Bond. Electron Transfer
F
Na
+
2
6
(2s p )
F
Na
2
6
(2s p )
+
F
-
Na
-
+
Ionic Bond. Electron Transfer
Cl
Mg
Cl
[Mg]2+
Cl
Cl
Mg
O
[Mg]2+
-
2-
O
2-
O
Al
Al
O
O
O
[Al]3+
[Al]3+
2-
O
2-
O
Ionicity vs. Covalency
Covalent Bond. Sharing e+ H
H
H
H
  Anti Bonding Molecular Orbital
Energy
H
1S1
1S1
 Bonding Molecular Orbital
H
Collinear orbitals form  bond
F
+
F
F
F
F
F
Two p AO = Two 
MO
Only bonding MO shown
Coplanar orbitals form p bond
O
+
O
O
O
O
Four p AO = Two MO, and two pMO
Only bonding MO shown
O
Linus Pauling Electronegativity
-d
+d
H
+
H
Cl
+d
F
0
F
H
Cl
Dipole
H
Cl
-d
Cl
Na+
Cl-
3.3
Lewis Structures of Compounds
Count valence electrons available.
number of valence electrons contributed by nonmetal
atom is equal to the last digit of its group number in the
periodic table.
(H = 1)
Add electrons to take into account negative charge.
Ex.
OCl– ion: 6 (O) + 7 (Cl) + 1 (charge) = 14 valence e–
CH3OH molecule: 4 (C) + 4(H) + 6 (O) = 14 valence e–
Lewis Structures of Compounds
Draw skeleton structure using single bonds
Note that carbon always forms four bonds.
Central atom is written first in formula.
Terminal atoms are most often H, O, or a halogen.
Ex.
H
O — Cl -
H — C — O— H
H
Lewis Structures of Compounds
Subtract two electrons for each single bond
O-Cl– ion: 14 – 2 = 12 valence e- left
CH3 OH molecule: 14 – 10 = 4 valence e- left
Distribute remaining electrons to give each atom a
noble gas structure (if possible).
H
O — Cl
-
H — C — O— H
H
Lewis Structures of Compounds
Too Few Electrons?
Form multiple bonds
Ex. What is the structure of the NO3 – ion?
valence e– = 5(N) + 18 (3O) + 1(charge) = 24 e–
Skeleton:
O
N
O
O
Nitrate Ion (cont.)
valence e– left = 24 - 6 (3 single bonds) = 18 e–
Adding a double bond and rearranging:
O
N
O
O
N
O
O
N
O
O
O
I
II
III
Resonance Structures
O
Molecular Geometry
VESPR principle:
electron pairs around a central atom tend to be
oriented so as to be as far apart as possible.
BeF2 linear (2 pairs of e-)
BF3 trigonal planar (3 pairs of e-)
CF4 tetrahedral (4 pairs of e-)
PF5 triangular bipyramid (5 pairs of e-)
SF6
octahedral (6 pairs of e-)
Molecular Geometry
VSEPR model
Molecule
H2 S
Lewis Str.
H S H
Molecular
Pairs of e- electron
arrangem. Shape
tetrahedral
4
tetrahedral tetrahedral
Cl
CCl4
Cl C Cl
Cl
bent
4