CHEMICAL BONDS
the forces that attract atoms to each other in compounds
1.
IONIC BONDING
Ionic compounds (commonly called salts) are formed when metals react with
non-metals.
 Metals have low “Ionization Energies” and easily lose valence electron(s).
 Non-metals have high “Electron Affinities” and tend to gain electrons.
Example: NaCl (s), sodium chloride
 Formed from a sodium cation, Na+, and a chloride anion, Cl -.
 Oppositely charged ions attract each other in an ionic bond.
A sodium atom loses an electron to a chlorine atom.
Lewis Structure
The sodium atom is now a cation and is isoelectronic with Neon.
The chlorine atom is an anion and is isoelectronic with Argon.
For any stable compound to form from its elements, there must be a
net lowering of potential energy resulting in greater stability.
Solid-state ionic compounds have their atoms arranged in a rigid crystal lattice.
Electrical Conductivity
The ions in this solid salt crystal
cannot move freely.
However, if this salt is melted to
liquid state, or if the ions are
dissolved in water, then the ions can
move freely and carry a current of
electricity.
NaCl (s)
The Octet Rule;
When atoms form ions, they like to form a noble gas configuration - atoms tend
to gain or lose electrons until they have obtained a configuration that is the same
as that of the nearest noble gas.
Example: Transferring Multiple Electrons
. .
[Mg] 2+ [:O:] 2..
Problem: Three ions bond - Draw the Lewis Structure for calcium bromide.
2. COVALENT BONDING
Most substances that we see are not ionic but are electrically neutral
combinations called molecules.
Example: hydrogen molecule (H2).
As two H atoms approach each other, the electron of each atom feels the pull
from the other atom. To lower the energy needed, the two nuclei share the
electrons.
Hydrogen is a diatomic element. The electrons are shared equally when two atoms
of the same element come together in a pure covalent bond.
The two nuclei are kept a certain distance away from each other, called a bond
length. When the bond is formed, an amount of energy is released as the
energies for this system are lowered.
Covalent Bonding and the Octet Rule
When atoms form covalent bonds, they tend to share electrons to achieve an
outer shell having eight electrons (except for hydrogen).
Lewis Structures:
Problem: Draw the Lewis Structures for :
a) O2 (g),
b) CO2 (g),
c) N2 (g)
3. Polar Covalent Bonds

0.5 < ΔEN < 1.7 ; atoms have significantly different electronegativities,
but not great enough for electron transfer.
 The bonding electron pair spends more time near the more electronegative
atom.
 The more electronegative atom is given a partial negative charge symbol, δ- .
 The slightly positive atom is given a partial positive charge symbol, δ+ .
Example:
H δ+ ─ F δ-
Predicting Bond Type
1. Physical Property Comparison, p.67 Table 3.1
Property
State @ R.T.
Melting Point
Electrical conductivity
as a liquid
Solubility in water
Electrical conductivity
when dissolved in water
Ionic Compound
Crystalline solid
High
Yes
Covalent Compound
Solid, liquid, gas
Low
No
Most have high
solubility
Yes
Most have low
solubility
Not usually
2. Electronegativity
Use the difference between the electronegativities to determine bond type.
See p. 71 –Electronegativity Table of Values
See p. 73, Pauling Scale. **Always subtract the smaller value from the
larger value.
3.3
1.7
Ionic
0.5
polar covalent
0
mostly
Covalent
Pure
Covalent