Section 9.1 – Bonding of Atoms
Ionic and Covalent Bonds
Ionic: Conduct electricity, high MP, high BP, dissolve in
water
Covalent: Do not conduct electricity, low MP, low BP, do
not dissolve in water
You can make predictions about the properties, not
always right because compounds can be partly covalent
and partly ionic
Pure ionic: Sharing is so unequal that it is best
described as a complete transfer of ePure covalent: e- are shared equally
Most compounds fall between these two extremes (they
have ionic and covalent characteristics)
Partly covalent and partly ionic – electrons may be
shared equally, only slightly (almost not at all)
ELECTRONEGATIVITY (EN)
Measure of the ability of an atom in a bond to attract e(tug-of-war for shared e-)
– Table on p. 302
Atoms w/ large EN values (Fig. 9.5 p. 304) attract valence
e- stronger than atoms w/ small electronegativities.
In General:
EN values increase as you move from left to right on the periodic table.
EN values decrease as you go down a group on the periodic table
Reasoning: Increase left to right b/c of the # of protons increases and the
attraction of the nucleus for valence e- increases.
 Decreases down because the number of energy levels
increase and the valence e- are farther from the
nucleus and the shielding effect (e- in the inner
levels tend to block the attraction of the nucleus for
valence e-)
EN values
The farther the bonding atoms are from each other on
the periodic table, the greater the EN difference.
The greater the EN difference (ΔEN), the more
unequally the e- are shared.
ΔEN = larger EN – smaller EN
Ionic Bonding
When ΔEN is 2.0 or greater, sharing of e- is so unequal
(e- transfer is occurring)
ΔEN ≥ 2.0
Ionic
The greater the ΔEN = the more ionic the bond
F-Cs
ΔEN = 4.0 (F) – 0.7 (Cs) = 3.3
Covalent Bonding
ΔEN = 0 <0.5
Covalent
No difference or small ΔEN:
Diatomic molecules: Pure covalent
ΔEN of C-H
C=2.5
H= 2.1
2.5 – 2.1 = 0.4
Polar Covalent Bonding
Polar Covalent (partial transfer of e- to the more
electronegative atom)
ΔEN = 0.5 - 2.0
Polar Covalent Bonds
Polar covalent bonds are called polar because unequal esharing creates two poles across the bond
(-) pole = more electronegative atom
(+) pole = less electronegative atom
Charges are not 1+ or 1-, instead δ+ and δ-
Water has polar bonds: ΔEN for O-H (3.5-2.1) bond is
1.4
O - H
3.5 - 2.1
Compounds with polar covalent bonds have different
properties from compounds with pure covalent bonds
H2O boiling point: 100˚C due to its polar bonds
e- on O repel each other to be as far apart from each
other as possible
Geometric shape: Bent- Water angles 105 ˚C
Water molecule:
Metallic Bonds
Name some properties of metals:
malleable - shaped
ductile – drawn into wires
conductivity – ease of electron flow
Why?
Metallic bonds can be described as:
a “sea of valence electrons”
shared pool of electrons
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