CHEMISTRY
January 7, 2015
CHEMICAL BONDS
SCIENCE STARTER
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• 5 MINUTES
ANNOUNCEMENT
• LAB ON MONDAY
• FINAL EXAM ON JANUARY 16
AND 20
• NO HOMEWORK ACCEPTED
AFTER JANUARY 22, 2015
REVIEW
• ELECTRONEGATIVITY IS THE ATTRACTION FOR
ELECTRONS
• ELECTRONEGATIVITY INCREASES ACROSS A
PERIOD
• ELECTRONEGATIVITY DECREASES DOWN A
GROUP
• ELECTRONEGATIVITY DOES NOT EXIST FOR
NOBLE GASES
ELECTRONEGATIVITY DIFFERENCE
• Electronegativity differences determine the
type of bonding that may occur.
COVALENT BONDING
• Usually, if the electronegativity between 2
atoms is less than 1.7 than the bond is a
covalent bond.
• For example: In CO2:
– C has an electronegativity value of 2.6 and O has
an electronegativity of 3.4.
– The electronegativity difference is 3.4 - 2.6 which
is 0.8.
– Since 0.8 is less than 1.7, the bond is a covalent
bond.
IONIC BONDING
• Generally, if the electronegativity difference is
greater than 2.0, then the bond is an ionic bond.
• For example: NaCl
– Electronegativity value of Na = 0.9
– Electronegativity value of Cl = 3.2
– Electronegativity difference (high number - low
number) = 3.2 - 0.9 = 2.3
– Since 2.3 is greater than 2.0, the bond is therefore an
ionic bond
IONIC OR COVALENT
• If the electronegativity difference is between
1.7 and 2.0, then
– if one of the atoms is classified as a metal, then an
ionic bond exist
– if both atoms are nonmetals, then a covalent bond
exists.
POLARITY
• Definition - a separation of charge
• Occurs in covalent bonds
POLARITY DETERMINATION
• Polarity is determined based on the
electronegativity of the atom.
• For example: HCl
– Electronegativity value of H = 2.2
– Electronegativity value of Cl = 3.2
– Even though the pair of electrons are shared
between H and Cl, the electrons are positioned
closer to Cl than H because Cl has the higher
electronegativity value.
NONPOLAR
• Nonpolar occurs when the electronegativity differences
between the two atoms in a compound is less than 0.5.
• Example :
• F2 - each F atom has an electronegativity value of
4.0. So, the electronegativity difference is 0. As a
result, F2 is nonpolar. Hence, F2 has a nonpolar
covalent bond
• CH4
–
–
–
–
Electronegativity value of C = 2.6
Electronegativity value of H = 2.2
Electronegativity difference = 2.6 - 2.2 = 0.4.
Thus, CH4 is nonpolar and has a nonpolar covalent bond.
DIPOLE
• Definition of Dipole: difference in charge
between the two atoms caused by a shift in
electron density in the bond.
DIPOLE
• Bond polarity is measured by its dipole moment.
• In other words, the electron density around the
higher electronegative atom results in giving the
atom a partial negative charge (designated as d-).
The less electronegative atom has some of its
electron density taken away giving it a partial
positive charge (d+).
• Example: HCl
– Cl has the higher electronegativity value so Cl will
have a greater electron density. As a result, Cl will
have a partial negative charge while H will have a
partial positive charge.
DIPOLE DIAGRAM
• Dipole moment for a polar covalent bond can
be shown as in the example below.
• For HCl, Cl has higher electron density. Thus,
more of the electrons are positioned closer to
the Cl. To illustrate this, draw the dipole arro
• w as shown below.
REMEMBER
• The greater the electronegativity difference,
the more polar the bond.
• Example:
– HCl vs. HF
– HF has a greater electronegativity
difference. Thus, HF will be more polar.