Happy
Valentine’s
Day on
Friday!
Wednesday, Feb. 12th: “A” Day
Thursday, Feb. 13th: “A” Day
Agenda
Chapter 5 tests/Chapter 5 Summary
Begin Chapter 6: “Covalent Compounds”
Sec. 6.1: “Covalent Bonds”
Covalent bond, molecular orbital, bond length,
bond energy, non-polar covalent bond, polar
covalent bond, dipole
Homework:
Section 6.1 review, pg. 198: #1-14
Concept Review: “Covalent Bonds”
Chapter 5 Tests
Class
2A
4A
1B
3B
Average Score
(out of 65)
54.63
52.56
54.29
58.56
Average
Percentage
84.05%
80.86%
83.52%
90.09%
Sec. 6.1: “Covalent Bonds”
When an ionic bond forms, electrons are
rearranged and are transferred from one atom
to another to form charged ions.
In another kind of bond, the neutral atoms
share electrons.
Sharing Electrons
The simplest example of sharing electrons is found
in diatomic molecules, such as hydrogen, H2.
Since the 2 atoms are the same, neither atom will
remove the electron from the other, they will share
them equally.
The result is that the H2 molecule is more stable
than either hydrogen atom alone.
By sharing their 1 electron, each hydrogen atom
obtains an electron configuration like the noble gas
helium.
Formation of a Covalent Bond
A molecular orbital is made when 2 atomic
orbitals overlap.
Sharing Electrons
Covalent bond: a bond formed when atoms
share one or more pairs of electrons.
The shared electrons move within a space called
a molecular orbital.
Molecular orbital: the region of high probability
that is occupied by an individual electron as it
travels with a wavelike motion in the threedimensional space around one of two or more
associated nuclei.
In plain English: a molecular orbital is the space
that the shared electrons move around in.
Energy Is Released When Atoms Form
a Covalent Bond
2 H atoms
When 2 bonded hydrogen atoms are at their
lowest potential energy, the distance between
them is 75 pm. (1 pm = 1 X 10-12 m)
Potential Energy Determines Bond Length
Bond length: the distance between two bonded
atoms at their minimum potential energy; the
average distance between the nuclei of two
bonded atoms.
The two nuclei in a covalent bond vibrate back
and forth and the distance between them is
constantly changing. That’s why the bond length
is the average distance between the two nuclei.
Bonds Vary in Strength
 Bond Energy: the energy required to break the
bonds in 1 mole of a chemical compound
At a bond length of 75 pm, the potential energy of
H2 is –436 kJ/mol.
That means that 436 kJ of energy must be supplied
to break the bonds in 1 mol of H2 molecules.
Bonds that have higher bond energies
(stronger bonds) have shorter bond lengths.
(shorter bond length= stronger bond)
Electronegativity and Covalent
Bonding
Remember: electronegativity is how much an
atom in a molecule attracts electrons.
Electronegativity values are a useful tool to
predict what kind of bond will form between 2
atoms.
Polar vs. Non-Polar Covalent Bonds
Depending on how the electrons are shared,
covalent bonds can be either polar or
non-polar.
Polar means: “opposite in character”
Think of the North pole and the South pole
Atoms Can Share Electrons 2
Different Ways
1. Equally
2. Unequally
Atoms Can Share Electrons Equally
When the electronegativity values of two
bonding atoms are similar, bonding electrons are
shared equally and a non-polar covalent bond is
formed.
Non-polar covalent bond: a covalent bond
in which the bonding electrons are equally
attracted to both bonded atoms.
Examples:
Cl2
O2
H2
Atoms Can Share Electrons Unequally
When the electronegativity values of two bonding
atoms are different, bonding electrons are shared
unequally and a polar covalent bond is formed.
The atom with the higher electronegativity “hogs”
the electrons away from the other atom.
Polar covalent bond: a covalent bond in which a
shared pair of electrons is held more closely by
one of the atoms.
Predicting Bond Character from
Electronegativity Differences
Use the table of electronegativities on pg. 194.
 Find the difference in electronegativity values
between the bonded atoms to predict bond character.
Predicting Bond Character
Examples
Predict what type of bond would form between
the following atoms:
1. H and Br
 Electrongegativity of Br = 3.0
 Electronegativity of H = 2.2
Electronegativity difference = 0.8
polar covalent
Predicting Bond Character
Examples
Predict what type of bond would form between
the following atoms:
2. Cl and Cl
 Electrongegativity of Cl = 3.2
 Electronegativity of Cl = 3.2
Electronegativity difference = 0.0
(same atom will share electrons equally)
non-polar covalent
Predicting Bond Character
Examples
Predict what type of bond would form between
the following atoms:
3. Li and Cl
 Electrongegativity of Cl = 3.2
 Electronegativity of Li = 1.0
Electronegativity difference = 2.2
ionic
Polar Molecules Have Positive and
Negative Ends
In a polar covalent bond, the ends of the bond
have opposite partial charges because one of the
atoms is “hogging” the electrons.
Dipole: a molecule or part of a molecule that
contains both positively and negatively charged
regions.
In a polar covalent bond, the shared pair of
electrons are more likely to be found near the
more electronegative atom.
Polar Molecules Have Positive and
Negative Ends
The Greek symbol  (delta) means partial.
  + is used to show a partial positive charge
  – is used to show a partial negative charge
 Example: H+ F– is polar covalent
Because the F atom has a partial negative
charge, the electron pair is more likely to be
found closer to the fluorine atom.
Polarity Is Related to Bond Strength
In general, the greater the electronegativity
difference, the greater the polarity and the
stronger the bond.
Polarity Is Related to Bond Strength
Think of it this way:
You know that ionic bonds are stronger than
covalent bonds.
The greater the difference in electronegativity,
the closer the covalent bond comes to being like
an ionic bond.
Electronegativity and Bond Types
Differences in electronegativity values provide
one model that can tell you which type of bond
two atoms will form.
Another general rule states:
An ionic bond forms between a metal and a
non-metal.
A covalent bond forms between two
non-metals.
Properties of Substances Depend on
Bond Type
The type of bond that forms (metallic, ionic, or
covalent) determines the physical and chemical
properties of the substance.
Metallic bonds: good conductors in solid state
because electrons are free to move around.
Ionic bonds: strong attraction between ions makes
them solids with high melting/boiling points.
Covalent bonds: weaker attraction between
molecules makes them gases/liquids at room
temperature or solids with low melting/boiling
points.
Homework
Section 6.1 review, pg. 198: #1-14
Concept Review: “Covalent Bonds”
Concept reviews MUST be turned back in
before you leave today!
Next time:
Sec. 6.1 quiz: “Covalent Bonds”
Lab write-up AND lab
Dress appropriately please!