Molecular Bonding
Honors
Do Now…
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Write the names of the following compounds..
A. LiF
B. PCl5
C. N2O3
• D. Mg(SO4)
Terms..
• Covalent bonds- elements can attain
noble gas configuration by sharing
electrons
• Molecules-Compounds composed
mostly of covalently bonded groups of
atoms
• Compounds- a pure substance
composed of 2 or more elements
Diatomic gases
• BrINClHOF= bromine, iodine,
nitrogen, chlorine, hydrogen,
oxygen, fluorine
Video Clip-Ionic vs Covalent
• http://www.youtube.com/watch?v=QqjcCvzW
www&feature=related
• Binary Ionic
– Metal and nonmetal
– High melting/boiling
pt
– Solids at room temp
– Conduct electricity
when melted or
dissolved in H2O
– Electrons transferred
– Electrostatic attraction
• Binary Covalent
– Nonmetals
– Low melting pt
– Low boiling pt
– Electrons shared
– Physically attached to
each other
Examine this picture of NaCl in H2O
• example of the formation of molecular and
ionic bonds
• http://www.youtube.com/watch?v=yjge1WdC
FPs&feature=related
TRUE OR FALSE:
• All compounds contain
molecules.
• There are no elements that exist as
molecules.
• Most molecular compounds
are composed of two or more
nonmetallic elements.
• Atoms in molecular compounds
share electrons.
• Molecular compounds tend to have
higher melting and boiling points
than ionic compounds
• Which of the following formulas
represent molecules?
–Br2
–KBr
–CH4
–SO3
–N2H2
–Ne2
–Answer: Br2, CH4, SO3,
N2H2
• Which of the following formulas
represent compounds?
–Br2
–KBr
–CH4
–SO3
–N2H2
–Ne2
–Answer: KBr, CH4, SO3, N2H2
Covalent bond….
• One atom donates one electron to
the bond, and the other atom
donates the second electron
• The shared pair represents the
bond
• If two pairs of electrons are shared:
double bond
• If three pairs of electrons are
shared: triple bond
Lewis Structures
• H2
• Cl2
• N2
• CCl4
Drawing the Lewis Structures
• 1. Determine the “skeleton”- usually a central
atom with surrounding atoms bonded to it
– Carbon is usually the central atom, in
compounds with more than one carbon
atom, the carbon atoms are joined in a
chain
– Ex CH4
C-C-C-C
• 2. Hydrogen is never the central atom
because it can form only one covalent bond
• 3. Halogens form only a single covalent bond
when oxygen is not present, and is usually not
the central atom
• 4. Oxygen forms only two covalent bonds and
is rarely a central atom, but may link 2 carbons
• 5. In the simpler molecules the atom that
appears only once in the formula will be the
central atom
Arranging electrons
• 1. Write the element symbols in the form of
the skeletal structure
• 2. Add all of the valence electrons of all atoms
together
• 3. If the substance is a polyatomic-don’t
forget the electrons used to form the ion
• 4. Draw a pair of electrons between each two
atoms in the skeletal structure-covalent bond
• 5. Remaining electrons used to complete the
octets of all outer atoms in the skeletonnonbonding pairs
• 6. If any electrons are left they are added in
pairs to the central atom- nonbonding pairs
• If the central atom has an octet, you are
finished
• For others-you may need to consider double
or triple bonds to satisfy the octet on the
central atom
• Exceptions to the octet rule:
– hydrogen- duet
– boron- sextet
– beryllium- quartet
Exceptions
• Less than eight-B is an exception-is satisfied
with 6 electrons
• More than eight-PCl5
8.2
Exceptions to the Octet Rule
• The electron dot structure for PCl5 can be written so
that phosphorus has ten valence electrons.
SF6
• Expanded octet in sulfur occurs because 4 of
the 12 electrons around it are promoted to
the 3d subshell – 3d and 3p have similar
energies
XeF6
• The 5d subshell is very close in energy to the
5p subshell
• Xe has 12 electrons around it, 4 of those are
promoted to the 5d subshell
• Iodine and chlorine expand their octets in the
same way
• Ex: PCl3 26 electrons all together
Cl
Cl
Cl
P
Cl
Skeleton
Cl
P
Cl
Bonding electrons
Cl
Cl
Cl
P
Outer octets
Cl
Cl
P
2 e unused
Last e pair
added to P
Cl
• SO2?
8.2
Coordinate Covalent Bonds
• In carbon monoxide, oxygen has a stable configuration
but the carbon does not.
8.2
Coordinate Covalent Bonds
• As shown below, the dilemma is solved if the oxygen
donates one of its unshared pairs of electrons for
bonding.
8.2
Coordinate Covalent Bonds
• A coordinate covalent bond is a covalent bond in
which one atom contributes both bonding
electrons.
• In a structural formula, you can show coordinate
covalent bonds as arrows that point from the
atom donating the pair of electrons to the atom
receiving them.
8.2
8.2
Resonance
• A resonance structure is a structure that occurs when it
is possible to draw two or more valid electron dot
structures that have the same number of electron pairs
for a molecule or ion.
– 1. In covalent bonding, atoms attain the
configuration of noble gases by
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losing electrons.
gaining electrons.
transferring electrons.
sharing electrons.
– 2. Electron dot diagrams are superior to
molecular formulas in that they
• show which electrons are shared.
• indicate the number of each kind of atom in the
molecule.
• show the arrangement of atoms in the molecule.
• are easier to write or draw.
– 3. Which of the following molecules would
contain a bond formed when atoms share three
pairs of electrons?
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Se2
As2
Br2
Te2
Formal Charge
• Sometimes there is more than one way to
draw a Lewis structure- how do you know
which one is correct?
• Calculate the formal charge can tell you which
structure is more stable
• The sum of the formal charges on each atom
will be zero for a neutral molecule
How to tell which structure is more
likely….
• 1. Chose the Lewis structure where the formal
charges on each atom sum up to values
closest to zero
• 2. If there are negative charges they should
reside on the more electronegative elements
in the structure
Formal
Charge =
# of valence e assigned to
the neutral atom
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The number of e assigned to the
atom in the structure
Number of e assigned to the atom in
the structure
• Lone pairs around atom + #of bonding e
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2
Ex: Formal charge
• SCN –
Who goes in the middle?
According to our rules, it should be carbon, how
come?
Two Lewis structures for SCN-
C
Val e
4
e assigned - 6
Formal chg -2
S
6
4
2
N
5
6
-1
C S
Val e
4 6
E assigned - 4 6
Formal chg
0 0
N
5
6
-1