COVALENT COMPOUNDS
Formed when two atoms share electrons
o Usually involve 2 or more
non-metal atoms
o Similar electronegativity
values (electronegativity
difference is less than 1.7).
Can be pure covalent –
equal sharing of e (<.3)
Polar covalent – uneven
sharing of electrons (.3-
1.7)
o Called molecules
o There are more covalent
compounds than ionic
compounds.
Binary covalent – include 2
elements only
Organic molecules (which
include carbon) are
covalent
Glucose C6H12O6
Sucrose C12H22O11
Hydrocarbon C6H6 etc.
The shared valence electrons
are found in a molecular orbital, formed by overlapping
atomic orbitals of the atoms
involved in bonding.
Recall the properties of
covalent molecules listed
previously in chapter 5 notes
(flow chart).
NAMING COVALENT COMPOUNDS
Covalent molecules are NOT
named like ionic compounds!!!
Steps to naming binary covalent
molecules: (only include 2
elements)
1. the first element (least
electronegative) is named first
2. the second element has the
ending –ide
3. Prefixes are used before the
element name to indicate the
number of each atom in the
molecule (the subscript)
# of atoms Prefix1 Mono
2 Di3 Tri4 Tetra5 Penta6 Hexa7 Septa8 Octa
9 Nona10 Deca
Do NOT use a prefix for the first
atom when there is only one
present.
When the element name begins
with a vowel, the (a) and (o) are
dropped from the end of the
prefix
Examples:
CCl4
carbon tetrachloride
CO
carbon monoxide
(eliminate the o from the prefix)
P2Cl5
diphosphorus pentachloride
(more than one of the first
element)
N2O3
dinitrogen trioxide
Si3N4
trisilicon tetranitride
H2O
dihydrogen monoxide
Steps for writing covalent
formulas from the name.
1. the elements appear in the
same order as in the name
2. the prefix indicates the
subscript in the chemical
formula
Examples:
Boron trifluoride BF3
Dinitrogen monoxide N2O
Dinitrogen tetroxide N2O4
ENERGY and STABILITY of COVALENT BONDS
Most atoms have relatively LOW STABILITY and HIGH POTENTIAL ENERGY.
When a compound forms, the atoms become MORE STABLE and the potential energy is at a MINIMUM.
Example:
The potential energy curve for hydrogen.
http://www.usm.maine.edu/~newton/Chy251_253/Lectures/LewisStructures/Dihydrogen.html
When the nuclei are farthest apart, the potential energy is ZERO.
As they get closer, the energy DECREASES.
When the potential energy is LOWEST (at -436 kJ//mole), the atoms bond.
The distance between the two nuclei at this lowest energy is called the BOND LENGTH and is 75 pm for the H2 molecule.
When the REPULSION of the two atoms perfectly balance the ATTRACTIVE FORCES between the two nuclei, a COVALENT BOND forms.
Since the potential energy DECREASED, energy has been RELEASED. For H2, the energy released is -436 kj/mol.
The energy released when forming the bond is the exact amount of energy that would be needed to break the bond. Energy needed = +436 kj/mol
The energy required to break a bond is known as the BOND ENERGY.
BOND ENERGY and
BOND LENGTH
Bond energ
Bond lengt
Electronegativity Difference
y(kj/
mol)
h (pm)
HF 570 92 1.8CF 552 138OO 498 121HH 436 75HC
l432 127 1.0
CCl
397 177
HBr
366 141 0.8
HI 299 161 0.5
BOND ENERGY (STRENGTH) is inversely related to the BOND LENGTH. The higher the bond energy, the SHORTER the bond.
Bond length is actually an AVERAGE DISTANCE between the two nuclei since the distance is constantly changing due to the bond being able to vibrate and bend.
BOND STRENGTH and POLARITY
The HIGHER the electronegativity difference, the STRONGER the bond.
o Ex. HF has a much stronger bond than HI.
o HF is nearly IONIC and HI is almost PURE COVALENT.
LEWIS DOT STRUCTURES
RESONANCE
when 2 or more equivalent lewis structures exist for a
molecule or compound
all structures should be represented with a double
arrow between
the actual bond strength is an average of all the
bonds.
Examples:
(see back of notes)
MOLECULARGEOMETRY: ARRANGEMENT and
SHAPE
VSEPR
POLARITY and DIPOLE MOMENT
POLAR BONDS:
Result from high ELECTRONEGATIVITY DIFFERENCES between atoms of a molecule.
Causes PARTIAL POSITIVE and PARTIAL NEGATIVE ends of the molecule (called DIPOLES).
SHAPE AFFECTS POLARITY:
DIPOLE MOMENT – overall direction of electron “pull” within a molecule. Show using a molecular model. Sometimes dipoles will CANCEL each other, and the result will be a molecule with NO NET DIPOLE due to the shape.
Molecule
Sketch Direction of dipole or no net dipole
HBrBeCl2BBr3
SeCl2CO2
H2O
POLARITY AFFECTS PROPERTIES:
For example: o CO2 is non-polar, so the ATTRACTIVE FORCE between CO2 atoms is VERY WEAK.
o This results in a lower MELTING POINT and BOILING POINT.
H2O is POLAR, the molecules interact with each other and attractive forces are greater.
This results in a higher MELTING POINT and BOILING POINT.
A very strong INTERMOLECULAR force exists between water molecules due to its polarity.
o This force is called HYDROGEN BONDING
It is not the bond within water….it is a bond between water molecules.
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