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Formation of Covalent Compounds
• Two non-metal atoms obtain a stable filled
energy level by sharing a pair of electrons.
• An attractive force between the 2 atoms
results when two or more electrons are
shared by the atoms. This attractive force is
called a covalent bond.
• Covalent compounds are also called
molecular compounds.
Molecular Compounds
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The Octet Rule
Molecular Compounds
Chemical compounds tend to form so that each
atom, by gaining, losing, or sharing electrons, has
eight electrons in its highest occupied energy
level.
• Are formed from two or more nonmetals.
An octet gives the atom the same number of
electrons as in the nearest noble gas
The first exception to this is hydrogen, which
follows the duet rule.
The second exception is helium which does not
form bonds because it is already “full” with its
two electrons
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Molecular Compounds
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Lewis Dot Diagrams for Elements
• Molecular Compounds: form when
electrons are shared between nonmetals
in order to achieve an octet;
• Are held together by covalent (aka:
molecular) bonds.
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Molecular Compounds
Drawing Covalent Compounds
• The number of unpaired valence electrons is
referred to as the element’s combining
capacity
Eg.
Combining capacity for oxygen
is 2
• Formulas come from the name – no criss-cross
involved since there are not ionic charges
involved.
• Covalent bonding can be represented by a
Lewis structure.
• Lewis structures show the valence electrons
surrounding each atom. A dash represents
the covalent bond (shared electrons).
• The pairs of electrons not involved in the
formation of covalent bonds are called lone
pairs.
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Carbon forms 4 covalent bonds
Multiple Bonds
In nitrogen molecule, N2,
In a CH4, methane, molecule
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•
•
•
• a C atom shares 4 electrons to
attain an octet.
• each H shares 1 electron to
become stable like helium.
each N atom shares 3 electrons.
each N attains an octet.
the bond is a multiple bond called a triple bond.
the name is the same as the element.
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Naming Molecular Compounds
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Molecular compounds are easier to
name!
• Molecular compounds:
– the name tells you the number of atoms.
– Uses prefixes to tell you the exact
number of each element present!
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Learning Check
Prefixes
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•
•
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1 = mono2 = di3 = tri4 = tetra5 = penta6 = hexa7 = hepta8 = octa9 = nona10 = deca-
Fill in the blanks to complete the following
names of covalent compounds.
CO
carbon ______oxide
CO2
carbon _______________
PCl3
phosphorus _______chloride
CCl4
carbon ________chloride
N2 O
_____nitrogen _____oxide
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Prefixes
• To write the name, write two words:
Prefixes
• To write the name, write two words:
Prefix name Prefix name -ide
Prefix & name
Prefix & name -ide
Prefixes
• One exception is we don’t write mono
if there is only one of the first element.
Nonmetals and their names
Group 14
C4-,carbide
• To write the name, write two words:
Group 15
N3-, nitride
Group 16
Group 17
O2-, oxide
F-, fluoride
S2-, sulfide
Cl-, chloride
Prefix name Prefix name -ide
Br-, bromide
• Normally, we do not have double vowels
when writing names (oa oo)
I-, iodide
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Naming Covalent Compounds
Naming Covalent Compounds
Name P4S3.
What is the name of SO3?
1. The first nonmetal P is phosphorus.
1. The first nonmetal is S sulfur.
2. The second nonmetal S is sulfide.
2. The second nonmetal is O named oxide.
3. The subscript 4 of P is shown as tetra.
3. The subscript 3 of O is shown as the prefix tri.
The subscript 3 of O is shown as tri.
SO3  sulfur trioxide
P4S3  tetraphosphorus trisulfide
The subscript 1 (for S) or mono is understood.
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Practice by naming these:
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Practice by naming these:
 N2O
 N2O
 NO2
 NO2
 Cl2O7
 Cl2O7
 CBr4
 CBr4
 CO2
 CO2
 BaCl2
 BaCl2
Write formulas for these:
 diphosphorus
pentoxide
 tetraiodine nonoxide
 sulfur hexafluoride
 nitrogen trioxide
 carbon tetrahydride
 phosphorus trifluoride
 aluminum chloride
= dinitrogen monoxide
(also called nitrous oxide or laughing gas)
= nitrogen dioxide
= dichlorine heptoxide
= carbon tetrabromide
= carbon dioxide
(This one will not use prefixes, since it is
an ionic compound!)
Write formulas for these:
 diphosphorus
pentoxide
 tetraiodine nonoxide
 sulfur hexafluoride
 nitrogen trioxide
 carbon tetrahydride
 phosphorus trifluoride
 aluminum chloride (Ionic compound)
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Ionic Versus Molecular Compounds
Shapes of Molecular Compounds
Ionic compounds are
composed of ions. The ratio
of cations to anions is always
expressed in the simplest
whole number ratio known as
a formula unit.
Examples:
NaCl
CaBr2
The formula for a molecular compound
shows the number of atoms that are
combined in one molecule of the
compound. Molecular formulas are not
necessarily expressed in the simplest
whole number ratio of atoms.
Examples:
NO
N2O4
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Properties of Molecular
Compounds
Learning Check
• Low melting and boiling points
• Can be solids liquids or gases at room
temperature
• Non – electrolytes when dissolved in water
• Soft and flexible
• Can be flammable
• Many are not soluble in water
Identify each compound as ionic or covalent and give its
correct name.
A. SO3
B. BaCl2
C. (NH4)3PO4
D. Cu2CO3
E. N2O4
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Diatomic Molecules
Solution
• There are seven elements that exist in
nature as diatomic molecules:
H2, F2, Cl2, Br2, I2, N2, O2
Identify each compound as ionic or covalent and give its
correct name.
A. SO3
covalent – sulfur trioxide
B. BaCl2
ionic – barium chloride
C. (NH4)3PO3
ionic – ammonium phosphite
D. Cu2CO3
ionic – copper(I) carbonate
E. N2O4
covalent – dinitrogen tetroxide
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