What are Compounds and Molecules?
INTRODUCTION: Since there are only some
ninety elements, but over two million known
compounds, most substances are not found as
independent atoms but as groups of atoms
known as molecules in compounds.
The compounds may be divided into two
groups. Those molecules that consist of charged
ions with opposite charges are called IONIC.
These ionic compounds are generally solids with
high melting points and conduct electrical
current. The other type of molecules are called
COVALENT and do not consist of ions.
Covalent compounds have low melting points
and do not conduct electric current.
COMPOUNDS
COMPOUNDS contain two or more
different elements in a chemically
combined form.
The simplest form of most matter, other than
simple elements which still retains specific
properties, is in the form of compounds. The
simplest form of a compound is a molecule.
MOLECULE
• A MOLECULE contains
two or more atoms.
• Since a compound
contains two or more
different elements, it also
contains two or more
different atoms, hence the
simplest form of a
compound is a molecule.
As an example, a
molecule of water
contains two hydrogen
atoms and one oxygen
atom, (written as H2O).
diatomic molecule
When an element consists of two
identical atoms it is called DIATOMIC
molecule.
The simplest form of some
elements is a molecule. For example
oxygen in air consists of molecules
containing two atoms of oxygen,
(written as O2).
COMPOSITION OF COMPOUNDS
 Compounds contain atoms of different elements
combined in whole number ratios as stated as the Law
of Definite Composition. The Law of Definite
Composition states that two or more elements combine
to form a compound in a fixed proportion by weight
without regard to method of preparation.
 For example water is always 88.9% by weight oxygen
and 11.1% by weight hydrogen. Through a fairly
complicated process the percentages by weight are
ultimately converted into simple whole number ratios.
Water molecules contain a ratio of two hydrogen for
every one oxygen atom . This is expressed in the form
of a formula.
 Water is always H2O and not HO or H3O.
COMPOSITION OF COMPOUNDS
A good contrast to the compound of water is the
compound hydrogen peroxide, a very similar,
but very distinct compound with one extra
oxygen atom. The ratio is two hydrogen atoms
to two oxygen atoms. The formula for hydrogen
peroxide is H2O2.
The composition of compounds is indicated by
a formula using symbols for the elements. The
number of atoms in a formula is given by a
subscript. If no subscript is given, then the
number of atoms is assumed to be one.
COMPOSITION OF COMPOUNDS
 The composition of
compounds is also
indicated by the names of
compounds. For example
carbon monoxide (CO)
consists of one carbon
and one oxygen atom
("mono" = one) . While
carbon dioxide (CO2)
consists of one carbon
and two oxygen atoms
("di" = two).
 As you study chemistry,
there will be constant
references to both
formulas and names of
compounds. You must
learn these as they are the
language of chemistry.
Ionic Bonding
Introduction to Ionic Bonding:
Ionic bonding is best treated using a simple
electrostatic model . The electrostatic model
is simply an application of the charge
principles that opposite charges attract and
similar charges repel.
An ionic compound results from the
interaction of a positive and negative ion,
such as sodium and chloride in common salt.
Ionic Bonding
The IONIC BOND results as a balance
between the force of attraction between
opposite plus and minus charges of the ions
and the force of repulsion between similar
negative charges in the electron clouds. In
crystalline compounds this net balance of
forces is called the LATTICE ENERGY.
Lattice energy is the energy released in the
formation of an ionic compound.
 DEFINITION: The formation of an IONIC
BOND is the result of the transfer of one or
more electrons from a metal onto a nonmetal.
Ionic Bonding
 Metals, with only a few electrons in the outer energy
level, tend to lose electrons most readily. The energy
required to remove an electron from a neutral atom is
called the IONIZATION POTENTIAL.
 Energy + Metal Atom ---> Metal (+) ion + e Non-metals, which lack only one or two electrons in
the outer energy level have little tendency to lose
electrons - the ionization potential would be very high.
Instead non-metals have a tendency to gain electrons.
The ELECTRON AFFINITY is the energy given off by
an atom when it gains electrons.
 Non-metal Atom + e- ---> Non-metal (-) ion + energy
 The energy required to produce positive ions
(ionization potential) is roughly balanced by the
energy given off to produce negative ions (electron
affinity). The energy released by the net force of
attraction by the ions provides the overall stabilizing
energy of the compound.
Introduction to Covalent Bonding
Bonding between non-metals consists of two
electrons shared between two atoms. In
covalent bonding, the two electrons shared
by the atoms are attracted to the nucleus of
both atoms. Neither atom completely loses
or gains electrons as in ionic bonding.
There are two types of covalent bonding:
 Non-polar bonding with an equal sharing of
electrons.
 Polar bonding with an unequal sharing of
electrons. The number of shared electrons
depends on the number of electrons needed
to complete the octet.
NON-POLAR BONDING
NON-POLAR BONDING results when
two identical non-metals equally share
electrons between them.
One well known exception to the
identical atom rule is the combination
of carbon and hydrogen in all organic
compounds.
IODINE
Iodine forms a
diatomic non-polar
covalent molecule.
The graphic on the
right shows that
iodine has 7
electrons in the
outer shell.
Since 8 electrons
are needed for an
octet, two iodine
atoms EQUALLY
share 2 electrons.
OXYGEN
• Molecules of oxygen,
present in about 20%
concentration in air are
also a covalent molecules .
See the graphic on the
right the Lewis symbols.
• There are 6 electrons in
the outer shell, therefore, 2
electrons are needed to
complete the octet. The
two oxygen atoms share a
total of four electrons in
two separate bonds, called
double bonds.
• The two oxygen atoms
equally share the four
electrons.
POLAR BONDING
• POLAR BONDING results when two different nonmetals unequally share electrons between them. One
well known exception to the identical atom rule is the
combination of carbon and hydrogen in all organic
compounds.
• The non-metal closer to fluorine in the Periodic Table
has a greater tendency to keep its own electron and
also draw away the other atom's electron. It is NOT
completely successful. As a result only partial
charges are established. One atom becomes
partially positive since it has lost control of its
electron some of the time. The other atom becomes
partially negative since it gains electron some of the
time.
WATER
• Water, the most universal compound on all of the
earth, has the property of being a polar molecule. As
a result of this property, the physical and chemical
properties of the compound are fairly unique.
• Hydrogen Oxide or water forms a polar covalent
molecule. The graphic on the left shows that oxygen
has 6 electrons in the outer shell. Hydrogen has one
electron in its outer energy shell. Since 8 electrons
are needed for an octet, they share the electrons.
• However, oxygen gets an unequal share of the two
electrons from both hydrogen atoms. Again, the
electrons are still shared (not transferred as in ionic
bonding), the sharing is unequal. The electrons
spends more of the time closer to oxygen. As a
result, the oxygen acquires a "partial" negative
charge. At the same time, since hydrogen loses the
electron most - but not all of the time, it acquires a
"partial" charge. The partial charge is denoted with a
small Greek symbol for delta.
Compare
Ionic, Polar, and Non-polar Bonds
• Whereas non-polar bonding involves the equal
sharing of electrons between identical non-metal
atoms, POLAR BONDING is the unequal sharing of
electrons between two different non metal atoms. A
proper understanding of polar bonding is gained by
viewing the types of bonding on a continuum as in
the diagram on the top left. Ionic bonding is on one
extreme with a complete transfer of electrons
forming charged ions. Non-polar covalent bonding
with equal sharing of electrons is at the other
extreme. Somewhere in the middle but favoring the
covalent side is polar bonding with unequal sharing
of electrons and partial but incomplete transfer of
electrons.
Comparison of Lewis Diagrams of
Ionic, Polar and Non-Polar Bonding:
• The best way to show and represent the unequal
sharing of electrons would be by comparison with
NaCl and HCl, and H2 using Lewis diagrams.
• The captions below correspond to the graphic on the
bottom left.
• IONIC: Complete transfer of electrons, therefore Na
becomes positive (lost e-) and Cl becomes negative
(gained e-).
• POLAR: Unequal sharing. Chlorine has a greater
tendency to keep its own electron and also draw
away hydrogen's electron. It is NOT completely
successful. As a result only partial charges are
established. Hydrogen becomes partially positive
since it has lost control of its electron some of the
time (H +). Chlorine becomes partially negative since
it gains hydrogen's electron some of the time (Cl -).
polar bond
In summary, a polar bond results when different
atoms share electrons. One atom will attract the
bonding electrons more strongly than the other
atom and will acquire more than a half share of
these electrons. This leaves the other atom with
less than a half share and makes the electron
distribution unsymmetrical. On a time-average
basis the electrons spending more time with
one atom and cause it to have a partial negative
charge. The other atom, deficient in electrons,
acquires a partial positive charge.
NON-POLAR
• Equal Sharing. Neither atom can
dominate the other, therefore the
electrons are shared equally between
them.