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What are Compounds and Molecules?

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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 ... – PowerPoint PPT presentation

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Title: What are Compounds and Molecules?


1
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.

2
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.

3
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).

4
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).
       

5
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.

6
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.

7
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.

8
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.

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10
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 non-metal.

11
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 ---gt 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- ---gt 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.

12
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.

13
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.

14
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.

15
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.

16
POLAR BONDING
  • POLAR BONDING results when two different
    non-metals 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. 

17
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.  

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20
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.

21
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 -).

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23
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.

24
NON-POLAR
  • Equal Sharing. Neither atom can dominate the
    other, therefore the electrons are shared equally
    between them.
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