LECTURE - 3
CHEMICAL BONDING
• A chemical bond is the attractive force that holds two atoms
together in a more complex unit.
• There are two types of chemical attractive forces (chemical bonds):
ionic bonds and covalent bonds.
• An ionic bond is a chemical bond formed through the transfer of
one or more electrons from one atom or group of atoms to another
atom or group of atoms.
• An ionic compound is a compound in which ionic bonds are
present.
• A covalent bond is a chemical bond formed through the sharing of
one or more pairs of electrons between two atoms.
• A molecular compound is a compound in which covalent bonds are
present.
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• VALENCE ELECTRONS AND LEWIS SYMBOLS
• A valence electron is an electron in the outermost electron shell of
a representative element or noble-gas element.
• A Lewis symbol is the chemical symbol of an element surrounded
by dots equal in number to the number of valence electrons
present in atoms of the element.
• Lewis symbols, named in honor of the American chemist Gilbert N.
Lewis
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• THE OCTET RULE
• A key concept in elementary bonding theory is that certain
arrangements of valence electrons are more stable than
others. The valence electron configurations of the noble
gases (helium, neon, argon, krypton, xenon, and radon) are
considered the most stable of all valence electron
configurations. All of the noble gases except helium possess
eight valence electrons, which is the maximum number
possible.
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The noble gases are the most unreactive of all the elements. They are
the only elemental gases found in nature in the form of individual
uncombined atoms.
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octet rule: In forming compounds, atoms of elements
lose, gain, or share electrons in such a way as to
produce a noble-gas electron configuration for each of
the atoms involved.
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THE IONIC BOND MODEL
• An ion is an atom (or group of atoms) that is electrically charged as a
result of the loss or gain of electrons. If an atom gains one or more
electrons, it becomes a negatively charged ion. If an atom loses one or
more electrons, it becomes a positively charged ion. The charge on an
ion depends on the number of electrons that are lost or gained. Loss of
one, two, or three electrons gives ions with+1, +2, or+3 charges,
respectively. A gain of one, two, or three electrons gives ions with -1, -2,
or -3 charges, respectively. Some examples of ion symbols are
• Positive ions: Na+, K+, Ca2+, Mg2+, Al4+
• Negative ions: Cl_, Br_, O2_, S2_, N3_
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POLYATOMIC ION
• There are two categories of ions:
• A monatomic ion is an ion formed from a single atom through loss
or gain of electrons. Like (Cl-, Na+, Ca2+, N3-, and so on).
• A polyatomic ion is an ion formed from a group of atoms (held
together by covalent bonds) through loss or gain of electrons. An
example of a polyatomic ion is the sulfate ion,
THE COVALENT BOND
• A covalent bond is a chemical bond resulting from two nuclei
attracting the same shared electrons.
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ELECTRONEGATIVITY
Electronegativity is a measure of the relative attraction
that an atom has for the shared electrons in a bond.
electronegativity values increase from left to right across
periods and from bottom to top within groups of the
periodic table.
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BOND POLARITY
• A nonpolar covalent bond is a covalent bond in which there
is equal sharing of electrons between two atoms.
• A polar covalent bond is a covalent bond in which there is
unequal sharing of electrons between two atoms.
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Differences between ionic and covalent bonding and
the resulting ionic and molecular compounds.
1.
Ionic bonds form between atoms of dissimilar elements (a metal and a
nonmetal). Covalent bond formation occurs between similar or identical
atoms. Most often two nonmetals are involved.
2. Electron transfer is the mechanism by which ionic bond formation occurs.
Covalent bond formation involves electron sharing.
3. Ionic compounds do not contain discrete molecules. Instead, such
compounds consist of an extended array of alternating positive and
negative ions. In covalently bonded compounds, the basic structural unit
is a molecule.
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4. All ionic compounds are solids at room temperature. Molecular
compounds may be solids (glucose), liquids (water), or gases (carbon
dioxide) at room temperature.
5. An ionic solid, if soluble in water, forms an aqueous solution that
conducts electricity. The electrical conductance is related to the
presence of ions (charged particles) in the solution. A molecular
compound, if soluble in water, usually produces a non-conducting
aqueous solution.
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