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CHEMICAL BONDS
•Atoms or ions are held together in molecules or compounds by chemical
bonds.
•The type and number of electrons in the outer electronic shells of atoms or
ions decide how atoms react with each other to form stable chemical bonds.
•Over the last 150 years scientists developed several theories to explain why
and how elements combine with each other.
•All chemical reactions involve breaking of some bonds and formation of new
ones where new products are formed.
TYPES OF CHEMICAL BONDS
• Ionic bonds
Loss / gain of electrons,formed by the attraction ofoppositely charged ions.-Larger charge = stronger attraction.-Smaller ion = stronger attraction.
• Covalent bonds
Electrons are shared by nuclei to attain octet
• Metallic bonds
Electronegativities (L.Pauling)
Charge distribution in a diatomic molecule
nonpolar covalentElectonegativity difference (ΔEN ) is < 0.4
polar covalent0.4 < ΔEN < 2
ionicΔEN > 2
ionic bonds ------------------------------- covalent bonds
complete complete
transfer sharing
100% 100%
most bonds are found
here (in between)
polar covalent bonds
POLARITY
Bonding Theories
• Lewis bond Theory
• Valence Bond Theory
• Molecular Orbital Theory
Lewis Symbols of Atoms
• Uses symbol of element to represent nucleus and inner electrons.
• Uses dots around the symbol to represent valence electrons.
– Put one electron on each side first, then pair.
Using Lewis Theory to Predict Chemical Strucure
Ionic bond
1) Predict the formula of the compound that forms between Na and Cl.
2)Draw the Lewis dot symbols of the elements.
3) Transfer all the valance electrons from the metal to the nonmetal,
adding more of each atom as you go, until all electrons are lost from the
metal atoms and all nonmetal atoms have 8 electrons.
Covalent bond
1)Predict the formula of the compound that forms between H and Cl.
2)Draw the Lewis dot symbols of the elements.
3)Allow sharing electrons for bond formation (nonmetal atoms 8 electrons)
Examples
F2
MULTIPLE COVALENT BONDS
More than one electron pair is shared (2 or 3)
Purpose: attaining octet (8)
try HCN
Expanded octets
P
Cl
Cl
Cl
••
••
••
Cl
ClS
F
F
F
••
••
••
F
F
F ••
••
••
Write Lewis forms for above reaction...
H
ı .. .. .. ..H - C – H + 2 : O : : O = C = O : 2 H – O - H
ı ..
H
Shape?
Molecular Geometry
• Molecules are three-dimensional objects.
• We often describe the shape of a molecule with terms that relate to
geometric figures.
• These geometric figures have characteristic “corners” that indicate the
positions of the surrounding atoms with respect to the the central atom.
• The geometric figures also have characteristic angles that we call bond
angles.
Valence Shell Electron Pair Repulsion(VSEPR Model)
• It is used to predict the geometries of molecules formed from nonmetals.
• Postulate: the structure around a given atom is determined principally by
minimizing electron pair repulsion.
• The bonding and nonbonding pairs should be positioned as far apart as
possible.
Predicting a VSEPR Structure
• Draw Lewis structure.
• Put pairs as far apart as possible.
• Determine positions of atoms from the
way electron pairs are shared.
• Determine the name of molecular
structure from positions of the atoms.
Molecule Polarity
Molecule Polarity
Bond Energies
• Bond breaking requires energy (endothermic).
• Bond formation releases energy (exothermic).
ΔH = Σ H (bonds broken) – Σ H (bonds formed)
energy required energy released
to break the bonds during bond formation
Bond Energy of CH4
Experimental result : 1652 kJ/mol
C(g) + 4H(g) → CH4(g) + 1652 kJ/mol
An average C-H bond energy per mole
of C-H bond: 1652/4= 413 (kJ/mol)
Metallic Bonding
• The model of metallic bonding
can be used to explain the
properties of metals.
• The luster, malleability, ductility,
and electrical and thermal
conductivity are all related to the
mobility of the electrons in the
solid.•The strength of the metallic bondvaries, depending on the chargeand size of the cations, so does the mp.