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.