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Intermolecular and

Intramolecular Forces

Introduction

• Atoms can form stable units called molecules by sharing electrons.

• The formation of molecules is the result of intramolecular bonding (within the molecule) e.g. ionic, covalent.

• Forces that cause the aggregation of the components of a substance to form a liquid or a solid are called intermolecular forces (between molecules) e.g. van der Waalsl forces as dipole-dipole forces (responsible for the physical properties of the material).

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Introduction

• These forces can be divided into attraction and repulsion forces.

• The force is repulsive when the molecules are brought close enough together that the outer charge clouds of the molecules touch, and this causes the molecules to repel each other.

• The repulsive forces are necessary so that the molecules do not destroy each other.

Introduction

• The attractive forces can be divided into two

types: – Cohesive forces: this term is used when like molecules attract

each other

– Adhesive forces: this term is used when different molecules

attract each other

• Attractive forces are divided into two groups: – The weak forces of attraction are: Van der Waals forces, Ion-

dipole forces, and Hydrogen bonds.

– The strong forces include the Ionic and Covalent (coordinate

type) bonds.

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Introduction

• Knowledge of these forces is important for:

1- Understanding of the properties of gases,

liquid, and solids.

2- Understanding of interfacial phenomena.

3- Understanding the hydrophobic effect.

4- Flocculation of suspensions

5- Stabilization of emulsion

6- Compaction of powders in capsules, and the

compression of granules to form tablet

Intramolecular interactions

• Ionic bond (could also be available as

intermolecular forces).

• Covalent bond.

• Metallic bond.

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Ionic Bond

• An ionic bond is a chemical bond formed by the

electrostatic attraction between positive and negative

ions.

• Ionic compounds result when a metal reacts with a

nonmetal

• Ions form due to valency changes in an atom.

• The atom that loses electrons become a cation (+ve ion),

and the atom that gains electrons becomes an anion (-ve

ion).

Ionic Bond

• Any given ion tends to attract as many neighboring ions of opposite charge as possible.

• When large numbers of ions gather together, they form an ionic solid. The solid normally has a regular, crystalline structure.

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Ionic Bond • Example 1: NaCl, a crystalline solid material

• Example 2: Magnesium Fluoride

Covalent Bond

• A covalent bond is a chemical bond formed by sharing of a pair of electrons between atoms.

• A molecule is a group of atoms, frequently nonmetal atoms, strongly linked by a covalent bond.

• Example:

Hydrogen (H2) →

The electrons are attracted simultaneously by the positive charges of the two hydrogen nuclei. This attraction that bonds the electrons to both nuclei is the force holding the atoms together.

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Covalent Bond

• Examples

Single, double and triple bonds

acetylene ethylene

Coordinate Covalent Bond

• A coordinate covalent bond is a bond formed

when both electrons of the bond are donated by

one atom:

• A coordinate covalent bond is not essentially

different form other covalent bonds; it involves

the sharing of a pair of electrons between two

atoms. Ex: formation of ammonium ion:

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Intermolecular interaction Polarity of molecules:

• In some molecules, one of the atoms in a covalent bond has the ability to attract shared electrons to itself resulting in a polar molecule (dipole).

• A dipole is a separation of two opposing charges over a distance r. and is generally described by a vector known as the dipole moment (µ).

• The dipole moment is a vector property where the symmetry of the molecules affects generally its dipole moment. For example, carbon dioxide has no net dipole.

Intermolecular interaction Polarity of molecules:

• Another example on the effect of symmetry on the net dipole moment: Benzene and p-dichlorobenzene are symmetric planar molecules and have a dipole moment of zero. Meta (m-) and ortho (o-) dichlorobenzene are not symmetrical and have significant dipole moment.

Benzene p-dichlorobenzene

o-dichlorobenzene m-dichlorobenzene

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Intermolecular interaction

Polarity of molecules:

• A molecule can maintain a separation of electric charge

(i.e. get polarized) either:

By having a permanent charge separation within a polar

molecule (permanent dipole moment).

Through induction by an external electric field or

surrounding ions. Induced polarization can occur for both

polar and nonpolar molecules (induced dipole moment).

Intermolecular interaction Intermolecular interactions include:

van der Waals forces.

Dipole-dipole forces.

Dipole-induced dipole forces.

Induced dipole-induced dipole forces.

Hydrogen bonds.

Ion-diople interactions.

Ion-induced dipole interactions.

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van der Waals forces

• van der Waals forces are non-ionic charge-

charge interactions between molecules. They

include:

i) Dipole-dipole interaction ( keesom).

ii) Dipole – induced dipole interaction (Debye).

iii) Induced dipole – Induced dipole interaction

(london).

van der Waals forces

• Permanent polar molecules (dipoles) can line up themselves so that partial +ve and –ve ends are close to each other dipole dipole attraction.

• Dipole-dipole forces are typically almost 1% as strong as covalent or ionic bonds, and they rapidly become weaker as the distance between the dipole increases.

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van der waals forces

• Permanent dipoles are capable of inducing an electric dipole in nonpolar molecules which are easily polarizable in order to produce dipole-induced dipole (Debye) interactions

van der Waals forces

• London or dispersion forces (induced dipole – induced dipole or instantaneous dipole) occurs in noble gas atoms and nonpolar molecules.

• It is sufficient to bring about condensation of nonpolar gas molecules to form liquids and solids when molecules are brought close enough together →induced dipole – induced dipole forces→ instantaneous dipole examples: H2 (hydrogen) gas, CCl4 (Carbon tetrachloride) , benzene

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Ion dipole forces

• Ion – dipole forces are attractions between ions and permanent dipoles. The attraction occurs because ions have a stronger charge than dipoles, so a partially charged end of a dipole will attract to an ion.

• This helps in part for the solubility of ionic crystalline materials in water. i.e. the cation attracting the relatively negative oxygen atom and vice versa.

• This is also important in the use of diuretics. Diuretics increase the volume of urine and remove excess electrolytes and fluid.

Ion- Induced dipole forces

• As in the formation of iodide complex: a

potassium ion can induce a dipole in a diatomic

iodine molecule. This is important in the

solubility of iodine in solution of potassium iodide.

I2 + K+I- K+I3-

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Hydrogen bonds • A strong dipole-dipole force are seen in molecules in

which hydrogen is bound to a highly electronegative

atom such as nitrogen, oxygen, or fluorine.

• Two factors account for the strength of this interaction:

1- the great polarity of the bond

2- close approach of the dipoles, allowed by the very

small size of the hydrogen.

• Effects on physical properties (especially with water):

1- high boiling point

2- low vapor pressure

3- high dielectric constant

Hydrogen bonds

• Intermolecular in water, intra molecular and intermolecular in Salicylic acid solution.

• Hydrogen bonds are relatively weak, with a value of about 2 to 8 Kcal/mole as compared with a value of about 50 to 100 kcal for the covalent bond and well over 100kcal for the ionic bond.

• The formation of dimer ( formic acid, acetic acid)

• For e.g. Ether {CH3OCH3 (dimethylether)} and Ethanol {CH3CH2OH}.

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Hydrogen bonds