1 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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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).
• 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}.