Intermolecular Forces Introduction Atoms can form stable units

Intermolecular 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 Waal forces, dipole-dipole forces (responsible for the physical properties of the material).

Intermolecular forces • They 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.

Intermolecular forces • 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

Intermolecular forces • 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.

Ionic Bond • An ionic bond is a chemical bond formed by the electrostatic attraction between positive and negative ions. • 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. • Example 1: Na. Cl, a crystalline solid material

Ionic Bond • Example 2: Magnesium Fluoride • Ionic compounds result when a metal reacts with a nonmetal.

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 (H 2) → 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.

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:

Intermolecular forces • § § Intermolecular interactions include: Van der Waals forces. Ion-dipole interactions. Ion-induced dipole interactions. Hydrogen bonds.

Van der Waals forces • Van der Waals forces includes: i) Dipole-dipole interaction ( Keesom). ii) Dipole – induced dipole interaction (Debye). iii) Induced dipole – Induced dipole interaction (London).

Van der Waals forces • In some cases, one of the atoms in a covalent bond in a molecule has the ability to attract shared electrons to itself. • These molecules can line up themselves so that +ve and –ve ends are close to each other dipole attraction • 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 • 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.

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: H 2 (hydrogen) gas, CCl 4 (Carbon tetrachloride) , benzene.

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 be attracted to an ion. • Accounts for the solubility of the ionic crystalline substances in water, e. g. the cation attracting the relatively negative oxygen atom and vice versa. • This is 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. I 2 + K +I- K + I 3 -

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 100 kcal for the ionic bond. • The formation of dimer ( formic acid, acetic acid) • For e. g. Ether {CH 3 OCH 3 (dimethylether)} and Ethanol {CH 3 CH 2 OH}.