Atomic Theory and Laws of Stoichiometry


Dalton's Atomic Theory (Dalton in 1808)


The main postulates of this theory are:

i. All matters are made up of extremely small indivisible particles called atoms.


ii. Atoms of same element are identical in all respects i.e. shape, size, weight and properties.


iii. Atoms of different elements are different in all respects and have different properties.


iv. Atom is the smallest unit which can takes part in chemical combination.


V. Atoms of two or more elements combine in a simple whole number ratio to form compound atoms (now-a-days called molecule).


vi. Atoms can neither be created nor be destroyed during any physical or chemical change.


Laws of Stoichiometry


In Greek, stoichiometry means to measure an element. The calculation which deals with the mass and volume relationship between reactants and products is called stoichiometry.



2. Molar heat capacity


It is defined as the amount of heat required to raise the temperature of 1 mole gas through 1°C Note: 1. 

2. Gases on heating show considerable change in volume, if heated under pressure and thus, according to Ist law of thermodynamics, heat given to a gas depends upon the condition of constant pressure (gp) or constant volume (9) Therefore, gases have two values of specific heat or molar heat capacity.


1) Specific heat at constant pressure : When heat is given to raise the temperature of I g gas through 1°C, at constant pressure, the term used is 

(ii) Specific heat at constant volume : When heat is .(cal/g)


given to raise the temperature of 1 g gas through 1°C, at


constant volume, the term used is รง. (cal/g) Similarly, for molar heat capacities, the terms used are C (molar heat capacity at constant pressure) and C, (molar heat capacity at constant volume). (In cal/mol)

The Intermolecular Forces


The strength of intermolecular forces of different substances varies over a wide range depending upon the physical state of substance and external conditions of pressureand temperature. However, these forces are much weaker than ionic or covalent bonds.When a substance changes from solid to liquid and then to gas, the molecules remain intact.


Many properties of liquids such as evaporation, boiling point, surface tension, viscosity as well as that of solids such as fusion ctc. reflect the strength of intermolecular forces. For example, a liquid boils when bubbles of its vapour form within the liquid. The stronger the attractive forces, the higher the boiling point, viscosity, surface tension of liquids. Similarly, the melting point of solids increases with increase in the strength of intermolecular forces.


van der Waals' Forces


1. Three types of intermolecular attractive forces are known to exist between neutral molecules: dipole-dipole forces, London dispersion forces and hydrogen bonding forces.