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Matter itself is made of different types of atoms. A human hair is roughly 5,00,000 times bigger than an atom. Democritus introduced the idea of an atom. It was found many years later that atoms of a single element are identical in size to each other but different in size to atoms found in other elements.
Dalton’s Atomic Theory (John Dalton,1803).
Atoms are small indivisible particles which matter is made of.
Atoms of the same element are identical in all respects.
Atoms combine in a simple form to form compounds.
Atoms are the tiniest particles, which can take part in a chemical reaction.
Atoms can neither be created nor be destroyed.
In some compounds like CO2 ,SO2 etc., the atoms are present in stoichiometric properties.
Berthollide compounds are non-stoichiometric compounds in which the exact analytical composition deviates from the atomic theory.
Laws of Chemical Combination
Laws of Conservation of Mass ( A Lavoisier,1774) – Matter is neither created nor destroyed during any physical or chemical change. The total mass of the reactants before the reaction is equal to the total mass of the products after the reaction.
Law of Definite Proportions or Law of Constant Composition ( Louis Proust,1799 ) – A chemical compound is always made up of the same elements combined together in a fixed proportion by weight.
Law of Multiple Proportions ( John Dalton,1804 ) – When two elements combine to form two or more compounds,the weights of one of the elements which combine with a fixed weight of the other bear a simple whole number ratio.
Law of Reciprocal Proportions (Richter,1792 ) – When two different elements separately combine with the same weight of the third element,the ratio in which they do so will be the same.
Gay Lussac’s Law of Gaseous Volumes ( Gay Lussac,1809 ) – Whenever gases react together they do so in volumes which bear a simple ratio to one another and to the volumes of the gaseous products,all measurements being made under identical conditions of temperature and pressure.
Valency
It is the combining capacity of an element and is numerically equal to the number of hydrogen or chlorine atoms or double the number of oxygen atoms which combine with or are displaced by one atom of the element.It is also equal to the number of electrons lost,gained or shared by an atom of the element with other atoms.
Atomic Weight
It is the average relative weight of an atom of an element as compared to the weight of Carbon (C12) taken as 12.It has no units.However it is expressed in a.m.u. which only signifies that it is taken on atomic mass unit scale.
Atomic Mass Unit (a.m.u)
It is the quantity of mass equal to 1/12th of the mass of a carbon atom which is equal to
1.6603 x 10 -24 g.
Gram Atomic Weight
It is the atomic weight of an element expressed in grams or it is the weight in grams which is numerically equal to the atomic weight of an element known as gram atom.
Example – Atomic weight of oxygen is 16 a.m.u. while gram atomic weight is 16g.
Relation between atomic weight , equivalent weight and valency is that Atomic weight is equal to the product of equivalent weight and valency.
Isomorphism
Substances having the same crystalline structure are said to be isomorphous. Isomorphous substances have the same number of atoms arranged similarly e.g. ZnSO4.7H2O and FeSO4.7H2O.
Law of Isomorphism
If AC and BC are isomorphous then weight of A which combines with
fixed weight of C /weight of B which combines with the same weight of C
=Atomic weight of A / Atomic wt. of B
Molecular Weight
It is the average relative weight of a molecule of an element or a compound as compared to the weight of Carbon ,C12 taken as 12 on the atomic mass unit scale. Molecular weight of a substance is calculated by adding the atomic weights of the atoms present in the molecule.
Formula Mass
It is the sum of atomic masses of various atoms represented by the formula of an ionic compound.
Equivalent Weight
It is the number of parts by weight of a substance which combines with or displaces directly or indirectly 1008 parts by weight of hydrogen or 8 parts by weight of oxygen or 35.5 parts by weight of chlorine called chemical equivalent since it is number only with no units.
Mole Concept
A mole represents 6.02 * 1023 particles irrespective of their nature.The number is called Avogadro’s number.
Thus 1Mole = 6.02 * 1023 particles
The mole is also related to the weight of the substance.The quantity of matter containing Avogadro’s number (6.02 * 1023) of particles represents one mole of that species.Weight of one mole atom of any element in grams is equal to its gram atomic weight or one gram atom.
Gram Molecular Weight
It is the weight of 6.02 * 1023 molecules, while the actual weight is the weight of one molecule.
Weight of a molecule
= Gram molecular weight/ 6.02 * 10 23.
Vapor Density
Vapor Density of a gas is the ratio of the weight of a certain volume of the gas to the weight of the same volume of hydrogen under similar conditions of temperature and pressure.
Empirical Formula
It is the formula of a compound which gives the simple whole number ratio of the atoms of various elements present in one molecule of the compound.
Normality
It is the number of gram equivalents of solute dissolved per liter of the solution.
N1V1 = N2V2
Molarity
It is the number of moles of solute dissolved per liter of the solution.
Normality = Eq. wt. / Mol. wt.
Formality
Formality of a solution is the number of grams of molecular weight of the ionic solute dissolved in one liter of the solution.
Molality
It is the number of moles of solute dissolved in 1000g of the solvent.
Limiting Reactant
It is the reactant which is completely used and determines the amount of products.
Conclusion –
Matter itself is made of different types of atoms. A human hair is roughly 5,00,000 times bigger than an atom. Democritus introduced the idea of an atom. It was found many years later that atoms of a single element are identical in size to each other but different in size to atoms found in other elements.
