Defination of atoms and molecule.What is an atom?

                        ATOMS AND MOLECULES
What is an Atom?
           Atom is the smallest particle of an element which may or may not have independent existence.Thus,atoms are building blocks of all matter.The word atom is a Greek word meaning indivisible.Indian and Greek philosophers gave the idea of Permanu and atoms but this was based on philosophical considerations.The term permanu was first used by Mahirishi Kanad,Indian philosopher.Another Indian Philosopher Pakudha Katyayama said that these particles normally exists in a combined form which gives us various forms of matter.Around the same era,anacient Greek philosophers Plato,Aristotle,Lucretius,Democritius, etc. also believed that matter is composed of exteremely small and indivisible particles called atom.
How big are atoms?
           Atoms are very small,smaller than anything that we can imagine or compare with.Atoms are very-very small in size.
                 1 nanometre (nm)= 10-9metre.
Defination of atoms and molecule.What is an atom?
Defination of atoms and molecule

Laws of Chemical Combination
           By the top of the eighteenth century,Chemists recognised the difference between elements and compounds.They also took an interest find how and why elements combine and what happens once they combine.
A chemical reaction is a process in which atoms of the same or different elements rearrange themselves to form a new substance.While they do so either by absorbing heat or giving it off.In chemical reactions,the substances which react are called reactants and the ones which are formed after reactions are called products.These chemical reactions takes place according to certain laws,called the law of chemical combination.
     Lavoisier and Joseph L.Proust after many experiments established two laws of chemical combination.These are :  
1.Law of conservation of mass
2.Law of constant proportions or law of definite proportions
1.Law of conservation of mass: According to Lavoisier(in 1774),this law stats that “matter is neither created nor destroyed in a chemical reactions”.
2.Law of constant proportions:
                 The law of constant proportions is also know as  law of definite proportions.
          This law was stated by Proust in 1797.According to this law.”In a chemical substance,the elements are always present in definite proportions by mass.”
Isotopes and Law of Constant Proportions
        Discovery of isotopes has proved that an element can have atoms with different atomic weights.Hydrogen has three isotopes having atomic mass 1,2 and 3 respectively.Oxygen has also three isotopes having atomic weights 16,17 and 18 respectively.A molecule of water may have the isotopes of hydrogen and oxygen.
         The term molecule was first used by Avogadro to describe the smallest particle of matter which capable of existing independently.Molecule is the smallest partical of the substance (element or compound) which can exist independently.Molecules are composed of two or more atoms of same or different types e.g.., Cl2,H2,NH3,H2O etc.
Types of Molecules:
Molecules are two types:
(1)Homoatomic Molecules:
          Molecules formed by combination of similar atoms are called homoatomic molecules.Molecules of elements are homoatomic.The number of atoms present in a molecule is called its atomicity.If a molecule has only one atom,it is called monoatomic molecule.The molecule of metals are monoatomic.Thr molecules of He,Ne,Ar,Kr, etc. are also monoatomic.
(2)Heteroatomic Molecules:
       Molecules formed by two or more of  different types of atoms are called heteroatomic molecules.For example CH4  methane is formed by one atom of carbon and four atoms of hydrogen,CO2 has one carbon atom and two oxygen atoms.

         Differences between an Atom and a Molecule
Major differences between an atom and a molecule are given in following table:
 1.Atom is the smallest particle of an element which generally cannot exist in the free state.
2.An atom does not get decomposed in a chemical reaction.
3.An atom is made up of some fundamental particles,i.e. electrons,protons and neutrons.   
1.Molecule is the smallest particle of the substance which can exist independently.
2.A molecule can get decomposed in a chemical reaction.
3.A molecule is made up of atom of similar or different elements.
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