Atomic scale

Introduction:- Many chemical phenomena occur around us and these are explained on the basis that matter is made up of molecules. Molecules,in turn, are made up of atoms. daltons atomic theory  that an atom is an indivisible particle.  but reaserch findings of the last hundred years on the study of gases in particular and then of solids,led to the discovery of the fundamental particles,viz., electron,proton and nutron. various atomic models  to indicte the arrangement of these fundamental particles in an atom were proposed. an atom consists of a nucleus at its center. protons and nutrons are present  in the nucleous while electrons revolve aroud the nucleous. the charges and masses of  the fundemental particles are listed below.  
         It is especially focus on the properties. There are two kinds of atomic units.

1. Hartree atomic units.
2.  Rydberg  atomic units.   The numerical values of the follwing four fundemental physical constants are all units by definition                                                             electron mass                                                                                                                                                                                                                       elementary charge                                                                                                                                                                                                              reduced plank's constant                                                                                                                                                                                                    columbs constant.

Atomic number:-

 Every atom of a given element consists of a definite number of electrons. this number is called the atomic number of the element. it is denoted by Z.Moseley proposed that a simple relation between the frequencies of the charecteristic X-rays given by element and its atomic number.When a meterial target,called anticathode,is placed in the path of cathode rays in a discharge tube,X-rays charecteristic of the metal are emitted from the metal target.

MASS NUMBER:-

 The total of the numer of protons and nutrons present in an atom  is reffrred to as the mass number(A) of the atom of that element.This is mathematically written as
                                              A=Z+N.
                                where Z is the atomic number and N is the number of nutrons.Hence mass number is always positive.Atoms ofan element which differ in their mass but have the same atomic number are  called isotopes of that element.The isotopes of an element thus have the same number of protons but differ in the number of nutrons present in them.

Fundamental atomic units 

Fundamental Atomic Units
Dimension	Name	Symbol	Value in SI units
mass	electron rest mass	me	9.1093826(16)×10−31 kg
charge	elementary charge	e	1.60217653(14)×10−19 C
angular momentum	Reduced Planck's constant	h-=h/2π	1.05457168(18)×10−34 J·s
electric constant	Coulomb force constant	1 / (4πε0)	8.9875517873681×109 kg·m3·s-2·C-2

Atomic models:-

Rutherford model:-
                                  
Rutherfords experiment showed that most of the space in an atom is empty and all the mass of the atom and its positive charge are concentrated at the center of   the atom which is spherical in shape.The center point of the positive charge is called "atomic nucleus". The electrons revolve round the nucleus in circular orbits just as planets revolve round the sun.
This model failes to explain two important facts:

• As per the law of electrodynamics ,a charged particle like electron in circular motion around the opposite charge should continously lose energy by emission and spiral down into the nucleus due to nucleus attraction.If this happens ,the atom should collaps which is not happening.

If the electron in an atom continuosly radiates energy ,the spectrom of that element should be continous spectrom.But the eseatoms give rise to discontinuous line specta with we defined lines.