4. ATOMIC MASS AND ATOMIC NUMBER ....

Atomic number-

             We know that any atom contains three type of fundamental particles i.e. electron ,proton and neutron.

     In any atom of protons indicates the Atomic number of that atom. According to Moseley (1913, working in Rurherford's lab)  "the number of unit positive charges carried by the nucleus of an atom is termed as the Atomic number of that atom" .

Symbol of Atomic number is Z. The atomic number is also equal to the number of electrons.

     It is an unique value, it means no two element can posses same atomic number.

Example: Atomic number of H is 1 and atomic number of carbon is 6.

Mass number or Atomic mass-

         "The sum of the number of proton and neutron in uncleus  is termed as Mass number of an atom".It is denoted by A. We know that the mass of each proton and neotron are almost equal to 1 on the atomic mass scale, therefore the atomic mass   of the atom is approximately equal to its mass number.

Example: Mass number of C is 12

Number of neutron in necleus N= A-Z

           (wher A=Atomic mass and Z= Atomic number )

 Isotopes-
       Atom with same atomic number but different neutrons number and hence different atomic masses called as Isotopes.

           The term isotopes formed from the greek roots  isos ( equal) and topos (place) , it means “the same place”. Thus , different isotopes of a single element occupy the same position on the periodic table.

       The number of proton is called atomic number,it is equal to the number of electron,but the atom can have different numbers of neutron, thus each isotope of a given alement has a different mass number. 

For example: carbon-12, carbon-13, carbon-14,  where 12, 13, and 14 is the mass number of carbon atom respectively . Here ,in each isotope of carbon atom the number of electron is same i.e. 6, and the number of proton is also the same as electron i.e. 6, but number of neutron is different .

                                                                                                                                                            

                                     

Carbon 12 = 6 neutrons

Carbon 13 = 7 neutron

1. Carbon 14 = 8 neutrons.

3. Atom and its characterstics...

    In previous blog we have discussed about Atom and its parts i.e. inner nucleus and outer space(electron cloud).Now we have to know that what is electron, proton  and  neutron,and which type of charges they have, and what are the explanations behind it?

Electron proton and neutron are also called fundamental particle of an Atom.

Electron-(symbol e⁻)

              It is discovered by j.j. Thomson.An electron is a subatomic part with a negative charge,having very small or negligible mass (approximately 9.109×10⁻³¹ kilograms) in comparison to proton and neutron.This mass is called the rest mass of an electron.

Charge on an electron is(−1.602×10⁻¹⁹ coulomb).  It is  determined by Millikan,in 1939 by using "Oil drop technique".  An electron is also the smallest charge of electricity i.e. unit negative charge.

Proton-( symbol p or p⁺)

            Proton was discovered by Ernest Rutherford in 1911 .Experiment with discharge tube containing perforated cathode indicates the presence of another type of rays that passed through the hole in cathode and carried positive charge,these rays were called Anode rays.

The determination of  ratio of charge to mass (e/m) by using different gases in discharge tube shows that Hydrogen gas gave highest value for e/m,it indicates that if  mass of each gas is same then mass of hydrogen gas is the lowest.it proves that hydrogen is the smallest fundamental particle having a positive sign to that of electron.This lightest positive particle is known as Proton.Mass of proton-1.672621×10⁻²⁷ kg

Charge of proton-1.602176565(35)×10⁻¹  Coulomb

Number of proton is called as Atomic number of an atom(or the number of electron present out side the neucleous).

Neutron-( n or n°)

      It is discovered by Chadwick in 1932.When thin sheet of boron or beryllium was bombarded by alpha particles,a stream of new particle which carried no charge  but had a mass almost equal to that of proton, was emitted.These neutral particles are called neutron. 

 

Mass of neutron-1.674927351(74)×10⁻²⁷ kg.

It is important to mention that the neutron was not discovered until 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle.

If you have any confusion or question please send .