THE ATOMIC NUCLEUS

INTRODUCTION:

The beginning of twentieth century. not only witnessed the successful development of atomic hypothesis the old quantum theory of radiation by plank, and Einsteins special theory of relativity; these years also marked the beginning of nuclear physics as a new field of scientific activity.

In 1886 Henri Bacqueral first observed the phenomena of radioactivity.A year after J.J Thomson discovered the electron.Later on Thomson demonstrated that cathode rays were constituted of electrons..In the way of notion of the atom as the ultimate indivisible unit of matter had to be discarded, and it become evident that atoms included electrons among their structure and these electrons could be liberated by electromagnetic excitation as in a gas discharge spontaneously as in radio activity.

In 1902 Rutherford and Soddy showed that in radioactive decay an atom transform itself into a different chemical element.This discovery led to the development of models for nuclear structure and a after a period three decades James Chadwick made another break through by discovering the neutrons which laid the foundation of a nuclear model namely a very small (of the order of 10e-14 m),roughly spherical in shape and highly dense object,comprising of protons and neutrons.

NUCLEAR STRUCTURE:

If the nucleus has any sort of structure then we must argue what are its constituent ? To answer question, let us make a study of the known chemical elements in the order of their atomic numbers.The hydrogen atoms which is the lightest atom has one electron and its atomic number Z=1, helium has two electrons and its Z=2 silver has forty seven electrons and its Z=47 and uranium has ninety two electrons and its atomic number Z=92 etc.The increase of atomic mass with atomic number suggests that all atoms are simply combination of hydrogen. Thus the helium atom with Z=2 should have 2 protons in its nucleus,silver atom should have 47 protons etc in order to make the atom neutral in charge.However, atomic masses are not found to increase in steps of the mass of hydrogen atom.Helium atoms for example, has mass four times that of hydrogen, Lithium has mass of about seven times etc. The proton is the nucleus of hydrogen atom.Its mass is 1836 times of the mass of electron and it carries a positive charge.  

One possible argument to overcome the above discrepancy in the behaviour of atomic masses is that there may be enough protons in the nucleus to account for its atomic mass and several electrons may also be present in the nucleus in order to neutralize the positive charge of the protons that are in excess of the required number.Thus the helium nucleus whose mass is four time's that of protons, through its charge is only +2e may be considered to have four protons and two electrons.This explanation gets support from the observed phenomena of the beta activity in which nuclei spontaneously emit electrons and whose occurrence can't be easily explained if the electrons are not assumed to present in the nuclei.

However there are many argument against this concept of the existence of electrons in the nuclei.One of the objection against the presence of electron in nucleus is that, in order to confine an electron inside a nucleus the electron must have an energy of about 10e3 MeV. Whereas the observed energies of the electrons in beta activity is of the order of 2 and 3 MeV only, As such the presence of electron inside the nucleus is ruled out.

The problem was resolved by the Chadwick discovery of neutron.The neutron has a mass slightly Greater than that of the proton and it carries no charge.The term nucleon is used as a generic name for either proton or neutron.

The atomic number Z of an atom is the number of atomic electrons in the neutral atom; it is also the number of protons in the nucleus.The atomic mass number A equals the number of nucleons in the nucleus often called as the nucleons number.Since Z is the atomic number or ( proton number ) the neutron number N is therefore defined as N=A-Z

A nucleus is completely specified by any two of those numbers.To identify a nucleus the conventional symbol is  zXA where x is the the chemical abbreviation for the particular element.

ISOTOPES:

The atomic or proton number z determines the chemical properties of the element. Many chemical elements have nucleai with more than one value of the mass number A . For example we have hydrogen deuterium and tritium for Z=1.

All such nuclei having same Z but different values of N are called isotopes. Deuterium is sometimes expressed by the symbol 

Out of about 1400 nucleai which are known to exist about one fifth are found to be stable.Let us now, analyze them on our chat according to the number of protons and neutrons. If A nucleus has an equal number of protons and neutrons its position on the graph will be 45 line.This is approximately the situation for the the commo-nest isotopes of light elements as the atomic number increases however the number of extra neutrons gets larger and larger it is to be noted that N must be greater than Z in order to achieve stability except for some of the very light nuclear we conclude that all nucleus contribute to the attractive forces that hold the nucleus together whereas the instability e is due to repulsive coulombs forces between the protons as Z increases an excess of N over Z is required in order to achieve stability

RADIOACTIVITY AND NUCLEAR CHANGES:

Radioactivity may be defined as the spontaneous disintegration of the nucleus of atoms it is self disrupting activity exhibited buy some naturally occurring elements the first discovery of a radioactive element was made by Henry Bacqueral in 8996 when he found that uranium atom emits radiations which are are penetrating to such an extent that uranium salts causing blackening of the photographic plates this effect appeared intrinsic to uranium because good grinding dissolving and heating the salts did not change the radiation effect the radiations could be penetrate not only paper but also glass and even thin sheets of aluminium.Bacqueral founded the more uranium the simple contained the more intense the immediate radiations 2 year letter Marie and Pierre Curie succeeded in chemically e isolating two new elements polonium and radium which were found to be radioactive it has been demonstrated that all the elements with Z greater then 83 a radio tape Rutherford and his coworkers were able to prove with the help of experiments that the radiations emitted by radioactive substances are of three different types a small amount of radioactive substance is is placed at the bottom of cavity drilled in a block of Lead When the narrow beam of radioactive rays is allowed to pass through the space between two charge plates the path of some of the rays Bends towards the positive plate and some race bends towards the negative plate while other go non deflected by the influence of electric field between the plates similar effect is observed in the presence of a transverse magnetic field the rays bending the negative plate indicate date that consists of positively charged particles while those bending towards the positive plate indicate negatively charged particles

further experiments confirmed that the positively charged particles are nuclei of helium atoms called alpha particles while the negatively charged particles are electrons and are called beta particles the race which go non deflected indicate no charge and are therefore energetic photons are the gamma rays

Is just a helium atom with both of its electron removed it has a atomic weight or mass number A=4 and the charged number Z=2 the beta particles have been identified as electrons with more more energy as compared with the ordinary electrons because their origin is nucleus and not the atomic orbit the gamma rays are electromagnetic waves of nearly same or same what higher frequency than x-ways the alpha rays are ejected with a speed of 1/10th to one 100 of the velocity of light where is the beta particles travel often with the higher velocity as about one fifth of the velocity of light the gamma rays being electromagnetic waves have the same velocity as that of the velocity of light.\

 

THE DISINTEGRATION OF RADIOACTIVE ELEMENTS:

A nuclear species corresponding to given values of A & Z is called a nuclide and is denoted by zx.where X is the chemical symbol for the particular element, e.g carbon is denoted by 6C 12 etc.The nuclear volume is directly proportional to A,which leads to the important result that the density of nuclear matter is essentially constant for all nuclides.The main cause of radio activity is the instability of nuclides of heavy elements. The unstable nuclides in nature decay by the phenomena of radio activity taking place by the process of emission of a or B particles.which may be accompanied by Y-rays. As the mass number of a-particle is A = 4 and the charge number 2 = 2.the decay product after alpha particle emission will have as mass number (A-4) and charge number (Z-4).

In this process the daughter nucleus may also remain unstable and under go further disintegration till it attains stability.

The process of beta particle emission involves no effect on mass number A, but it does change the charge number Z by +1 depending upon whether the emitted particle is negative beta particle or a positive beta particle.

Most frequently the alpha and beta emission leaves the daughter nuclide in an excited state and that one or more gamma rays are emitted as it goes back to the ground or normal state.Since the gamma rays are mass less photons, gamma emission will cause no change on wither A to Z.