Nuclear physics





·         Things around us are made up of atoms.

·         Greek philosopher ‘Democritus’ in 400 BC believed that matter is made up of tiny indestructible units called atom.

·         In 1803, John Dalton considered that element consist of atoms, which are identical in nature.

·         JJ Thomson experimentally discovered cathode rays – Electrons

·          Goldstein discovered positive rays, which were named as protons by Rutherford.

·         In 1932, James Chadwick discovered the chargeless particles called neutrons.

·         Presently, a large number of elementary particles like photon, meson, positron and neutrino have been discovered.

·         In 1911, the British scientist, Ernest Rutherford explained that the mass of an atom is concentrated in its central part called Nucleus 



Composition of nucleus;

·         Atoms have a nucleus surrounded by electrons.

·         Nucleus contains protons and neutrons.

·         Neutrons are electrically neutral (q=0) and the protons have positive charge (q= +e) equal in magnitude of the charge of the electron (q= -e)







                                                                                                                                      

Electrons = -e

Neutrons = 0

Nucleus = protons and neutrons

Protons = +          

 

 

·         Number of protons in the nucleus – atomic number, denoted by Z

·         Number of neutrons in the nucleus – neutron number (N)

·         Total number of neutrons and protons in the nucleus – mass number, denoted by A. Hence, A=Z+N.

·         Two constituents of nucleus namely neutrons and protons. are collectively called nucleons

·         Mass of a proton is 1.6726*10-27 kg which is roughly 1836 times the mass of the electron      

·         Mass of a neutron is slightly greater than the mass of the proton and it is equal to 1.6749*10-27kg.

·         To specify the nucleus of any element, we use the following general notation,

Where         X is the chemical symbol of the element

                     A is the mass number

                     Z is the atomic number

·         Nucleus is made up of positively charged protons and electrically neutral neutrons, the overall charge of the nucleus is positive and it has the value of +Ze

·         But the atom is electrically neutral which implies that the number of electrons in the atom is equal to the number of protons in the nucleus.           

 

Isotopes

·         In nature, there are atoms of a particular element whose nuclei have same number of protons but different number of neutrons. These kinds of atoms are called isotopes.

·         In other words, isotopes are atoms of the same element having same atomic number Z, but different mass number A 

 

Isobars;

·         Isobars are the atoms of different elements having the same mass number A, but different atomic number Z

·         In other words, isobars are the atoms of different chemical element which has same number of Nucleon.

·         Isobars are chemically different element.

·         They have different physical and chemical properties.

 

Isotones;

·         Isotones are the atoms of different elements having same number of neutrons.  

 

Radioactivity – Discovery;

·         In 1896, French physicist Henri Becquerel discovery,

·         Uranium radiated something that could affect a photographic plate

·         Phenomenon was called as radioactivity.

·         Uranium was identified to be a radioactive element.             

·         The radioactive elements emit harmful radioactive like radiations like alpha rays or beta rays or gamma rays.    

 

Definition of radioactivity;

·         Nucleus of some elements is unstable. Such nuclei undergo nuclear decay and get converted into more stable nuclei.

·         During this nuclear reaction, the nuclei emit certain harmful radiations and elementary particles.

·         The phenomenon of nuclear decay of certain elements with the emission of radiations like alpha, beta and gamma rays is called ‘radioactivity’ and the elements, which undergo this phenomenon are called ‘radioactive elements’.

 

Natural radioactivity;

·         The elements such as uranium and radium undergo radioactivity and emit the radiations on their own without any human intervention.

·         This phenomenon of spontaneous emission of radiation from certain elements on their own is called ‘natural radioactivity.

·         The elements whose atomic number is more than 83 undergo spontaneous radioactivity.

·         Example: uranium, radium, etc.,

·         There are only two elements, which have been identified as radioactive substances with atomic number less than 83, they are technetium (Tc) with atomic number 43 and promethium (Pm) with atomic number 61.           

 

Units of radioactivity – Curie;

·         It is the traditional unit of radioactivity.

·         It is defined as the quantity of a radioactive substance which undergoes 3.7 * 1010 disintegrations in one second. This is actually close to the activity of 1g of radium 226.

                            1 curie = 3.7 * 1010 disintegrations per second

 

Rutherford (Rd);

·         It is another unit of radioactivity. It is defined as the quantity of a radioactive substance, which produces 106 disintegrations in one second.

                             1 Rd = 106 disintegrations per second

 

Becquerel (Bq);

·         The SI unit of radioactivity is becquerel. It is defined as the quantity of one disintegration per second.

Roentgen (R);

·         The radiation exposure of ᵞ and x-rays is measured by another unit called roentgen. One roentgen is defined as the quantity of radioactive substance which produces a charge of 2.58*10-4 coulomb in 1kg of air under standard conditions of pressure, temperature and humidity.