Introduction To Atoms And Nuclei

                   ATOMS     AND    NUCLEI

                                                                                     

                                                                                  

Author: Proloy Bhaduri

Introduction:

An  atom  is the  smallest  constituent  unit  of any matter.John Dalton (1805) considered that all matter was composed of small particles called atoms,which has  the   properties same as that of a chemical element.Every  solid ,liquid,gas and plasma  is composed of atoms. In general   solid,liquid and gasses are composed of  neutral  atoms and plasma is composed of ionized atoms.

Atoms are very small ,typical  size of an atom  is nearly 100 pm(1pm= 10^-12 m)  ,i.e.,  ten  billionth  of a meter,in the short scale. Clearly you can't guess the size. 

                                                   Because  of this smaller size  of atoms, it is very difficult  to analyse and make decisions on the various properties of atoms individually.In other words in Classical Mechanics,  behavior of atoms cannot be predicted.Hence  another branch  of mechanics i.e., Quantum  Mechanics(Discussed later on this topic ) is practiced to study the properties of atoms.

Let  us  conceive atoms as billiard balls, e.g, gives noticeably incorrect predictions  due to quantum effects(It is  discussed  later in Quantum Mechanics Chapter).Through the development of physics , atomic models have incorporated quantum principles  better explain and predict their behavior.

Every atom  composed of a nucleus(positively charged)  and electrons(negatively charged) revolves around the nucleus .There is a lot of propositions on the  Structure  Of Atom.At the end of the nineteenth century there accumulated enough experimental evidence to show that the atom is made of still smaller particles. These subatomic particles are called the fundamental particles. The number of subatomic particles now known is very large. For us, the three most important are the proton, neutron and electron. 
                   How these fundamental particles go to make the internal structure of the atom, is a fascinating story. The main landmarks in the evolution of atomic structure are :

                           1896   J.J. Thomson’s discovery of the electron and the proton
                           1909   Rutherford’s Nuclear Atom
                           1913   Mosley’s determination of Atomic Number
                           1913   Bohr Atom
                           1921   Bohr-Bury Scheme of Electronic Arrangement
                           1932   Chadwick’s discovery of the neutron
                                                                                       (source:https://www.google.com/)
Diameter  of an  atom  is  approximately between the range  62 pm(He) to 520 pm(Cs)(Source: Wikipedia).
           We just got to know that, every atom is consisted of  a nucleus and electrons bound to the nucleus . The nucleus  is composed of the protons and neutrons and  99.94% of the total mass of  an atom is due to the nucleus  . Remaining  is the mass of electrons.So many of experiments
and theories are there to explain the same.
We have previously  learned that electrons are negatively charged and protons are the positively charged sub atomic particles.
                       
 Charge of  proton = + Q_1.
 Charge of electron  =  - Q₂
Condition 1: If  |Q₁| = |Q₂| ,  the atom is neutral.
Condition 2: If   Q₁ > Q₂ ,   the atom is a cation.
Condition 3: If  Q₁ < Q₂   , the atom is  an anion.  

 Whereas neutrons don't have any charge. The electrons of an atom are attracted to the protons in an atomic nucleus by this Electrostatic force. The protons and neutrons in the nucleus are attracted to each other by a different force, the Nuclear force.

                                                    (Strength)_Nuclear > (Strength)_{Electrostatic}