ATOMIC STRUCTURE
Atom
   The word ‘atom’ has been derived from the Greek word ‘a-tomio’ which means ‘uncutable’ or
    ‘non-divisible’.
   It is the basic unit of an element.
   It is a form of matter which cannot be further broken down using any chemical changes.
   Nucleus is the very dense region consisting of protons and neutrons at the center of an atom.
   Atom is made up of three types of sub-atomic particles; these are electrons, protons and neutrons.
         Properties              Electron (-1e0)        Proton (1p1)              Neutron (0n1)
   Discovered by
                            J. J. Thomson in       Ernest Rutherford in      James Chadwick in
                            1897                   1920                      1932
   Symbol                   e                      p                         n
   Position or located      Outside the nucleus    Inside the nucleus        Inside the nucleus
   Electric Charge          −1 e or                +1 e or                   0 e or
                            −1.6 × 10−19 C         1.6 × 10−19 C             0 C (No Charge)
   Mass                     9.1 × 10−31 kg or      1.672 × 10−27 kg or       1.674 × 10−17 kg or
                            5.4 × 10−4 u           1.00727 u                 1.00867 u
Cathode Rays and Anode Rays                                  through minutely thin gold foils and
                                                             detecting them using screens coated with
For details about cathode and anode rays,                    zinc sulfide. He found that although the
Please Check ATOMIC AND NUCLEAR                              vast majority of particles passed straight
PHYSICS in Physics.                                          through the foil approximately 1 in 8000
                                                             were deflected leading him to his theory
Rutherford’s Atomic Model
                                                             that most of the atom was made up of
   This model describes the structure of                    "empty space".
    atoms proposed (1911) by Ernest
                                                        Atomic or Chemical Symbol
    Rutherford.
   It described the atom as a tiny, dense,                 The names of elements have been chosen
    positively charged core called a nucleus,                for a variety of reasons over the course of
    in which nearly all the mass is                          human and chemical history. It is
    concentrated, around which the light,                    represented by a unique chemical symbol,
    negative constituents, called electrons,                 X.
    circulate at some distance, much like                   The atomic symbol is used to identify the
    planets revolving around the Sun.                        element to which an atom belongs and the
   Gold foil experiment : This experiment                   number of electrons, protons and neutrons
    involved the firing of radioactive particles             it contains. As noted above, every neutral
                                                  260