Atom Sadiq Fdk

ATOMIC STRUCTURE Submitted By: Class 9th

G.S.S.S Sadiq (Faridkot)

ATOMIC NUCLEUS 



The Establishment of the identity of the electron as a charged particle, which is emitted by atoms under several conditions, led to the condition that the remaining part of the atom must be positively charged. Also, a comparison of the values of e/m for electron and the charge to mass ratios for H+, Na and other ions showed that the nucleus should be more than 2000 times heavier than the electron. J.J Thomson Gave a model of atom in which electrons were uniformly distributed in the entire volume of atom the mass of the atom was supposed to be uniformly distributed.

The size of the atom had beeneEstablished indepenently as about 10-10 m or 1A. At this stage Ernest Rutherford made very fundamental Contributions in evolving structure of the atom and establishing the presence of a small nuclcus in the atom where practically all  the mass of the atom is concentrated. Rutherford and his associates investigated scattering of α-particles from very thin foils of heavy metals. 

ELECTRON

E1

(a)

C

(B) E1 E2

C (C)

E3

E2

Structure of Atom The Rutherford Model Rutherford combined all the experimental results regarding the properties and the characteristics of the electrons and the nuclei as well as the emission spectra of atoms to develop a model of the atom.He proposed a model in which the electons were assumed to be revolving around the sun in fixed orbits just like the planets move around the sun in fixed orbits in our solar system.However, the orbital revolution of electrons is not expected to be stable.Any charged particle when accelerated is expected to radiate energy.To remain in a circular orbit,

 The

electron would need to und ergo acceleration.Therefore,it will radiate energy.The loss in energy would lead to shrinking of the size of the orbit.In a short time,it will hit the nucleus.Therefore,such an atom cannot be expected to be stable.

The Bohr Model 

Niels Bohr in 1912 proposed a model of the atom, which was based on radically new concepts.He made a bold suggestion that particles at atomic level would behave differently from the macroscopic objects.Bohr proposed thet at the dimensions of atomic level, electrons could revolve in stable orbits without continuously radiating energy in the form of electromagnetic waves.A or ccording to Bohr,s model, an electron revolves in an orbit, with well-defined energy. Under normal conditions the energy of the system will be lowest, let us say E1. There are other possible orbits with higher energies E2,E3 etc.

 The electron can go into these orbits when energy is provided to it. The electrons can occupy orbits with discrete energy levels falls to an orbit of lower energy level, the difference in energy is radiated in the form of electro-magnet radiation or light. Since,each atom has its specific energy levels it can emit radiation of specific wavelenths or energy. With this model, it was possible to explain the characteristics spectra of different atoms and other properties. When an energetic electron, like those in the cathode rays, hits an atom it can lead to ejection of atomic electrons.

 The energy of an electron in a stable orbit may increase due to collision to such a level that it is able to overcome the attractive force between it and the positively charged nucleus and move out of the atom. This could explain ionization of the gases and also formation of cathode rays in the discharge tube.  According to Bohr model, the number of electrons that can be accomodated in different orbits or shells is fixed.The shell emphasizes that an atom is a three dimensional enitity. The first shell can occupy two electrons.The second shell can occupy eight electrons; the third shell can also occupy 18 electrons and so on.



We know that the lightest atom is the hydogen atom. It contains one electron and its nucleus consists of a proton whose charge is equal to that of the electron but it is positively charged. The mass of proton is nearly 1800 times that of the electron. The one electron revolves in the first shell. The next atom is that of the helium which contains two electrons. To maintain charge neutrality, the nucleus of helium atom was known to be nearly four times that of the proton. Therefore, there should be some other particles, which could account for the remaining

 half

of the atomic mass. As described in the following section, the discovery of the neutron solved this problem. The neutron is nearly as heavy as a proton but without any charge. We know now that the nucleus primarily consists of protons and neutrons. The nucleus of helium atom  Contains two protons and two neutrons.Table 3.1 shows distribution of electrons in various shells in 20 lightest elements of the periodic table.