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Atomic Models Dr.P.Suriakala Assistant Professor Department of Physics. History of an Atom. The word ‘Atom’ means uncuttable was coined by Greek philosophers Leucippus and his pupil Democritus.
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Atomic ModelsDr.P.SuriakalaAssistant ProfessorDepartment of Physics
History of an Atom The word ‘Atom’ means uncuttable was coined by Greek philosophers Leucippus and his pupil Democritus During 442 B.C. , Democritus proposed that all the particles in the world were made up of a tiny particles surrounded by an empty space, these tiny particles vary in size and shape Depending on the substance they composed. His ideas were opposed by many great philosophers by stating that the atom was made up of air, water, fire.
Dalton’sAtomModel In 1800’s , the chemist John Dalton formulated the first modern description of an Atom Dalton discovered the law of multiple proportion to explain this model According to this theory, 1) All matter is made up of atoms ,atom is indivisible and indestructible 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by the combination of two or more different kinds of atoms 4) A chemical reaction is a rearrangement of atoms
J J Thomson’s AtomModel After the discovery of subatomic particles, J J Thomson discovered the existance of negative charge particles by cathode ray experiment in 1897.He called this particles as ‘Electron’. After that he assumed that an atom is composed of a cloud of negative charge in a sphere of positive charges According to this model, 1) Atoms are neutrally charged 2) Negatively charged particles called electrons are present in an atom Limitations 1) It fails to explain how positive charged holds an electron inside an atom 2) It does not explain the scattering experiment of Rutherford
Rutherford Atom Model Since above model fails to explain certain experimental results related to the atomic structure of elements. Ernest Rutherford one of the student of Thomson, conducted an experiment where he bombarded α-particle in a thin sheet of gold, he studied the deflected ray using zinc sulfide around the gold, it was seen that large number of α-particles were not deflected by the gold sheet. From this he said that the positive charge of an atom is concentrated in a very small volume at the centre of an atom and the negative charged electrons were revolving around the cloud of central positive charge. He named this central positive cloud as ‘Nucleus’. Limitations 1) This model was unable to explain the stability of nucleus 2) It does not mention the arrangement of electron
Bohr’s Atom Model In 1913, a Danish Physicist named Neils Bohr modified the problems and limitations associated with Rutherford Model of atom. Postulates of Bohr Model: 1) Electron revolve around the nucleus in a fixed circular path called “Orbits” or “Shells” or “Energy level”, the electrons will not radiate energy as long as they continue to revolve around the nucleus. The different energy levels or orbits are represented by integers n=1,2,3….. Where n is called quantum number 2) The change in energy occurs when the electrons jump from one energy level to other. In an atom, the electrons move from lower to higher energy level by acquiring the required energy. However, when electron loss energy it moves from higher to lower energy level
Bohr calculated the formula for radius and energy of the nth orbit, Radius of an electron in nth orbit,rn= n2h2ԑ0/(ᴨ e2m) Energy of an electron in nthorbit, En=-me4Z2/(8ԑ02 n2h2 ) Introduction of Bohr’s Atom Model let to the explanation of Excitation and Ionisation of an atom and hydrogen spectral lines
Sommerfeld Atom Model According to Bohr, the lines in the hydrogen spectrum should each have a well-defined wavelength. Spectrograph of high resolving power showed that Hα , Hβand Hγin the hydrogen spectrum are not single, Michelson found that Hαline can be resolved into two close components with wavelength separation of 1.3 nm. To explain the observed fine structure of spectral lines sommerfeld modified bohr’s theory 1) According to sommerfeld, the path of an electron around the nucleus in general is an ellipse with the nucleus at one of the foci. The circular orbit of Bohr are also a special case of this 2) the velocity of the electron moving in an elliptical orbit varies considerably at different parts of the orbit. This causes relativistic variation of mass of the moving electron Therefore he took into account the relativistic variation of the mass of the electron with velocity. Hence , this is known as the relativistic atom-model.
In this Model, electrons are considered to be moving in an elliptical orbits thus there are two coordinates one is angle φ and the other is radius r, sommerfeld calculated the relation , b/a=nφ/ n ,by using the bohr’s quantum conditions = nφ h ; dr = nr h . From this relation , sommerfeld calculated the shape of the electron orbits for a given value of n.
Fine structure of the Hα line Hα line is due to the transition from n=3 to n=2 state of the hydrogen atom For n=3, there are three possible energy levels corresponding to the three values of nφ ,1,2,3. Similarly ,there are two possible energy levels for n=2. Theoretically, six transitions are possible : 33 - 22 ;33 – 21; 32 – 22 ; 32 – 21 ; 31 - 22 ; 31 - 21
Limitations of sommerfeld Atom Model 1) It could not explain the fine structure of spectral lines. Moreover, it gave no information about the relative intensities of the lines. 2) It could not explain the complex spectra of alkali metals like sodium 3) It could not explain the distribution and arrangement of electrons in an atom 4) It could not explain the zeeman effect and stark effect
Vector Atom Model To overcome the drawbacks in the above models , Vector Atom Model introduce two concepts 1) Spatial Quantization 2) Spinning of electron 1) Spatial quantization According to Bohr’s theory, the orbitals are quantized as regard to their magnitude (i.e. size and form) only, But according to quantum theory, the direction or orientation of the orbits in space also should be quantized , To specify the orientation of the electron, we need a fixed reference axis .the direction of the external magnetic applied to an atom is taken as reference axis. The different orientation of an electron orbits are determined the fact that the projection of the quantized orbits on the field direction must themselves be quantized. The introduction of such a spatial quantization does not change shape or size of the Bohr-sommerfeld orbits.
2) Spinning of electron The electron spins about an axis of its own, While it also move around the nucleus of an atom in its orbit. In other words , the electron is endowed with a spin motion and an orbital motion. According to quantum theory, the spin of the electron also should be quantized. Hence, a new quantum called the spin quantum number (s) is introduced. The spin endows the electron with spin angular momentum and a spin magnetic moment. N Hence, the total angular momentum of an atom should be vector sum of the orbital angular momentum and spin angular momentum . Similarly, the total magnetic moment of an atom should be the vector sum of the orbital magnetic moment and spin magnetic moment.
