Atomic Stucture

An atom consists of extremely small and dense nucleus and an extra nuclear space. The nucleus contains positively charged protons and neutrons, and these particles are collectively called nucleons. In the extra nuclear space, negatively charged electrons revolve around the nucleus. As the magnitude of the charge of an electron is the same as that of a proton, the number of electrons is equal to that of protons in an atom, the atom being neutral.The number of protons present in the nucleus of an atom is termed as the atomic number of the element (Z). The sum of the number of protons and neutrons is called the mass number (A). The term 'nuclide' refers to a nucleus having a specific atomic number and specific mass number.To calculate the radius (r) and energy (E) of a permissible orbit for one electron species Bohr derived equations based on the following postulates.
Bohr's Postulates
1. The electrons revolve around the nucleus in certain orbits without losing energy because the energy in a fraction of a quantum can neither be lost nor gained.
2 Energy is absorbed or emitted only when an electron in an atom jumps from one orbit to another.
3 The electron is restricted to those orbits in which its angular momentum is an integral multiple of h/2pie Angular momentum = mvr= n x h/2pie
Failure of Bohr Theory
1 It does not explain the spectra of species having more than one electron.
2 It does not explain the fine spectral lines obtained under a spectroscope of strong resolution. However, it can be explained by Bohr-Sommerfeld theory of elliptical orbits.
3 It does not explain Zeeman effect, that is, splitting of spectral lines under magnetic field, and Stark effect, that is, splitting of spectral lines under electric field.