Introduction
The evidence of the electrical nature of matter preceeded the first theories about the structure of matter. In 1832, Michael Faraday discovered electrolysis. He gave the first important clue relating electricity to matter and about the electric nature of atoms.
Discovery of Atomic Particles
Electron - Discharge Tube experiments
These experiments were conducted in detail by J.J. Thomson and he studied the passage of electricity through gases at extremely low pressure in a cylindrical glass tube. It was discovered that at extremely low pressure, gases become conductors of electricity and emit streaks of light, which flows in the form of cathode rays.
Electromagnetic Nature of Light
In 1856, James Clark Maxwell proposed that light and other forms of radiant energy propagate themselves in the form of waves, through space. These waves have electric and magnetic fields associated with them and are called electromagnetic waves or electromagnetic radiation.
Electromagnetic Radiation and Energy(Planck's quantum theory)
In 1900, Max Planck presented the results of his famous black body radiation experiments, which showed that light has a dual character, behaving like a particle as well as a wave. He gave the Quantum Theory of Radiation explaining electromagnetic radiation and energy.
Atomic Spectra
A spectrum is an assembly of energy levels in the form of radiations emitted by an atom in its excited state. Every atom gives discontinuous line spectra. Each line in the spectra corresponds to a specific wavelength and it is unique to a given element. No two elements give same pattern of lines in their spectra.
Models of Atom - J.J Thomson's and Rutherford's Models
After the discovery of electrons and protons it was necessary to know how these particles are arranged within the atom. Various scientists tried to describe this structure and after subsequent improvements through experiments, the structure of an atom is well defined today.
Bohr's Atomic Model
In 1913, Neils Bohr proposed a model of an atom based on the Planck's quantum theory of radiation. The basic postulates of Bohr's theory are:
* An atom consists of a small, heavily positively charged nucleus around which electrons revolve in definite circular paths called orbits.
* These orbits are associated with definite energies called energy shells/energy levels. They are designated as K, L, M, N, …. etc. shells or numbered as 1, 2, 3, 4, …..etc. from the nucleus.
Quantum Mechanical Model of the Atom
Wave nature of material objects
In 1924, de Broglie's suggested that all material objects including an electron have a dual character; they behave as particles as well as waves. The wavelength associated with a particle of mass 'm', moving with velocity 'v' is given by de Broglie's relation as:
The discovery of the wave like character of the electron helped in the making of the modern electron microscope.
Atomic Orbitals
Electrons cannot exist at a particular point or in a well-defined orbit (path), according to the above new approach called wave mechanics. This led to the concept of 'most probable regions'. We talk about 'certain regions in space around the nucleus called atomic orbitals, where the probability (chances) of finding the electron is maximum (90%-95%)'.
Quantum Numbers
Orbitals of electrons in atoms differ in size shape and orientation. Definite energies and angular movements characterize atomic orbitals. The state of an electron in any atom is defined by certain permissible values of energy and angular momentum, which describe its location with respect to its nucleus and its energy level. These permissible states are called orbitals and are expressed by a set of four numbers 'n', 'l', 'm' and 's' called quantum numbers. These numbers serve as the signature of the electrons, uniquely describing its position in the atom. The 'n', 'l' and 'm' indicate the spatial distribution while 's' indicates the spin orientation of the electrons.
Shapes of Atomic Orbitals
An atomic orbital is the space around the nucleus in which the probability of finding the electron is maximum. These most probable regions can be diagrammatically represented by cloud density (dot) diagrams. The density of dots (or lack of them) in any region of the cloud diagram, indicates the degree of probability of finding the electron in that region. It is not always convenient to draw dot diagrams of orbitals, since the probability of finding an electron decreases with distance (but does not become zero), thus not giving it any definite shape. Drawing boundary surfaces, which enclose 95-99% of the probability of locating an electron, is the method generally used, to show the shape of an orbital.
Relative Energies in Atomic Orbitals
The relative energies of various orbitals of single electrons depend on the value of the principal quantum number 'n' and is independent of the value of 'l'. This can be shown by an arrangement known as energy level diagram. The diagram given below illustrates the relative energy of various energy levels for hydrogen and hydrogen like atoms.
Electronic Configuration
The distribution of electrons in different orbitals is known as its electronic configuration. This characterizes each electron in an atom. The electronic configuration is expressed by indicating the principal quantum number and its respective orbital along with the number of electrons present in it. For example the notation 3px1 indicates that in the third principal shell there is one electron in the 'px' orbital.
