Atoms are the building blocks of elements. They are the smallest parts of an element that chemically react. The first atomic theory, proposed by John Dalton in 1808, regared atom as the ultimate indivisible particle of matter. Towards the end of the nineteenth century, it was proved experimentally that atoms are divisible and consist of three fundamental particles: electrons, protons and neutrons. The discovery of sub- atomic particles led the proposal of various atomic models of explain the structure of atom.
Thomson in 1898 proposed that an atom consists of uniform sphere of positive electricity 🔌 evenly spread over the atom ⚛️ was proved wrong by Rutherford's famous alpha- particle scattering experiment in 1909. Rutherford concluded that atoms is made of a tiny positively charged nucleus, at its centre with electrons revolving around it in circular orbits.
Rutherford model, which resembles the solar system, was no doubt an improvement over does not fall into the nucleus. Further, it was also silent about the electronic structure of atoms i.e., about the distribution and relatives energies of electrons around the nucleus.
The difficulties of the Rutherford model were overcome by Niels Bohr in 1913 in his model of the hydrogen atom. Bohr postulated that electron moves around the nucleus in circular calculated the energy of electron in various orbits and for each orbit predicted the distance explaining the spectra of the hydrogen atom, could not explain the spectra of multi- electron atoms.
The reason for this was soon discovered. In Bohr model, an electron is regarded as a character of the electron is ignored in Bohr's theory. An orbit is a clearly defined path and the electron at the same time are known. This is not possible according to the Heisenberg duel behaviour of electron but also contradicts Heisenberg uncertainty principle.
Erwin Schrodinger, in 1926, proposed an equation called Schrodinger equation to describe the electron distribution in space and the allowed energy levels in atoms. This equation uncertainty principle.
When Schrodinger equation is solved for the electron can occupy (and the electron associated with function (s) (◻) ( which in fact are the mathematical functions) of the electron associated with each energy state). These quantized energy states and corresponding wave functions which are charterized by a set of three quantum number (principal quantum number n, azimuthal quantum number l and magnetic quantum number m1) arise as a natural consequence in the solution of the Schrodinger equation.
The restrictions on the values of these three quantum number also come by naturally from this solution. The quantum mechanical model of the hydrogen atom successfully predicts all aspects of the hydrogen atom spectrum including some phenomenon that could not be explained by the Bohr model.
According to the quantum mechanical model of the atom, the electron distribution of an atom containing a number of electrons is divided into shells . The shell, in turn, are through more orbitals which the electrons occupy. While for hydrogen like systems (such as He+, Li+, etc) all the orbitals within a given shell have same energy, the energy of the orbitals in a multi-electron atom depends upon the values of n and l: The lower the value of (n+1) for an orbital, the lower is its energy.
