John Dalton’s atomic theory to the modern structure of an atom was a long and arduous one. Dalton considers an atom as the smallest building block of matter but modern atomic structure clearly indicates an atom composed of subatomic particles like electron, proton, and neutron. Today electrons hold the key to chemical reaction and structure and chemical bonding or reaction is the change of electron configuration of atom in reacting compounds. Probably the most important factor for the determination of the structure has been the study of the electromagnetic spectrum radiation emitted and absorbed by atoms. Therefore, modern structure investigated by electron configuration and the atomic spectrum is different from Dalton’s atomic theory, and the Daltonian atomic model no longer granted his position in science (chemistry and physics).
Dalton’s atomic theory or model based on the following postulates or assumption, all Matter made up of atoms are indivisible and indestructible. All the atoms of a given element are identical atomic mass and properties. Compounds are formed by the combination of two or more same or different kinds of atoms. A chemical reaction is a rearrangement of atoms in the matter or compounds.
Rutherford and Bohr’s Atomic Structure
Science in chemistry education goes step by step and every scientist depends on the work of his predecessor. Therefore, this century Rutherford alpha ray scattering experiment has given valuable information about the nucleus of an atom and extranuclear electrons. Rutherford atomic model based on the following assumption, all the positive charge and the almost entire mass of an atom concentrated in a very small central core. He named this part as a nucleus of an atom. The nucleus of an atom is surrounded by negatively charged particles carrying negligible mass, called electrons arranged in a definite structural arrangement.
Rutherford’s model does not conform to the classical model of electromagnetic radiation. When a moving charged particle will emit radiation, the particle loses kinetic energy and hits the nucleus. Therefore, Niels Bohr proposed the structure of hydrogen atom (one electronic atom) based on the following postulates.
An atom possesses several stable circular orbits in which an electron can stay. Therefore, this orbit or orbits is called energy level or hydrogen energy level where electron neighbor emits nor absorbed energy. An electron can jump from one orbit to another with higher energy on the absorption of energy. From one orbit to another lower energy orbit with the emission of energy. The angular momentum of an electron moving in an orbit is an integral multiple of h/2π.
Modern Structure of Atom
Bohr’s and Sommerfield theories could not explain the fine structure of the atoms where electrons have both particles and wave properties in the de Broglie equation. Bohr and Sommerfield’s theory indicated the principal and azimuthal quantum number of atoms. These two quantum numbers not sufficient to specify the fine structure of the atoms. Therefore, along with these two quantum numbers another two quantum numbers need to explain the fine structure of the atoms. These quantum numbers are the identification numbers for electrons.
The modern structure of atoms is based on quantum numbers of electron orbitals and electron configuration. These factors explain the chemical properties and position of the elements on the modern periodic table. Therefore, the modern atomic structure classified the periodic elements on the basis of the number of electrons in the outer quantum shell or energy levels in inorganic chemistry.