What is Dalton’s atomic theory?

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 elementary particles like electron, proton, and neutron. Today electrons hold the key to chemical reaction and structure and chemical bonding. A chemical reaction is the change of electron configuration of atom of the periodic table elements. 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 electronic 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).

Postulates of Dalton’s atomic theory

Dalton’s atomic theory or model is based on the following postulates or assumptions,

  • All Matter made up of atoms is 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 matter or compounds.

Rutherford atomic model

Science in chemistry learning goes step by step and every scientist depends on the work of his predecessor. This century Rutherford model or alpha ray scattering experiment has given valuable information about the nucleus of an atom and extranuclear electrons. Rutherford atomic model is based on the following assumption, all the positive charges and the almost entire mass of an atom are 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. Dalton atomic theory to modern structure model of atom in matter

Bohr’s model of atom

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) is called Bohr model. It is 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.

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Bohr’s and Sommerfield’s theories could not explain the fine structure of the atoms where electrons have both particles and wave properties in the de Broglie relation. Bohr and Sommerfield’s theory indicated the principal and azimuthal quantum number of atoms. These two quantum numbers are 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 configurations. These factors explain the chemical properties and position of the elements on the modern periodic table. The modern atomic structure is based on s-, p-, d-, f-block elements classified on the basis of the number of electrons in the outer quantum shell or energy levels in inorganic chemistry.

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