What is Dalton atomic theory?

Dalton’s atomic theory proposed that an atom was the smallest indivisible and indestructible building block of matter. Dalton’s atomic theory was given by English physicist and chemist John Dalton in 1808. At the turn of this century, many valuable information about atoms of the periodic table elements was given. It clearly indicated that Dalton’s atomic theory no longer granted its position.

The modern atomic structure clearly indicates an atom composed of subatomic elementary particles such as electron, proton, and neutron.

Today electrons hold the key to structure and chemical bonding. The most important factor in physics and chemistry for the determination of the structure has been given by the study of the electromagnetic spectrum emitted or absorbed by atoms. Therefore, the modern structure investigated by electronic configuration and atomic spectrum is different from Dalton’s atomic model.

Postulates of Dalton’s atomic theory

For learning chemistry or physics, 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.

Limitations of Dalton’s Atomic Theory

• Dalton’s atomic theory does not account for subatomic particles such as electrons, protons, and neutrons. It states that atoms are indivisible and indestructible parts of matter.
• It does not explain the isotopes of elements because Dalton’s postulates state that all atoms of an element have identical masses.

Rutherford atomic model

Science 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.

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.

Niels Bohr proposed the structure of the 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.

Structure of atom in chemistry

Bohr’s and Sommerfield’s theories could not explain the fine structure and wave properties of atoms. The particles and wave properties of electrons can be explained by 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.