John Dalton Atomic theory and His Contributions

JOHN DALTON: (1766 – 1844)                                       

John Dalton Atomic theory

John Dalton (1766 - 1844)

An English chemist, meteorologist and physicist. The first modern atomic theory was proposed by John Dalton in the year 1808. He published his theory in a paper titled “A New System of Chemical Philosophy”. His theory based on the Law of conservation of mass and the Law of constant proportions.

Dalton’s theory included several ideas from Democritus, such as atoms are indivisible, Matter is made of tiny particles called atoms. 

Dalton’s additions to the theory:

• All matter is made of atoms, which are indivisible.
• The atoms of same element are similar in mass, and their physical or chemical properties.
• The atoms of different elements show different properties, and they have different masses and chemical properties.
• The atoms cannot be created or destroyed but can be transformed one form to another. (Antoine Lavoisier previously stated this in terms of the Law of conversation of mass, in1789)
• The atoms of different elements combined in small whole number ratios, to form stable compounds. (Law of multiple proportions, this law was developed by Dalton, by studied and expanded upon this previous work of Antoine Lavoisier and Joseph Louis Proust)
• The relative number and the kinds of atoms in a given compound are always in a fixed ratio. (Joseph Louis Proust previously stated this in terms of the Law of constant proportions. In 1799)

Dalton Interested in meteorology – extensive studies regarding nature of atmospheric gases. Daily weather records interest in the gases and ultimate components in the air.

Dalton studied colour blindness in 1794.

Dalton estimated the atomic weights according to the mass ratios in which they combined, with the hydrogen atom taken as unity. 

Drawbacks of Dalton’s atomic theory: (Failure of Dalton’s Atomic theory)

• The indivisibility of an atom, later this concept changed when discovery of electrons, protons and neutrons, these are the sub atomic particles.
• The atoms of same element are similar in mass, later this concept changed when discovery of isotopes, which are atoms of the same element but of different masses. For example, Chlorine (Cl) has 2 isotopes with the mass numbers of 35 and 37. 
• The atoms of different elements show different masses, later this concept changed when discovery of isobars, which are atoms of different elements but of the same mass. For example, Argon (Ar) and Calcium (Ca) atoms, each have an atomic mass of 40 amu.
• The atoms of different elements combining in simple whole number ratios. But this is not true in case of complex organic compounds.
• Nothing about the structure of atom was appropriately is explained.

 

Dalton's Model of an Atom :

Dalton represented as a tiny indivisible uniformly dense, solid sphere. 

Dalton's model of an Atom

Dalton gave a specific symbol to each atom and weight (see below). He became one of the first scientists to assign such symbols.

Dalton's Symbols of Different Atoms

Dalton's Symbols of Different Atoms

Dalton’s atomic weights, 1808

Dalton’s atomic weights, 1808