John Dalton's Atomic Theory

Though the earliest atomic theories were proposed by Democritus and Aristotle, the first accepted theory was proposed by John Dalton. John Dalton (1766-1844), an English Chemist, Physicist and meteorologist, is honored mainly because of his contribution to modern atomic theory and color blindness. It was Dalton's research studies on the properties of atmosphere and gases in 1803, that made him realize about the particles (later called atoms) and their weight. The same research paper was published in 1805.

John Dalton published the first table on relative atomic weights that included six elements viz. hydrogen, oxygen, carbon, nitrogen, phosphorous and sulfur. He stated the atomic weight of hydrogen to be 1. It was not confirmed as to how he derived the atomic weights. However, his laboratory notebook (dated September 1803) indicated that he found out the relative weights from the studies of the compounds - water, carbon dioxide and ammonia. In his notebook, the elements were represented by symbols. He was the first person to propose the idea of identifying an element by its symbol. Later on, elements are represented by their abbreviations, for example, 'H' is the abbreviation of the element hydrogen.

As per the lecture given by John Dalton in 1803, in the Royal Institution (London), atoms of one element differed from those of other elements by their relative weights. Following are some of the important points, proposed by John Dalton in his atomic theory:

• All elements are made up of tiny indivisible particles, known as atoms
• Atoms of the same element are identical with respect to their weights
• Atoms of different elements are different from each other and can be identified by their relative weights
• Atoms can neither be divided into smaller particles nor destroyed
• Chemical reactions occur due to the rearrangement of the atoms
• Atoms combine in the ratio of whole numbers such as 1:1, 1:2, 2:3 etc.
• Atoms of two or more different elements combine together to form chemical compounds

In 1808, John Dalton published a list of elements along with their atomic weights in his 'New System of Chemical Philosophy', for which he received the Royal Medal in 1826. He referred the masses of elements as weights. Since then, chemists used either 'atomic weight' or 'atomic mass'. In spite of the fact that there are some flaws in his work (for example, the atomic weight of oxygen that he had assigned was 7) John Dalton was the pioneer who had given a pathbreaking view to future experimentations. In order to honor his work, scientists referred one atomic mass unit as 'Dalton' (abbreviated as Da) for many years.

In 1886, Eugen Goldstein, a German Physicist, discovered positive charged particles - the protons in atoms. J. J. Thomson, a British Physicist, discovered the electron by using a cathode ray tube in 1897. In contrary to John Dalton's assumptions that atoms are indivisible, he suggested that atoms are made up of smaller particles.

In 1911, Rutherford, a student of Thomson, put forth the view that an atom consisted of a dense positive core at the center or nucleus, surrounded by electrons. He named the positive particles in the nucleus as protons and stated that protons were larger than the electrons and carried an equal and opposite charge to electrons. Later in 1932, James Chadwick discovered the electrically neutral particle in the atom and named it as the neutron. Thus, modern atomic theory was developed.