The atomic theory is a scientific theory put forward to shed some light on the nature of matter. Matter is defined as anything that has mass and occupies space. The atomic theory states that all matter is made up of discrete particles called atoms. When the atomic theory was first proposed, an atom was thought to be the smallest ‘indivisible’ part of matter.
The idea of substances being made up of atoms started as a philosophical concept in ancient Greece. In fact the word ‘atom’ was derived from the Greek word ‘atomos’ which means that which cannot be subdivided. For many centuries, the idea remained a mere philosophical concept confined to obscurity until the early part of the nineteenth century when chemists realized that matter did in fact behave as if it is actually composed of atoms. Consequently, the concept was assimilated into mainstream scientific thought.
The initial concept of the atom being indivisible persisted for a while being encapsulated in Dalton’s Atomic Theory. In his theory published in the eighteenth century, John Dalton, an English Chemist made several postulations. Dalton propounded that matter is composed of tiny particles called atoms and that atoms can neither be changed nor destroyed. He also said that elements can be definitely identified by their atomic mass. Dalton proposed that reacting elements combine in simple discrete ratios which sometimes may be more than one ratio. Most of Dalton’s postulates have remained true though with modifications when later discoveries in the fields of Chemistry and Atomic Physics were made. For instance, it is now known that atoms are composed of sub elementary particles, protons, electrons and neutrons. Also, an atom may be destroyed through nuclear reactions but it remains unaffected by ordinary chemical reactions.
Electrons were discovered by J.J. Thompson through his experiments on cathode rays. The atomic nucleus was discovered by Rutherford and his colleagues by taking note of the heavy deflection of hitherto unknown protons using fluorescent screens. The discovery of isotopes in 1913 by Fredrick Soddy, a radio chemist, led to the modification of Dalton’s postulation that elements could be uniquely identified by the mass of their atoms. As of today, the generally accepted model is the Quantum Physical models of the atom.
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