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16 chemistry Fig. 1.9 Two volumes of hydrogen react with one volume of oxygen to give two volumes of water vapour cannot combine and molecules of oxygen or Dalton’s theory could explain the laws hydrogen containing two atoms did not exist. of chemical combination. However, it could Avogadro’s proposal was published in the not explain the laws of gaseous volumes. It French Journal de Physique. In spite of being could not provide the reason for combining correct, it did not gain much support. of atoms, which was answered later by other After about 50 years, in 1860, the first scientists. international conference on chemistry was 1.7 A tomic and Molecular Mass es held in Karlsruhe, Germany, to resolve After having some idea about the terms various ideas. At the meeting, Stanislao Cannizaro presented a sketch of a course of atoms and molecules, it is appropriate here chemical philosophy, which emphasised on to understand what do we mean by atomic the importance of Avogadro’s work. and molecular masses. 1.6 Dalton’s Atomic Theory 1.7.1 A tomic Mass Although the origin of the idea that matter is The atomic mass or the mass of an atom is composed of small indivisible particles called actually very-very small because atoms are ‘a-tomio’ (meaning, indivisible), dates back extremely small. Today, we have sophisticated to the time of Democritus, techniques e.g., mass spectrometry for a Greek Philosopher (460– determining the atomic masses fairly 370 BC), it again started accurately. But in the nineteenth century, emergingasaresultofseveral scientists could determine the mass of one experimental studies which atom relative to another by experimental led to the laws mentioned means, as has been mentioned earlier. above. Hydrogen, being the lightest atom was In1808,Daltonpublished John Dalton arbitrarily assigned a mass of 1 (without (1776–1884) any units) and other elements were assigned ‘A New System of Chemical masses relative to it. However, the present Philosophy’, in which he proposed the following : system of atomic masses is based on 1. Matter consists of indivisible atoms. carbon-12 as the standard and has been agreed upon in 1961. Here, Carbon-12 is 2. All atoms of a given element have identical one of the isotopes of carbon and can be properties, including identical mass. Atoms 12 12 of different elements differ in mass. represented as C. In this system, C is assigned a mass of exactly 12 atomic mass 3. Compounds are formed when atoms of unit (amu) and masses of all other atoms are different elements combinein a fixed ratio. given relative to this standard. One atomic 4. Chemical reactions involve reorganisation mass unit is defined as a mass exactly equal of atoms. These are neither created nor to one-twelfth of the mass of one carbon – 12 destroyed in a chemical reaction. atom. Reprint 2026-27