An atom is the smallest indivisible particle of an element which can take part in a chemical change. Matter is discrete (discontinuous) and made up of very small particles called atoms.Thus, Dalton’s Atomic Theory can be stated as follows: If the same numbers and kinds of atoms are present in both reactants and products, then the masses of reactants and products must also be the same. For the law of conservation of mass to be valid, atoms must be chemically indestructible. Chemical reactions only rearrange the way that atoms are combined the atoms themselves are unchanged.Fractional parts of atoms cannot be involved in chemical reactions. Different samples of a pure compound always contain the same proportion of elements by mass (the law of definite proportions) only if whole numbers of atoms combine. Chemical combination of elements to make different substances occurs when atoms join together in small whole-number ratios.As the law of definite proportions had shown that elements always combine in specific mass ratios, the distinguishing feature between atoms of different elements must be mass. Atoms of the same element have the same mass, but atoms of different elements have different masses. An element is characterized by the mass of its atom.The concept of atoms explained why there were so many different elements. Elements are made of tiny particles called atoms.The explanation for the law of conservation of mass and the law of definite proportions was provided by the English schoolteacher John Dalton in 1808. Dalton’s Atomic Theory of Matter Run up to the theory In both cases, the mercury to oxygen ratio is the same, thus conforming to the law of definite proportions. ∴ Mass of oxygen in the compound = 17.28 − 16.00 = 1.28 g ∴ Mass of mercury in the compound = 2.16 − 0.16 = 2.00 g Show that these data conform to the law of definite proportions. In another experiment 16 g of mercury was obtained by the decomposition of 17.28 g of mercuric oxide. – and in each case, the ratio copper :oxygen by mass is always constant.Ģ.16 g of mercuric oxide gave on decomposition 0.16 g of oxygen. Dissolving copper in nitric acid, precipitating cupric hydroxide, and strongly heating the cupric hydroxide.Dissolving copper in nitric acid and igniting the cupric nitrate formed.The compound cupric oxide may be prepared by any one of the following methods – What this law means is that when elements react chemically, they combine in specific proportions, not in random proportions.Ī sample of pure water, whatever the source, always contains 88.9% by mass of oxygen and 11.1% by mass of hydrogen. In a given compound, the constituent elements are always combined in the same proportions by mass, regardless of the origin or mode of preparation of the compound. Law of Definite Proportions (Law of Constant Composition)
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