Atoms and molecules
The first idea of divisibility of matter The an Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (padarth) we shall get smaller and smaller particles. Ultimately, a time will come when we shall come across the smallest particles beyond which further division will not be possible. Democritus called these indivisible particles atoms (meaning indivisible) All this was based on philosophical considerations and not much experimental work to validate these ideas could be done till the eighteenth century.
LAW OF CONSERVATION OF MASS Is there a change in mass when a chemical change (chemical reaction) takes place? Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. EXPLANATION: The total mass of the product is same as the total mass of the reactants during a chemical reaction. For the chemical reaction: A+B C+D If x and y are masses of reactants A and B respectively and n and m are masses of products C and D respectively then according to this law total mass of the reactant- total mass of products. x+y=n+m
EXAMPLE: When carbon burns in air, carbon dioxide is formed. C+ O2 CO2 Molecular mass of C= 12gm Molecular mass of O2 32 gm Molecular mass CO2-12+ 2(16)= 44 gm Total mass of reactants 12+ 32 44 gm Total mass of products= 44 gm Total mass of reactants- total mass of products There is no loss in the mass
LAW OF CONSTANT PROPORTION This law was stated by Proust as "In a chemical substance the elements are always present in definite proportions by mass". EXPLANATION: Composition of a chemical compound is always constant irrespective of the source from which the compound is obtained. Example: H2 O is always found to contain two elements hydrogen and oxygen ratio in which hydrogen and oxygen are present is also found to be be fixed 1: 8 by mass.
DALTON'S ATOMIC THEORY British chemist John Dalton provided the basic theory about the nature of matter. Dalton picked up the idea of divisibility of matter, which was till then just a philosophy. He took the name 'atoms' as given by the Greeks and said that the smallest particles of matter are atoms. His theory was based on the laws of chemical combination. Dalton's atomic theory provided an explanation for the law of conservation of mass and the law of definite proportions.
DALTON'S ATOMIC THEORY POSTULATES The postulates of this theory may be stated as follows: (i) All matter is made of very tiny particles called atoms. (ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. (ii) Atoms of a given element are identical in mass and chemical properties. (iv) Atoms of different elements have different masses and chemical properties. (v) Atoms combine in the ratio of small whole numbers to form compounds. (vi) The relative number and kinds of atoms are constant in a given compound.
Merit's of Dalton's atomic theory: Explanation of the law of conservation of mass Dalton's atomic theory explains matter can neither be created nor be destroyed. When a chemical reaction occurs only rearrangement of atoms tales place. Since no of atoms remains the same, their mass remains unchanged too. .Explanation ofthe law of definite proportion According to Dalton's theory the no and kind of atoms in a given compound is fixed. Now since in a compound the number and kind of atoms is fixed therefore a compound will always have the same kind of atoms in same numbers.
Drawbacks of Dalton's atomic theory of matter -The indivisibility of an atom was proved wrong: an atom can be further subdivided into protons, neutrons and electrons. According to Dalton, the atoms of same element are similar in all respects. However, atoms of some elements vary in their masses and densities. These atoms of different masses are called isotopes. For example, chlorine has two isotopes with mass numbers 35 and 37. Dalton also claimed that atoms of different elements are different in all respects. This has been proven wrong in certain cases: argon and calcium atoms each have an atomic mass of 40 amu. These atoms are known as isobars.
According to Dalton, atoms of different elements combine in simple whole number ratios to form compounds. This is not observed in complex organic compounds like sugar (C12H22011). -The theory fails to explain the existence of allotropes; it does not account for differences in properties of charcoal, graphite, diamond.