9th Class Science Atoms and Molecules Daltons Atomic Theory

        Dalton's Atomic Theory

John Dalton provided the basic theory about the nature of matter. He used the idea of divisibility of matter and called the smallest particle of matter as atoms. It was based on the theory of chemical combination. He also provided the explanation on laws of conservation of mass and laws of constant proportion. According to Daltons atomic theory all the matters is composed of smallest particles called atom.

The postulates of this theory are:

• All the matters are made up of smallest particles called atoms.
• Atom are indivisible and can neither be created, nor be destroyed.
• Atoms of same elements are identical in shape, size and mass.
• Atoms of different elements have different mass and chemical properties.
• Atoms always combine together in fixed proportion by mass to form the compound.
• In a given compound the relative number and kinds of atoms are constant.

           Merits of Dalton Atomic Theory

• It can easily explain the reason for the validity of laws of conservation of masses.
• It also explain the laws of constant proportion.
• It can also explain the difference between the atoms of elements and that of compound.

            Demerits of Dalton Atomic Theory

• According to Daltons atomic theory atoms are indivisible, but according to modern atomic theory, atom consists of sub atomic particles called electron, proton, and neutron.
• It says that atoms of same elements are similar in shape and properties. But, it is not so, as the occurrence of isotopes proves that it is not to be so.
• It says that atoms of different elements have different mass, but occurrence of isobars proves that it is not as such.
• It also fails to explain why atoms of different elements shows different properties.
• It also fail to explain why the atoms are together in a compound and which binding force is responsible for this
• It also fails to explain Gay Lussac's law.

          To form water. Hydrogen and oxygen combine in the ratio 1:8. Find the mass of oxygen required to react completely with 3 g of hydrogen.

(a) 4 g

(b) 8 g

(c) 16 g

(d) 24 g

(e) None of these

Answer: (D)

          Certain mass of carbon burns with a given mass of oxygen to form certain mass of carbon dioxide. Which law of chemical combination is used in this process of formation of compound.

(a) Laws of constant proportion

(b) Laws of multiple proportion

(c) Laws of conservation of mass

(d) Gay Luassc's law

(e) None of these

Answer (A)

       Which postulates of Dalton Atomic Theory gives laws of conservation of mass?

(a) Atom of same elements are similar

(b) Atoms of different elements are different

(c) Atom can neither be created nor be destroyed

(d) Atom combine in fixed ratio to form compound

(e) None of these

Answer: (C))