John Dalton in 1808 put forward theory known as Dalton’s atomic theory. The main points of this theory are:
1)Matter is made up of extremely small indivisible particles called atoms.
2)Atoms of the same element are identical in all respect i.e. shape ,size and mass.
3)Atoms of different elements have different masses, sizes and also possess different chemical properties.
4)Atoms of the same or different elements combine together to form molecules.
5)When atoms combine with one another to form molecules ,they do so in simple whole number ratios.
6)Atoms of two elements may combine in different ratios to form more than one compound.
7)An atom is the smallest particle that take parts in a chemical reaction.
8)An atom can neither be created nor destroyed.
1)It could explain the law of chemical combination by Mass but failed to explain the law of gaseous volume.
2)It could not explain why atoms of different elements have different masses ,size ,valencies.
3)It could not explain the nature of binding forces between atoms and molecules which accounts for the existence of three States of matter.
Modern Atomic Theory
1)Atom is no longer considered to be indivisible: It is made up of electrons, protons and neutrons.
2)Atoms of the same element may have different atomic masses: Atoms of the same element which possess different atomic masses are called isotopes.
1)Isotopes of Hydrogen
1H1 1H2 1H3
2)Isotopes of Carbon
14C 12C 13C
3)Atoms of different elements may have same atomic masses: Atoms of the different elements which have the same atomic masses are called isobars.
For Ex: Calcium and Argon have atomic masses of 40 with atomic number as 20 and 18.
4)The ratio in which the different atoms combine with one another may be fixed and integral but may not be simple :
For example In sugarcane the ratio in which the elements carbon ,hydrogen and oxygen combine together 12 : 22 : 11 which is not simple.
5)Atom is the smallest particle that take part in a chemical reaction.
6)Atom is no longer Indestructible: By carrying out nuclear reactions atom of an element may be changed into another.