The word atom is originated from the Latin word atomos which means that something which cannot be divided. But certain accidental discoveries in the world of science changed Dalton's concept.
John Dalton's Model of Atom
The propound of atomic theory in modern period is John Dalton(1807).
Atoms according to John Dalton
* All matter is made up of small particles called atoms
* Atom is the smallest particle which can engage in a chemical reaction
* All the atoms of an element have the same properties ie they have the same mass, size and character.
* Atoms of different elements have different characteristics
* Atom can neither be created nor be destroyed
* Atoms are indivisible
Rutherford's Model of Atom
Rutherford proposed planetary model of atom
Planetary Model of Atom
Atoms according to Rutherford
* Atoms has a central part he called it nucleus
* Nucleus has positive charge
* Compared with total volume of an atom the volume of the nucleus is very meager
* Mass of an atom is concentrated in the nucleus
* Electrons are distributed in the space around the nucleus
* Almost whole of the mass of an atom is concentrated in the nucleus
Bohr's Model of an Atom
* An atom consists of a positively charged nucleus responsible for almost entire mass of the atom
* The electrons revolve round the nucleus in certain permitted circular path called orbits or stationery states
* The permitted orbits are those for which the angular momentum of an electron is an integral multiple of h/2∏
* When electrons move in permitted discrete orbit they do not radiate energy such orbits are called stationery orbits. In this way Bohr overcome Rutherford's difficulty to account for the stability of an atom.