We got to know about Atom when scientists try to give to the appropriate explanation of the "Law of Conservation of Mass". And the law of conservation of mass is "the matter neither be created not be destroyed". Then the English school teacher John Dalton proposed the theory about the nature of the matter.
1. If the mass was to be conserved, then all elements must be made up of extremely small particles, called atoms.
2. If the law of constant proportions is to be followed, the particles of the same substance can not be different.
Dalton's Atomic Theory
Postulates of Dalton Atomic Theory:
- Matter consists of indivisible particles called atoms.
- Atoms are neither created nor destroyed in a chemical reaction. Chemical reaction involve reorganization of atoms.
- All the atoms of a given element have identical mass and identical properties.
- Compounds are formed when atoms of different elements combine in simple whole-number ratios.
Until this theory we know that atom is a small particle that can't be divided into further parts, but when Michael Faraday raised a few questions about the indivisibility of atoms. That how could a neutral atom become electrically charged? It is a contradiction to Dalton's theory that the atom was indivisible. This led to the idea that there must be some tiny particles inside of an atom which is responsible for an atom to behave sometimes as a charged particle.
As an atom is considered as electrically neutral, then it probably has some positive constituents and some negative constituents to maintain its electrical neutrality. This gives the idea of sub-atomic particles.
Sub-Atomic Particles:
The particles smaller than the atom and are present inside of the atom are called "sub-atomic" particles. Since atoms are neutral they should have at least two types of sub-atomic particles, one is positively charged and another is negatively charged. In fact, three different sub-atomic particles are discovered.
Electrons, Protons, and Neutrons
In 1987, a British Physicist Joseph John Thomson demonstrated on the basis of his experiments that negatively charged particles are present in the atoms. Initially, Thomson thought that the negative particles would be different for each element. But after carrying out experiments with many different materials, he found that the negatively charged particles from all the materials were always identical are called "Electrons". Electrons are the first sub-atomic particles discovered and studied, and represented as "e-".
The other sub-atomic particle would have charge that balances the charge of the electrons. This sub-atomic particle was named as Proton in 1920. Its mass was approximately 1836 times greater than that of the electron. It is represented as "p+" and its charge is taken as one unit positive.
In 1932, James Chadwick discovered another sub-atomic particle which had no charge and mass nearly equal to the proton. It is eventually named as Neutron and is represented as "n0" which is neutral.
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