### Theory:

Matter is around us everywhere. The matter is made of atoms.
Matter is made up of atoms

Curiously, Greek philosophers developed the idea of the atom in the fifth century BC (BCE). On the other hand, their theory was more philosophical than scientific. John Dalton proposed the first scientific theory of the atom.
According to Dalton, atoms are indivisible. Later on, one of the first indications is that atoms are not indivisible since there are smaller particles inside an atom.
Later while researching, few of Dalton's atomic postulates were proved to be wrong by J. J. Thomson, Rutherford, Neils Bohr, and Schrodinger. The majority of the limitations of Dalton's theory were removed as a result of the research, and a new theory known as the modern atomic theory was proposed.

Postulates of modern atomic theory:
• After the discovery of the electron, proton, and neutron, an atom is no longer indivisible.
• Atomic masses of the same element can differ (the isotopes $$_{17}\textrm{Cl}^{35}$$ and $$_{17}\textrm{Cl}^{37}$$ were discovered).
Isotopes are chemical elements that have the same atomic numbers, but different mass numbers.
• The discovery of isobars $$_{18}\textrm{Ar}^{40}$$ and $$_{20}\textrm{Ca}^{40}$$ revealed that atoms of different elements can have the same atomic mass.
Isobars are chemical elements that have different atomic numbers but have the same mass number.
• It is possible to convert atoms of one element into atoms of other elements. In other words, the atom is no longer indestructible (discovery of artificial transmutation).
• Atoms do not always mix in a simple whole-number ratio.
Example:

 Glucose ($$C_6H_{12}O_6$$) $$C:H:O$$ $$=$$ $$6:12:6$$ or $$1:2:1$$ Sucrose ($$C_{12}H_{22}O_{11}$$) $$C:H:O$$ $$=$$ $$12:22:11$$
• An atom is the smallest particle involved in a chemical reaction.
$$E=mc^2$$ is the formula for converting an atom's mass into energy.
The modern atomic theory is the basis for all the studies of chemical and physical processes that involve atoms. In your lower classes, you have learnt the most fundamental concepts about atoms. In this session, we will look into some more concepts about an atom.