What is Atomic theory?

Hint:Atomic theory was used to define the types of particles present inside the atoms, their size, shape, masses, etc. The ability of atoms to form compounds, their ratios, chemical nature, etc. Dalton was the first to give the atomic theory which was modified later.

Complete step-by-step answer:Atomic theory was used to define the types of particles present inside the atoms, their size, shape, masses, etc. The ability of atoms to form compounds, their ratios, chemical nature, etc.
There were many people who gave atomic theory in which Dalton was the first one, in which he said that matter consists of atoms which is the smallest particle and building block; size, shape, and mass of the atoms of the same element are the same; chemical properties of the atoms depend on the size, shape, and mass that means if the size is different then, the chemical properties are also different; compounds are formed from atoms so, the compounds are formed in simple ratio, etc.
But this theory has many limitations, so this theory was modified to form Modern atomic theory which says that atom contains electrons, protons, and neutrons; the concept of isotopes; concept of isobars; the ratio of formation of compounds will be integral but not always simple; in a chemical reaction, the smallest thing that participates is an atom.
After this many scientists modified the theory according to their discoveries like Rutherford's hypothesis, Bohr's theory, Quark theory, etc.

Note:The concept of isotopes means the same element has different atoms having different masses and the concept of isobars means different elements have atoms having the same mass.