### Theory:

In our lower classes, we have studied about the atom. Let us recall them.
• Atom is the smallest particle.
• Everything around us is made of atoms.
• Atoms are indivisible.
A series of experiments by scientists resulted in the discovery of an atom's structure.

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.

According to Thomson, atoms are indivisible, but they contain at least one subatomic particle, the electron.

According to E. Goldstein, gas discharge contains new radiations known as canal rays. The rays had a positive charge. Canal rays helped the discovery of another subatomic particle. The charge of this subatomic particle was the exact opposite of the charge of the electron.

What are subatomic particles?
The particles that build an atom are called subatomic particles.
During the twentieth century, discoveries were made that atoms of all elements are made up of smaller components such as electrons, protons and neutrons.

Lets us tabulate the name of the scientists who discovered the particles.

 Subatomic particles Scientists name Symbol Charge Mass (amu) Electron J. J. Thomson $$e^-$$ $$-1$$ $$0$$ Proton Eugen Goldstein $$p^+$$ $$+1$$ $$1$$ Neutron James Chadwick $$n^0$$ No charge $$1$$

Neutrons are present in all atoms except hydrogen.

The sum of the masses of protons and neutrons in the nucleus defines the mass of an atom.

In this chapter, we will discuss about the topics listed below:
• Subatomic particles inside the atom.
• Arrangement of the particles inside an atom.
• Various models.
• Atomic and mass number of the atom.
• Valency.
• Isotopes and isobars.