According to the Dalton's Atomic Theory, atoms of the same element are identical in all respects. But according to the Modern Atomic Theory, this postulate is proved wrong. Explain.

Write down :

(i) the mass number of the atom having 20 neutrons and 15 protons.

(ii) the number of neutrons in the nucleus of an atom having atomic number 17 and mass number 37.

What is a proton ? What is the significance of the number of protons found in the atoms of each of the different elements.