Atoms of all elements are composed of three fundamental particles protons, neutrons and electrons. The mass of the electron is 9.10939×10−31kg. All isotopes have same atomic number, which means they have the same number of electrons. So, they bear similar chemical properties. Protons and neutrons are collectively known as nucleons. According to Dalton's atomic theory, all matter is made of atoms, which are indivisible. All atoms of a given element are identical in mass and properties. Compounds are combinations of two or more different types of atoms.