Isotopes do not have a variable number of electrons. They have different number of neutrons in the nucleus of the atom. The extra neutrons in the nucleus make the atoms heavy, unstable and radioactive.Continue Reading
The extra neutrons in the nucleus increase the mass of the atom. Thus, isotopes are atoms of the same element having the same atomic number but slightly different atomic masses.
Not all elements have isotopes. Some elements have more than one isotope. A normal atom of carbon has six neutrons in the nucleus. The C-14 isotope of carbon has eight neutrons in the nucleus.
Uranium has two isotopes: U-235 and U-238. Normal hydrogen has two isotopes, known as deuterium and tritium.Learn more about Atoms & Molecules