Question

How did the anomaly of putting isotopes together has been resolved in the modern periodic table?

Answer 1

All isotopes of an element have the same number of protons and hence the same atomic number, though their mass numbers are different. Since the modern periodic table is arranged according to increasing atomic number, all isotopes are placed at one position in the same group.

Example: Chlorine-35 and Chlorine-37 both have atomic number 17, so they occupy the same position in the periodic table.