Why do elements in the same group have similar physical and chemical properties?

Why do elements in the same group have similar properties ?

Although Zirconium belongs to 4d transition series and Hafnium to 5d transition series even then they show similar physical and chemical properties because...

Compounds havign same molecular formula but different physical and chemical properties are called…………………

Periodic table in a chart in which elements having similar chemical and physical properties are grouped together. Out of Fe^(2+) and Fe^(3+) ions which is smaller in size? Explain.

Periodic table in a chart in which elements having similar chemical and physical properties are grouped together. Group the following elements in pairs that you would expect to show similar chemical properties K, F, O, Na, Cl, S.