Why do elements with similar properties occur in the same group?

Evaluate the Mendeleev's periodic table and find the following. Are the same elements showing similar properties arranged in the same group or same period?

Mendeleev's arranged the then known elements in horizontal rows and vertical columns of a table in the order of their increasong_________in such a way that the elements with similar properties occupied the same vertical column or group.

Why do elements in the sae group show similar properties but the elements in different groups show different properties?

Why do Zr and Hf exhibit similar properties ?