Atomic number is considered to be a more appropriate parameter than atomic mass for classification of elements in a periodic table. Why ? How does atomic size of elements vary on moving from:
Give reasons for your answers.
from top to bottom in a group.

Explanation

Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number.
Atomic size of elements increases on moving from top to bottom in a group.
Reason: In a group, the new shells are being added when going down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases inspite of the increases in nuclear charge.