Question

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

Answer 1

The properties of elements depend upon their valence electrons which in turn depends on the total number of electrons, i.e., atomic number. Therefore, atomic number is considered to be a more appropriate parameter than atomic mass for the classification of elements.

(i) On moving from left to right in a period, the atomic size decreases due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

(ii) On moving from top to bottom in a group, atomic size increases because new shells are being added as we go down the group. This increases the distance between the valence shell and the nucleus.