Question

atomic mass is not useful to construct the periodic table give reasons with an example

Answer 1

Atomic number is the number of protons, while atomic mass is the number of protons and neutrons (and presumably the nearly weightless electrons). The chemical behavior of the element is determined by the number of protons, not so much because of the protons per se, but because in a neutral atom, there are a matching number of electrons. The electrons are arranged in orbitals or shells, and those in the outer shell are responsible for chemical reactivity at earthly temperatures.

The number of neutrons in the atom has little effect on chemical behavior, because they do not influence the number of outer shell electrons. Many atoms have multiple isotopes -- for example, hydrogen has no neutrons, while its isotopes deuterium and tritium have one or two, respectively. Each of these isotopes have a different atomic mass. So when we say "the atomic mass of hydrogen is x," we need to take into account the relative natural occurrence of each isotope to produce an average value. (Or we need to talk about the atomic mass of each isotope separately, but that would be a table of the isotopes.)

So atomic mass is not a natural way to order a periodic table, since periodic tables were (and are) principally aimed at capturing the chemical relationships between elements.

Answer 2

because it is not a periodic function of its chemical and physical properties