CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 9

Atomic number is more appropriate because the chemical properties of an element depend on its electrons, and the number of electrons in a neutral atom equals its atomic number (the number of protons), not its atomic mass. Arranged by atomic number, all the isotopes of an element (same atomic number, different masses) share one place, and anomalies of the order by mass disappear: cobalt ($58.9$ u) must come before nickel ($58.7$ u), and it does, since cobalt has $Z=27$ and nickel $Z=28$. (i) From left to right in a period, metallic character decreases. The nuclear charge increases while electrons are added to the same shell, so the valence electrons are pulled in more strongly and the tendency to lose them decreases. (ii) From top to bottom in a group, metallic character increases. A new shell is added at each step, so the valence electrons are farther from the nucleus, the effective nuclear charge on them decreases, and they are lost more easily.