CBSE Class 10 Science 2019 · Paper 31/3/1 · Question 8

The chemical properties of an element depend on the number of electrons in its outermost shell (its electronic configuration). This is fixed by the atomic number, which equals the number of electrons in a neutral atom, and not by the atomic mass. For X (atomic number $13$): (i) Electronic configuration: $2,8,3$. (ii) It has $3$ shells, so it is in period $3$, and $3$ valence electrons, so it belongs to group $13$. (iii) It loses its $3$ valence electrons easily to form ${\text{X}}^{3+}$, so its valency is $3$ and it is a metal with electropositive character (X is aluminium; its oxide is ${\text{X}}_2{\text{O}}_3$). All of this follows from the atomic number alone; the atomic mass ($27$ u) predicts none of it. Hence the atomic number is the more appropriate parameter, as the Modern Periodic Law states.