JEE Main · 2013 · 07 Apr 2013 · Offline · Classification of Elements and Periodicity

First ionisation enthalpy grows across a period and shrinks down a group. Barium sits below calcium in group 2, so $\mathrm{Ba}<\mathrm{Ca}$; selenium sits below sulphur in group 16, so $\mathrm{Se}<\mathrm{S}$; the group-16 non-metals lie above the group-2 metals, and argon, a noble gas, tops them all. Putting these together, the increasing order is $\mathrm{Ba}<\mathrm{Ca}<\mathrm{Se}<\mathrm{S}<\mathrm{Ar}$.