Entropy is the measure of randomness. As the randomness (disorder) increases, entropy increases,
The order of entropy is
$\underset{\underset{\text{orderlyarranged})}{(\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{s}}}{\mathrm{S}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}}<\mathrm{L}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}<\underset{\underset{\text{disordered})}{(\mathrm{m}\mathrm{o}\mathrm{s}\mathrm{t}}}{\mathrm{G}\mathrm{a}\mathrm{s}}$
Thus, in reaction
${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\to 2\mathrm{N}{\mathrm{H}}_3\left(\mathrm{g}\right)$
Entropy decreases because number of gaseous molecules decreases.