The electron density on the central atom decreases with the increase in the size of the central atom. Hence its ability to donate a pair of electrons decreases thus reducing the basic character. … So, as the size of the central atom increases, the stability decreases. Thus the thermal stability decreases down the group.
Why does reducing character increase down group 15?
As we move down in group 15 the size of the group 15 element increases which causes the length of the bond between the hydrogen atom and the group 15 element to increase. This in turn decreases the strength of the bond between hydrogen atom and the group 15 element, hence making it easier to donate a hydrogen atom.
Why does basicity decrease down the group?
Going down the group, size of the atom increases. And hence, electron density over the group 15 elements decreases. Thus tendency to donate electrons decreases and basicity decreases.