Why `Ag` is a noble metal and `K` is a highly reactive metal ?

Explanation

`K` is an alkali metal with low `IE`, and thus forms `K^(o+)` readily by losing an electron. But in case of `Ag`, due to imperfect screening effect of inner `4d` orbitals, the valence electrons are not lost easily. Thus the `IE` of `Ag` is very high. That's why `Ag` is relatively inert.