why the oxidizing character of elements increases from left to right across a period & decreases down a group ?

Oxidising power is the tendency of an element to accept electron.The greater is its ability greater will be its oxidising power
Down the group the effective nuclear charge decreases whereas across the period the oxidising power increases as the effective nuclear charge increases across the perid
The periodic table tendency for effective nuclear charge:

Increase across a period (due to increasing nuclear charge with no accompanying increase in shielding effect).
Decrease down a group (although nuclear charge increases down a group, shielding effect more than counters its effect)