The most acidic oxide formed by second period elements is

Cl, S are not second period elements;

In second period, oxygen cannot have oxides;

In second period,oxides of fluorine are unstable and cannot form oxo acids;

HOF is a very unstable oxo acid;

So nitrogen forms most acidic oxide;

${\mathrm{N}}_2{\mathrm{O}}_5>{\mathrm{CO}}_2>{\mathrm{B}}_2{\mathrm{O}}_3>\mathrm{BeO}>{\mathrm{Li}}_2\mathrm{O}....\mathrm{acidic} \mathrm{strength};$

In fact ${\mathrm{Li}}_2\mathrm{O} \mathrm{is} \mathrm{basic} \mathrm{and} \mathrm{BeO} \mathrm{is} \mathrm{amphoteric};$