Question

which element is less reactive element A with atomic number 18 or element b with atomic number 20. why ?

Answer 1

the less reactive element is element A because it has atomic number as 18 which makes it a stable compound with configuration 2,8,8 but the element B have atomic number as 20 hence the configuration 2,8,8,2 is not stable hence it reacts easily.

Answer 2

electronic configuration of element A is :2,8,8
It's octet is filled so it can't combine with other elements . That's why it's less reactive.
electronic configuration of element B is :2,8,8,2
It's octet is not filled so it can reactive with more elements and hence is more reactive.