An unknown element, X, reacts with potassium to form the compound K2X. In other compounds this element also can accommodate up to 12 electrons rather than the usual octet. What element could X be

Se

Explanation:

First of all, we must note that any element that we must choose is an element that is in group sixteen.

Elements of groups 16 have six electrons in their outermost shell which can be used for bond formation thereby yielding a total of twelve electrons on the valence shell.

However, this is only possible for the heavier members of the group 16 (from sulphur downwards) which are able to expand their octet.

Oxygen can not expand its octet hence it is not the answer.

dynamic equilibrium is the answer.