An unknown element, x, reacts with rubidium to form the compound rb2x. in other compounds this element also can accommodate up to 12 electrons rather than the usual octet. what element could x be?

There are few elements that can form 12 electron rather than the usual 8. Therefore these are the possible answers:

Sulfur = 2Na + S --> Na2S 
Selenium = 2Na + Se --> Na2Se 
Tellurium = 2Na + Te --> Na2Te

the number of protons in the nucleus of a given element. has slight luster and conducts electricity at high temperatures like metal.but like a nonmetal,boron is brittle and does not conduct electricity well at low temperatures. in graphite, carbon is a soft,brittle solid.it is reflective,ductile,or malleable.