An unknown element, x , reacts with rubidium to form the compound r b 2 x . in other compounds this element also can accommodate up to 12 electrons rather than the usual octet. what element could x be?

Answer : The Correct answer is Tellurium ( Te)

In the compound Rb₂X , Rb is Rubidium which belongs from s block and group 1. Also its electronic configuration is [Kr] 5s¹ , which suggest that oxidation state of Rb is +1 . Since there are two Rb atoms present so total positive charge is +2 .

The compound is neutral means it has zero total charge so total negative charge is -2 . Also the there is one atom of X , so oxidation state of X must be -2 . Element with -2 oxidation state belong to oxygen family or group 16 . Electronic configuration of oxygen family ends with ns² np⁴ , means if the element accommodate 12 electrons , the configuration will become ns² np⁶ nd⁴ , which in turn says that element should have d orbitals .

Oxygen and sulfur belongs to period 2 and 3 respectively so they cannot have d orbital . Tellurium belongs to period 5 ( same period of Rb), it can extend its configuration to d orbital . So the X element can be "Tellurium " (Te) .