Thalium (Tl) exhibits monovalency whereas Al exhibits trivalency. This is due to:

• A

  The energy required to unpair outer s-electrons in Tl exceeds the energy involved in bond formation.

• B

  Tl has only one electron in its outermost orbital.

• C

  Al can use its vacant d-orbitals for the bond formation

• D

  Tl is a non-metal.