Aluminium is placed in ________ period and group _______ in the periodic table.

A period is determined by the number of shells and the group is determined by the number of electrons present in the outermost shell. Aluminium has atomic number 13 and its electronic configuration is 2,8,3. It has three orbits and hence 3 electrons in the outermost shell, hence it is placed in the third period and group 3 in the periodic table.