Predict the position of the element in periodic table having valency shell electronic configuration

Question

(a) Predict the position of the element in periodic table having valency shell electronic configuration of (n-l)d¹ns²; n = 4. 

(b) Why noble gases have bigger atomic size than halogens? Why electron gains enthalpy of noble gases are positive?

Answer 1

(a) (n -l)d¹ns², with n = 4, 3d¹4s² is Scandium belongs to 4th period and group 3. 

(b) It is because we can measure van der Waal’s radii in noble gases which are bigger than covalent radii. It is because electron has to enter the next higher energy level leading to very unstable electronic configuration.