Explain why:(i) All the elements of a group have similar chemical properties. (ii) All the elements of a period have different chemical properties. (b) The atomic radii of three elements X, Y and Z of a period of the periodic table are 186 pm; 104 pm and 143 pm respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.

(a) (a) (i) All the elements of a group have similar chemical properties because they have same number of valence electrons in their outermost shell.

(ii) All the elements of a period have different chemical properties because they have different number of valence electrons in their atoms.

(b) As the atomic number increases in a period from left to right, the size of atoms goes on decreasing and the order of atomic numbers of elements will be X < Z < Y.