Dobereiner made groups of elements, having similar chemical properties and called them _______.
(a) 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 group of three elements with similar physical and chemical properties is called ------------