(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) (i) All the elements of a group have similar chemical properties because they have the same number of valence electrons in their outermost shell.
(ii) All the elements of a period have different chemical properties because they have a different number of valence electrons in their atoms.
(b) Order of atomic numbers of an element: X<Z<Y. Because as the atomic number increases in a period from left to right the size of atoms goes decreasing.