(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 same no. of valence electrons in their outermost shell.
(ii) All the elements of a period have different chemical properties because they have different no. of valence electrons in their atoms.
(b) Order of atomic numbers of elements: X < Z < Y. Because as the atomic number increases in a period from left to right, the size of atoms goes on decreasing.