a) State two main characteristics of elements on which modem periodic table is based.
(b) No fixed position can be assigned.to hydrogen in the periodic table. Why ?
(a) On the basis of atomic number and number of valence electrons.
(b) Like metals hydrogen combine with halogens, oxygen and sulphur. Like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to form covalent compounds so no fixed position can be given to hydrogen.