Answer:

POSITION of isotopes: all the isotopes of an element have the same atomic no. therefore they can be placed in the same GROUP of the periodic table.eg- all isotopes of carbon i.e. C12,C13,C14 are placed in same position as that of carbon(gr-14) of the periodic table. anamolous position of some PAIRS of elements: in mendleev's periodic table Co slight higher mass (58.71u). This limitation has been removed in modern periodic table, elements are ARRANGED in increasing order of atomic no. therefore cobalt with lower atomic no. comes first and nickel with higher comes later position of hydrogen:  hydrogen resembles alkali metals in elec. configuration(1 VALANCED elec.) and some other properties. therefore it can be placed in gr-1 along alkali metals. at same time H also resemble halogens in its elec. configuration . therefore it can be placed in gr-17 along with halogens. but at same time it differs from both alkali metals and halogens in some other properties. eg- oxide of Hydrogen H2O which is neutral oxides of  alkali metals are basic while that of halogen is acidic. hence the position of hydrogen still remain to be contriversial in the modern periodic table. however for convinience it is placed just above gr-1 in separate block and period1 because it has lowest at. no. i.e. 1 and mass 1.008u