Q1)What do A and B represent in Mendeleev's periodic table?
Q2)In​Mendeleev'speriodic table, the atomic masees do not increase in a regular manner in going from one element to the next. How does it create a problem to predict new elements?

1) In Mendeleev's periodic table, there are 8 groups and each group is divided into two sub-groups designated as A and B. Mendeleev arranged the elements according to their atomic weights. He found that there occurs a periodic recurrence in certain elements in terms of their physical and chemical properties. He found that elements present even in same group have different properties. So he divided the group into two subgroups containing elements having same physical and chemical properties and named them A and B. The properties of elements of a sub-group resemble each other more markedly than the properties of those between the elements of the two sub-groups.
2) As the atomic mass does not increase on regular manner it is difficult to predict the atomic mass and position of new element. For example : If one element has atomic mass 4 then we cannot predict that at what atomic mass the new element will exist and what would be the exact position of new element.