(a) What were the two major shortcomings of Mendeleev's periodic table? How have these been removed in the modern periodic table?
(b) Two elements X and Y have atomic numbers 12 and 16 respectively. Write the electronic configuration for these elements. To which period of the modern periodic table do these two elements belong? What type of bond will be formed between them and why?
(c) To which groups of the modern periodic table do the elements X and Y belong? Give reason for your answer.
(d) What could the elements X and Y be? Write their names and symbols.

(a) Two major shortcomings of Mendeleev's periodic table:
(i) Isotopes position was not confirmed
(ii) Position of hydrogen was doubtful

These anomalies were removed in modern periodic table:
(i) Isotopes were placed with their atoms, as all of them have same atomic numbers.
(ii) Hydrogen was assigned a position at the top of the period which is different but its nature matches with the group elements.

(b) Electronic configuration of X = 2, 8, 2 and Y = 2, 8, 6.
Both of them belong to the third period of the modern periodic table because three shells available in both of them.
To achieve noble gas configuration X will lose electrons while Y will gain electrons. So, they will form an ionic bond.

(c) X has two valence electrons so it comes under alkaline earth metal family (1) and Y has 6 valence electrons so it comes under oxygen family (16).

(d) Electronic configuration of X resembles magnesium (Mg) while the electronic configuration of Y resembles sulphur (S). So the ionic compound formed will be of XY type i.e. MgS.