The correct answer is 40 u. Concept: The Modern Periodic Table: In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass. Modern Period Law: The physical and chemical properties of elements are the periodic function of their atomic number. The modern periodic table contains 18 vertical columns known as groups and seven horizontal rows known as periods. On moving from left to right in a period, the number of valence electrons increases from 1 to 8 in the elements present but the number of shells remains the same. All the elements of a group of the periodic table have the same number of valence electrons. [ alt="RRB Group-D 27th Sep 2018 Shift 1 (English) Sunny (Type) Madhu(Dia) D1 utkarsha" src="//storage.googleapis.com/tb-img/production/20/12/RRB_Group-D_27th%20Sep%202018%20Shift%201%20%28English%29_Sunny%20%28Type%29_Madhu%28Dia%29_D1_utkarsha.PNG" style="width: 400px; height: 300px;"> Explanation: Period: The periods have the same elements that have the same valence shell or the energy shell. The number of elements present in a period can be determined by the formula 2n2, where n is the number of the shell from the nucleus. For example, the first period has two elements only as they have only 1 valence shell and the second period has 8 elements with 2 shells. Also, the elements have the same no. of shells as their period no. So, an element belonging to period 4 will have 4 shells. ​Groups: The group consists of elements that have the same number of electrons in the valence shell or outermost shell of the atom. The number of shells increases by one unit as the elements move down in the periodic table in a group. Group number indicates the valence electrons of the atom. Group1 and 2 have 1 and 2 valence electrons in the outermost shell respectively. But for group no. 13-18 it is calculated by subtracting 10 from group no. For example, an element belonging to group 13 will have 13-10 = 3 valence electrons. Element X belongs to the 3rd period and 2nd group. 3rd period means there are 3 shells. 2nd group means 2 valence electrons in its outermost shell. So, its electronic configuration will be 2, 8, 2. Element X is Magnesium with atomic number 12 and an atomic mass of 24 u. Element Y belongs to the 2nd period and 16th group. 2nd period means there are 2 shells. 16th group means 16-10 = 6 valence electrons in the outermost shell. Electronic configuration = 2, 6 Element Y is Oxygen with atomic number 8 and atomic mass 16 u. The molecular mass of compound XY = mass of X + mass of Y = 24 + 16 = 40 u.​