Five elements A, B, C, D, and E have atomic numbers of 2, 3, 7, 10, and 18 respectively. The elements which belong to the same period of the periodic table are

From the given elements A, B, C, D and E with atomic numbers 2, 3, 7, 10 and 30, respectively. The elements belonging to the same period are B, C, and D.
Electronic configuration of A = 2
Electronic configuration of B = 2, 1
Electronic configuration of C = 2, 5
Electronic configuration of D = 2, 8
Electronic configuration of E = 2, 8, 18, 2
The electronic configuration of the element gives an idea about the atomic number, group and period number of the element. The number of valence electrons indicates the group number of the elements. The number of shells in which electrons are filled in an atom indicates its period number. Elements B, C and D have a similar number of shells, so they belong to the same period (period 2).