An ion X2+ contains 18 electrons and 20 neutrons. Calculate the atomic number and mass number of element X. Name the element X

Number of electrons in X2+ion = 18 Since positive ion is formed by the loss of electrons from the neutral atom and the number of electrons lost is equal to the number of units of positive charge on the ion.
Number of electrons in the neutral atom = 18 + 2 = 20
Now, for a neutral atom, Atomic number = Number of protons = Number of electrons. Thus, atomic number of element X=20 Mass number of the element X = Number of protons + Number of neutrons = 20 + 20 = 40 The given element X with atomic number 20 is calcium.