The correct answer is statements (ii) and (iv). Concept: Atomic number: It is the number of electrons or protons found in an atom. Electronic configuration: The arrangement of electrons in different shells around the nucleus. There are 7 shells K, L, M, N, and so on. K shell can accommodate only 2 electrons while others can accommodate 8 electrons. Ionic compounds: The compounds formed by the complete transfer of electrons from one element to another. Explanation: Element X Atomic no. of X = 20 ( Calcium) Electronic configuration = 2, 8, 8, 2. Valency = +2, since it will lose 2 electrons Element Y Atomic no. of Y = 8 (oxygen) Electronic configuration = 2, 6. Valency = -2, since it will gain 2 electrons Compound Z 2 electrons will be transferred from element X to element Y and an ionic bond is formed. The formula for Z will be XY. The compound will be Calcium oxide (CaO). Statement True / false The formula of 'Z' is XY2. False, because the formula for Z is XY. The melting point of 'Z' is very high. True, because Z is an ionic compound and these bonds are very strong. The molten form of 'Z' does not conduct electricity. False, because ionic compounds conduct electricity in their molten or aqueous state. Z' is soluble in water but insoluble in petrol. True, because ionic compounds are soluble in polar solvents like water but insoluble in non-polar solvents like petrol Therefore, statements (ii) and (iv) are correct.