X is an element with atomic number 20.
(i) Is it a metal or a non-metal?
(ii) Among X or Mg, which one is more reactive?
(iii) What is the valency of X?
(i) The element X has atomic number=20, it means that the element is Ca. It has electronic configuration as: 2,8,8,2. The number of valence electrons is 2 which it can lose easily. Thus element X (Ca) is a metal.
(ii) Mg has atomic number 12 and electronic configuration as: 2, 8, 2. Also, it has a lesser number of shell and thus its valence electrons lie closer to nucleus as compared to X(Ca) which has its valence electrons in 4th shell, so Mg is less reactive than X(Ca). Thus X(Ca) is more reactive than Mg.
(iii) Since X contains 2 electrons in last shell and it can readily lose them so its valency is 2.