Answer:
i) The size of X ion is greater than that of X atom even though both contain the same number of protons because the ion has fewer electrons compared to the atom. When an atom forms an anion (negative ion), it gains electrons, which causes increased electron-electron repulsion. This repulsion causes the electron cloud to expand, and as a result, the ion becomes larger than the neutral atom.
In the case of element X, when it forms an ion with a -3 charge, it will gain 3 more electrons, increasing the total number of electrons to 18. This will cause the size of the X ion to be larger than the neutral X atom.
ii) To determine the compound of X in the -3 oxidation state, we first need to determine the element's identity. We know that X has 15 electrons in total (2 in the K shell, 8 in the L shell, and 5 in the M shell). Therefore, X has an atomic number of 15, which corresponds to phosphorus (P).
Since phosphorus is in the -3 oxidation state, it gains 3 electrons and becomes P^3-. To form a compound, we need a cation that can balance the negative charge. A common example is aluminum (Al), which has a +3 charge (Al^3+). When phosphorus and aluminum combine, they form the compound aluminum phosphide with the formula AlP.