Atomic number of X is 10, so this must be Ne (Neon), a noble or inert gas. This gas has complete octet configuration. Atomic number of Y is 17 as [2, 8, 7], so this must be chlorine (Cl). Chlorine is a non-metal. Non-metals have 5 to 7 electrons in their valence shell. Atomic number of Z is 12 as [2, 8, 2], so this must be magnesium (Mg). Magnesium is a metal. Metals have 1, 2 or 3 electrons in their valence shell.