Q.11. Elements X, Y and Z have atomic no. 13, 18 and 17 respectively. Which one will form a cation? Identify the inert gas among them.

Here, X has atomic number 13 so it's electronic configuration is  2, 8, 3 .
Y with atomic number 18, has electronic configuration 2,8,8.
Z atomic number 17 has electronic configuration 2,8,7.

So, here Z is anion with valency -1.
X is cation with valency +3.
Y has octet fulfil electrons.Hence it will be the inert gas.