(a) Lithium is an alkali metal (Group 1). It has only one electron in the valence shell, therefore, its valency is 1. Oxygen is a group 16 element with a valence of 2 . Therefore, formula of the compound formed would be \(\mathrm{Li}_2 \mathrm{O}\) (Lithium oxide).
(b) Magnesium is an alkaline earth metal (Group 2) and hence has a valence of 2 . Nitrogen is a group 15 element with a valence of \(8-5=3\). Thus, the formula of the compound formed would be \(\mathrm{Mg}_3 \mathrm{~N}_2\) (Magnesium nitride).
(c) Aluminium is group 13 element with a valence of 3 while iodine is a halogen (group 17) with a valence of 1. Therefore, the formula of the compound formed would be \(\mathrm{A} 1_1 \mathrm{I}_3\) (Aluminium iodide).
(d) Silicon is a group 14 element with a valence of 4 while oxygen is a group 16 element with a valence of 2 . Hence the formula of the compound formed is \(\mathrm{SiO}_2\) (Silicon dioxide).
(e) Phosphorus is a group 15 element with a valence of 3 or 5 while fluorine is a group 17 element with a valence of 1 . Hence the formula of the compound formed would be \(\mathrm{PF}_3\) or \(\mathrm{PF}_5\).
(f) Element with atomic number 71 is a lanthanoid called lutetium (Lu). Its common valence is 3 . Fluorine is a group 17 (halogen) element with a valence of 1 . Therefore, the formula of the compound formed would be \(\mathrm{LuF}_3\) (Lutetium fluoride).