Using the Modern periodic table, predict the formules as compounds which might be formed by the following pairs of elements : (i) Boron & Oxygen (ii) Calcium & Fluorine.

Answer

Boron is group `13` element with a valence of `3`; Oxygen belongs to group `16` (chalcogen family) with a valence of `2`. Hence, the formula of the compound formed would be `B_(2)O_(3)`(by criss-cross method).
Calcium belongs to group `2` (alkaline earth metals) with a valence of `2`; Fluorine belongs to group `17` (halogen family) with a valence of `1`. Hence the formula of the compound formed would be `CaF_(2)` (by criss-cross method).