An element X belongs to period 3 and group 13 of the modern periodic table and another element Y belongs to period 4 and group 17 of the modern periodic table. The formula of the compound formed between X and Y will be:

As element X belongs to group 13 and period 3, it contains 3 valence electrons in the valence shell. Another element Y belongs to group 17, so it contains 7 valence electrons.

Element Y will only require 1 electron to complete the octet, hence it will try to accept one electron from element X. So, element X being metallic in nature will donate 3 valence electrons to 3 Y atoms and will form an ionic compound having formula XY₃