Their formula will be XYXY.
Notice that the number of electrons located in their outermost shell varies significantly for the two elements.
On one hand, XX has only 2 electrons in its outermost shell. This means that it's easier for XX to lose these electrons In order to complete its octet, i.e. have a total of 8 electrons in its outermost shell, than to gain 6 more.
On the other hand, YY has 6 electrons in its outermost shell, which means that it can easily gain two electrons to complete its octet.
Element XX can thus form 2+ cations, X^(color(red)(2+))X2+, and element YY can form 2- anions, Y^(color(blue)(2-))Y2−.
When these two elements react, XX will lose its 2 valence electrons and YY will take them. The two elements will thus form an ionic compound with the formula
{: (X^(color(red)(2+))), (Y^(color(blue)(2-))) :}} =>X_(color(blue)(2))Y_(color(red)(2)) <=> XY
