Aragonite, with a density of 2.9 g/cm3, has exactly the same chemical composition as calcite, which has a density of 2.7 g/cm3. other things being equal, which of these two minerals formed under higher pressure?

Aragonite is most likely be formed under high pressure.

Explanation:

Relating the densities of Aragonite and Calcite which both has the same chemical composition, it shows that the density of Aragonite is more than Calcite which says to some large extent that there is every possibility that Aragonite is most likely to be formed under high pressure.

if you mean hydrogen and oxygen then water is the product

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.