Rb reacts with O in a mole ratio of 2 to 1, forming the ionic compound Rb20. Which of the following elements will react with O in a mole ratio of 2 to 1, forming an ionic compound, and why? A)
S, because it is in the same group as O.

B)
Cs, because it is in the same group as Rb.

C)
Sr, because it is in the same period as Rb.

D)
Br, because the atomic mass of Bris similar to that of Rb.​

the answer is barium and copper bromide

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.