What elements make O stable?
Two hydrogen make Oxygen stable.
How can you predict which elements are stable under natural conditions and which elements tend to undergo chemical reactions?
You can tell when they are stable because it depends n the number and arrangement of electrons in their atoms. An atom is chemically stable when its outermost energy level is filled. Most atoms are not stable in their natural stat. Thus, they tend to react, or combine with other atoms, in ways that make the atoms more stable.
Some elements are stable on their own because they have a complete outer energy level. These elements are called noble gases. Hydrogen and Helium have a complete energy level with two electrons and all of the other elements are stable with eight. When an element doesn't have a complete outer energy level it is stable in a compound. This is because it is being combined with another element to complete it's energy level.