Why H2 react with O2?

Hydrogen atoms have one electron each. They combine into molecules of two so that they can share two electrons (one each). This is because the inner-most electron shell is at a lower energy state (and therefore more stable) when occupied by two electrons.
Oxygen atoms have eight electrons each. Two electrons in the inner shell and 6 electrons in the outer shell of each oxygen atom. The oxygen atoms combine together to form a molecule of two atoms by sharing four electrons so that their outer-most electron shells are fully occupied by eight electrons for each oxygen atom . However, a far more stable alignment of electrons arises when two hydrogen atoms share an electron with one oxygen atom. Only a small amount of energy is needed to "bump" the electrons of the reactants "out" of their orbits so that they can realign in the more energetically stable alignment, forming a new water molecule, H2O.

chemical reaction i assume?