As seen in the following reaction, O2 + 2H2 à 2H2O, what particles composing the oxygen atoms (O2) and hydrogen atoms (H2) were directly involved in making the chemical reaction which created the water molecules?

The particles that make up the oxygen and hydrogen atoms that participate in the chemical reaction are electrons.

Explanation:

The water molecule is formed by two atoms of hydrogen and one of oxygen joined by a covalent bond. The arrangement of electrons communicates an electrical asymmetry due to the electronegativity of hydrogen and oxygen. Because oxygen is more electronegative than hydrogen, electrons are closer to the oxygen atom than the hydrogen atom, which makes hydrogen atoms have a positive charge and oxygen atoms a negative one.

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).