Stableelements, whose atoms have full outer electron shells, are found as single atoms. Most elements have atoms with incomplete outer electron shells, and these atoms form chemicalbonds with other atoms in order to gain full outer shells and become stable. These chemical bonds can be broken or made in chemicalreactions to produce different combinations of elements that we call compounds.
As the chemical reaction progresses, there will be observable and/or measurable changes, such as change in colour, temperature, flames, production of gas, production of solid precipitate, etc.