Atoms combine with each other to form various compounds. The smallest unit of a substance which can exist independently is called a molecule. So, atoms combine with each other to form molecules.
The chemical behavior of any element depends upon the position of the element in the periodic table. The position of an element in the periodic table is related to its electronic configuration. So, the chemical reactivity of an element depends upon its electronics configuration, i.e., the chemical reactivity of an element depends upon the distribution of electrons in its atom.
It has been found that the atoms having a total of 2,10,18,36,54, and 86 electrons are the most stable, i.e., such atoms do not show any chemical reactivity. These electrons configurations are those of the noble gases, viz., Helium, Neon, Argon, Krypton, xenon and Radon, respectively. These elements called noble gases do not form compounds either among themselves or with other elements. Xenon, however, forms fluorides and oxyflourides under controlled conditions. This non-reactivity of noble gases is due to their stable electronic configurations.
These elements have electronic configurations so stable that these electronic distributions cannot be changed. As a result, these elements do not show chemical reactivity.
All other elements having less than eight electrons in their outermost shell (except helium which has been two electrons in its outermost shell) show a reasonable chemical activity. For example, sodium and chlorine are quite reactive because their outermost shells are incomplete, i.e., Na and Cl contain less than eight electrons in their outermost shell.
Thus, were seeing that the elements having less than eight electrons in their outermost shell (except helium) are chemically reactive.