Final answer:
The number of electrons in the outer shell of an atom, known as valence electrons, determines its chemical properties and reactivity. Helium, with a full outer energy level, is very stable and unlikely to react with other atoms.
Step-by-step explanation:
The number of electrons in the outer shell of an atom can tell us about its chemical properties and reactivity. These electrons, known as valence electrons, are the ones involved in forming bonds with other atoms. The outermost energy level of an atom, where the valence electrons are located, determines the energy and stability of the atom. For example, helium with its full first energy level is very stable and unlikely to react with other atoms.