The correct option is **D. the outer electron shell is completely filled**. ### Detailed Explanation: 1. **Understanding Helium's Electron Configuration**: - Helium (He) is a noble gas with an atomic number of 2, which means it has 2 protons and, in a neutral atom, 2 electrons. - The electron configuration of helium is 1s², indicating that both of its electrons occupy the first energy level (the 1s orbital). 2. **Electron Shells and Stability**: - Atoms are most stable when their outermost electron shell is filled. For helium, the first shell can hold a maximum of 2 electrons, and since it has 2 electrons, this shell is completely filled. - A filled outer shell means that the atom has achieved a stable electronic configuration, which is a key reason why helium does not readily react with other elements. 3. **Chemical Reactivity**: - Chemical reactions typically occur when atoms seek to achieve a more stable electron configuration, often by gaining, losing, or sharing electrons. - Since helium already has a full outer shell, it does not need to gain or lose electrons, making it chemically unreactive. ### Why the Other Options Are Incorrect: - **Option A: There are no electrons around the nucleus**: - This statement is incorrect because helium has 2 electrons. The presence of electrons is crucial for chemical reactivity, and helium has a complete set of electrons in its outer shell. - **Option B: The number of protons equals the number of electrons**: - While it is true that helium has an equal number of protons (2) and electrons (2), this fact alone does not explain its lack of reactivity. Many elements with equal numbers of protons and electrons are reactive. The key factor for helium's unreactivity is the filled outer shell, not just the balance of protons and electrons. - **Option C: There are equal numbers of protons and neutrons in the nucleus**: - Helium does have 2 protons and typically 2 neutrons (in its most common isotope, helium-4), but this balance does not influence its chemical reactivity. The number of neutrons affects the stability of the nucleus and isotopic properties, but it does not determine how the atom interacts chemically. ### Summary of Key Points: - Helium is chemically unreactive because its outer electron shell is completely filled (1s²). - A filled outer shell provides stability, preventing helium from gaining or losing electrons. - Other options do not address the core reason for helium's lack of reactivity, focusing instead on unrelated aspects of atomic structure. - Understanding electron configurations and stability is crucial for predicting chemical behavior in elements.