No chemical compound of helium is known.

Answer

### Step-by-Step Solution:

1. **Understanding Helium's Electron Configuration**:
- Helium has the electron configuration of 1s². This means it has two electrons in its outermost shell, which is the first shell (n=1).
- Hint: Remember that the first shell can only hold a maximum of 2 electrons.

2. **Absence of D Orbitals**:
- Helium does not have any d orbitals available for bonding because d orbitals begin to appear in the third shell (n=3). Since helium only has electrons in the first shell, it lacks the ability to form compounds that require d orbitals.
- Hint: Consider the significance of d orbitals in the formation of complex compounds.

3. **High Ionization Enthalpy**:
- Helium has a very high ionization enthalpy (the energy required to remove an electron). This high ionization energy makes it difficult for helium to lose or share electrons to form chemical bonds.
- Hint: Compare the ionization enthalpy of helium with other elements like fluorine (F) and oxygen (O) to understand why it remains unreactive.

4. **Inert Nature of Helium**:
- Due to its complete outer shell (1s²) and high ionization energy, helium is chemically inert. This means it does not readily react with other elements to form compounds.
- Hint: Recall the properties of noble gases and how they relate to reactivity.

5. **Conclusion**:
- Based on the above reasons, we conclude that no chemical compounds of helium are known due to the lack of d orbitals and its high ionization enthalpy, which prevent it from participating in chemical reactions.

### Summary:
Helium's lack of d orbitals and its high ionization enthalpy are the primary reasons why it does not form any chemical compounds.