Why Hydrogen and Helium Cannot Form a Molecule

Understanding the chemical behavior of elements like hydrogen and helium requires an examination of their atomic structures, particularly in relation to their orbital configurations. In this article, we explore why these elements cannot form molecules despite their seemingly complementary requirements.

Atomic Structure and Orbital Configurations

Hydrogen (H) has an atomic number of 1, indicating it has a single electron. According to its electronic configuration, hydrogen's electron is in the 1s orbital. This single electron means hydrogen can either give away its electron to form a positively charged ion (H ), accept an electron to form a negatively charged hydride ion (H-), or share its electron to form a covalent bond. When hydrogen is present in its pure form, it combines with another hydrogen atom to form the H2 molecule. This molecule has a single covalent bond, which is as pure as possible given hydrogen's electron configuration.

In contrast, helium (He), with an atomic number of 2, has a full outer shell, with its two electrons in the 1s orbital. Helium's stable configuration means it does not have a need to gain or share electrons. As a result, helium is the least reactive element on the periodic table, often referred to as a "noble gas."

Molecular Stability and Octet Rule

A crucial factor in understanding why hydrogen and helium cannot form a molecule lies in the concept of the octet rule. The octet rule indicates that atoms prefer to have 8 electrons in their outermost shell to achieve a stable configuration similar to that of noble gases.

Hydrogen, with one electron, has a valance of 1 and needs one more electron to complete its octet. To achieve this, it shares its single electron with another hydrogen atom, forming the H2 molecule. This sharing creates a single covalent bond, which is the purest form of covalent bonding possible for hydrogen.

Helium, however, already has a full outer shell of two electrons, represented as 1s^2. This complete 1s orbital leaves helium with no need to acquire or share electrons. Therefore, helium does not form any molecules, as it is stable in its current configuration. According to molecular orbital theory, the molecular orbital diagram for helium would show a bond order of zero, indicating a high degree of instability in any hypothetical helium molecule.

Formation of H2 Molecule

The H2 molecule has a bond order of 1, calculated by the formula bond order 1/2[Nb - Na], where Nb is the number of bonding electrons and Na is the number of antibonding electrons. In the case of H2, Nb is 2 (representing the two bonding electrons), and Na is 0, resulting in a bond order of 1. This bond order indicates a single covalent bond between the two hydrogen atoms. The H2 molecule is also dimagnetic with a bond dissociation energy of 438 kJ/mol, which is the energy required to break the bond between the two hydrogen atoms.

Hence, the H2 molecule is a stable form, while the hypothetical He2 molecule is highly unstable and does not exist.

Conclusion

The inability of hydrogen and helium to form a molecule is a direct result of their respective orbital configurations and the principles governing their chemical behavior. While hydrogen requires an additional electron to achieve stability and can thus form molecules, helium's complete outer shell means it does not have the chemical drive to bond with other atoms.

Understanding these fundamental aspects of atomic structure and molecular stability is crucial for comprehending the vast array of chemical reactions and molecular forms that we observe in the natural world.