Question

Why hydrogen molecule exists helium remains monatomic?

Answer 1

Explanation:

Chemists call these molecules “homonuclear” referring to the fact that both atoms have the same nuclear structure. Nitrogen stands out because its atoms share a strong triple bond, making it a very stable substance. The noble gases, such as helium and neon, rarely form molecules at all; they are monatomic.

Answer 2

Answer:

Helium is considered monoatomic because it has 1 atom in its molecule, but hydrogen is considered diatomic because it has 2 hydrogen atoms in its molecule.

The ionization enthalpy of the hydrogen atom is very high (1312 kJmol)

Therefore, it is very difficult to remove its single electron. Consequently, its tendency to exist in monatomic form is rather low.

The electron configuration of the helium molecule is such that it has bond order zero. So a value of "zero" means that the helium molecule He does not exist, which means that diatomic helium does not exist.

Hydrogen atom has atomic number of 1 which means that it has only one electron in the s-orbital ($1s^1$). These two half-field atomic orbitals combine from a molecular orbital that contains both of these electrons. But helium (Z=2) already has a full  ($1s^2$) orbital.

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