Two hydrogen atoms can combine to form H2 but two He atoms cannot form He2 —...

Two hydrogen atoms can combine to form H2 but two He atoms cannot form He2 — Justify from MO viewpoint.

Expert Solution

Electrons are added to molecular orbitals, one at a time, starting with the lowest energy molecular orbital. The two electrons associated with a pair of hydrogen atoms are placed in the lowest energy, or sigma bonding, molecular orbital, as shown in the figure below. This diagram suggests that the energy of an H2 molecule is lower than that of a pair of isolated atoms. As a result, the H2molecule is more stable than a pair of isolated atoms.

Combining a pair of helium atoms with 1s²electronconfigurations would produce a molecule with a pair of electrons in both the bonding and the* antibonding molecular orbitals. The total energy of an He₂ molecule would be essentially the same as the energy of a pair of isolated helium atoms, and there would be nothing to hold the helium atoms together to form a molecule.

The fact that an He2 molecule is neither more nor less stable than a pair of isolated helium atoms illustrates an important principle: The core orbitals on an atom make no contribution to the stability of the molecules that contain this atom. The only orbitals that are important in our discussion of molecular orbitals are those formed when valence shell orbitals are combined.

queen_honey_blossom answered 6 months ago

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