Question

Bond energy has been defined in the text as the amount of energy required to break a chemical bond, so we have come to think of the addition of energy as breaking bonds. However, in some cases the addition of energy can cause the formation of bonds. For example, in a sample of helium gas subjected to a high-energy source, some He2 molecules exist momentarily and then dissociate. Use MO theory (and diagrams) to explain why He₂molecules can come to exist and why they dissociate.

Short answer

When two atomic orbitals combine they form a molecular orbital which consists of bonding and nonbonding electrons, through which we can calculate the bond order. For helium molecule, the bond order is 0, which depicts that it is an unstable molecule and dissociates easily.

Step-by-step solution

Molecular orbital configuration for Helium

Helium atom consists of 2 valence electrons, therefore in ${\text{He}}_2$a total of 4 electrons are present. So, the molecular orbital configuration for ${\text{He}}_2$ will be $(\sigma 1s{)}^2({\sigma }^{*}1s{)}^2$ and diagram will be formed as shown

$$\begin{gathered} BondorderofH{e}_2=\frac{No.ofbondingelectrons-no.ofnonbondingelectrons}{2} \\ =\frac{2-2}{2} \\ =0 \end{gathered}$$

Step 2:

As the bond order is zero, the helium cannot exist for longer time and dissociates easily.