Helium molecule (He2) –
He (2) – 1s2.
Helium has two electrons in the 1s orbital. Thus, when we draw the MO diagram, two electrons occupy the BMO and the other two have to occupy the ABMO.
∴Bond order = 1/2 (2-2) = 0.
The bond order is zero which indicates that there is NO bond between two helium atoms.In reality too this dimer does not exist. This bond is so so weak that it breaks even if the molecule vibrates or rotates.At normal temperature, this molecule keeps rotating and vibrating as the thermal energy at room temperature is sufficient to rotate or vibrate the bonds in the molecule .So, this molecule can only exist if the vibrations and rotations can be ceased.This happens at very very low temperatures(cryogenic temperatures), where there is no sufficient thermal energy to bring about changes in rotational or vibrational levels of the molecule.Thus, under normal circumstances HELIUM MOLECULE DOES NOT EXIST.
Before proceeding further we need to have a look at the following figure –
The above figure shows the s and p orbital mixing in the second row elements of the periodic table. These second row elements have s and p orbitals as their valence orbitals ,which take part in bonding.
As seen in the above figure, the diatomic molecules like Li2 , Be2 , B2, C2 and N2 the energy gap between the 2s and 2p is less. This is because the effective nuclear charge(Zeff) increases as we move from left to right in the second row of the periodic table.
What is effective nuclear charge, Zeff ?
The outer/ valence electrons of an atom with more than two orbitals do not experience the full nuclear pull from the nucleus of that atom as the inner electrons shield/protect the outer electrons from the nuclear charge.We can say that , the inner electrons don’t let the outer electron feel the entire pull from the nucleus.Shielding can also be described as the net effect of repulsion between inner an outer electrons and the attraction of the outer electron by the nucleus.
In the following figure , the two inner electrons experience the nuclear charge completely as there is nothing coming in the way between them and the nucleus.So, they are tightly bound to the nucleus.However, the outer electron is shielded by these two electrons and the nuclear charge it experiences is less (as inner electrons come in the way).This net nuclear charge the outer electron experiences is the effective nuclear charge, Zeff .
What happens as we move to the right along the second row of the periodic table? The atomic number(Z) increases, but the no. of inner electrons which are responsible for shielding(S) DONOT increase.In the second row, there are only two inner electrons , as shown in the figure below.The # of outer electrons increase as Z increases from left to right.
So, in the formula above if Z goes on increasing and S does not then the Zeff will increase.This is exactly what is observed.
Zeff increases across a period (due to increasing nuclear charge with no accompanying increase in shielding effect).
Small effective nuclear means that there isn’t a great energy difference between 2s and 2p orbitals.So, 2s and 2p orbitals are closer in energy to each other and thus, the π2p orbital is lower in energy than the σ2p orbital.
From O onwards, Zeff is more. So, there is a considerable energy difference between the 2s an 2p orbital energies.As seen in the figure above, the order changes from N(Z=7) to O(Z=8) and from O onwards the σ2p orbital is lower in energy than the π2p orbital.
How do we remember this?
We will study nitrogen and oxygen MO diagrams in the next post and see how the diagrams differ.Till then,
Be a perpetual student of life and keep learning..
Image source –
References and Further Reading –