Why Do He2 Molecules Don T Exist
This molecular orbital model can be used to explain why He2 molecules don't exist. Combining a pair of helium atoms with 1s2 electron configurations 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 …show more content…
If we arbitrarily define the Z axis of the coordinate system for the O2 molecule as the axis along which the bond forms, the 2pz orbitals on the adjacent atoms will meet head-on to form a 2p bonding and a 2p* antibonding molecular orbital, as shown in the figure below. These are called sigma orbitals because they look like s orbitals when viewed along the oxygen-oxygen bond.
Diagram
The 2px orbitals on one atom interact with the 2px orbitals on the other to form molecular orbitals that have a different shape, as shown in the figure below. These molecular orbitals are called pi () orbitals because they look like p orbitals when viewed along the bond. Whereas and * orbitals concentrate the electrons along the axis on which the nuclei of the atoms lie, and * orbitals concentrate the electrons either above or below this axis. …show more content…
There is a significant difference between the energies of the 2s and 2p orbitals on an atom. As a result, the 2s and *2s orbitals both lie at lower energies than the 2p, 2p*, x, y, x*, and y* orbitals. To sort out the relative energies of the six molecular orbitals formed when the 2p atomic orbitals on a pair of atoms are combined, we need to understand the relationship between the strength of the interaction between a pair of orbitals and the relative energies of the molecular orbitals they form.
Because they meet head-on, the interaction between the 2pz orbitals is stronger than the interaction between the 2px or 2py orbitals, which meet edge-on. As a result, the 2p orbital lies at a lower energy than the x and y orbitals, and the 2p* orbital lies at higher energy than the x* and y* orbitals, as shown in the figure
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 …show more content…
If we arbitrarily define the Z axis of the coordinate system for the O2 molecule as the axis along which the bond forms, the 2pz orbitals on the adjacent atoms will meet head-on to form a 2p bonding and a 2p* antibonding molecular orbital, as shown in the figure below. These are called sigma orbitals because they look like s orbitals when viewed along the oxygen-oxygen bond.
Diagram
The 2px orbitals on one atom interact with the 2px orbitals on the other to form molecular orbitals that have a different shape, as shown in the figure below. These molecular orbitals are called pi () orbitals because they look like p orbitals when viewed along the bond. Whereas and * orbitals concentrate the electrons along the axis on which the nuclei of the atoms lie, and * orbitals concentrate the electrons either above or below this axis. …show more content…
There is a significant difference between the energies of the 2s and 2p orbitals on an atom. As a result, the 2s and *2s orbitals both lie at lower energies than the 2p, 2p*, x, y, x*, and y* orbitals. To sort out the relative energies of the six molecular orbitals formed when the 2p atomic orbitals on a pair of atoms are combined, we need to understand the relationship between the strength of the interaction between a pair of orbitals and the relative energies of the molecular orbitals they form.
Because they meet head-on, the interaction between the 2pz orbitals is stronger than the interaction between the 2px or 2py orbitals, which meet edge-on. As a result, the 2p orbital lies at a lower energy than the x and y orbitals, and the 2p* orbital lies at higher energy than the x* and y* orbitals, as shown in the figure