Sketch a molecular orbital energy diagram for CO and place the electrons in the levels appropriate for the ground state. The AO ionization energies are 025: 32.3 eV; 02p: 15.8eV; C2s: 19.4 eV; and C2p: 10.9 eV. The MO energies follow the sequence (from loest to highest) la, 2a, 3 a, 4a, Ik, 5 a, 2n, 6a. Assume that the first two a orbitals originate from the Is AOs on C and O. Connect each MO level with the level of the major contributing AO on each atom.
The Correct Answer and Explanation is :
To sketch a molecular orbital (MO) energy diagram for carbon monoxide (CO), we need to understand the bonding and anti-bonding molecular orbitals that form from the atomic orbitals (AOs) of carbon (C) and oxygen (O). The ionization energies of the atomic orbitals (AOs) you provided help us determine the relative energy levels of the orbitals that will combine to form MOs.
Step 1: Understand Atomic Orbitals
The atomic orbitals (AOs) for C and O include:
- C 1s: 32.3 eV
- C 2s: 19.4 eV
- C 2p: 10.9 eV
- O 1s: 32.3 eV
- O 2s: 15.8 eV
- O 2p: 12.1 eV
Step 2: Molecular Orbitals (MO) Energy Ordering
The molecular orbital energy ordering for CO is typically given as:
- σ(1s) (lowest energy)
- σ*(1s)
- σ(2s)
- σ*(2s)
- π(2p) and π*(2p)
- σ(2p)
We use these labels to connect the atomic orbitals (AOs) with the molecular orbitals (MOs) in the diagram.
Step 3: Constructing the MO Diagram
- The lowest energy molecular orbitals correspond to the 1s orbitals of both carbon and oxygen, which will combine to form σ(1s) and σ*(1s).
- The 2s orbitals from carbon and oxygen will combine to form σ(2s) and σ*(2s).
- The 2p orbitals from both carbon and oxygen combine to form the π(2p) (degenerate), π*(2p) (degenerate), and σ(2p) orbitals.
The electron configuration of CO in the ground state consists of 10 valence electrons (5 from C and 5 from O). Here’s how the electrons fill the orbitals:
- σ(1s): 2 electrons (from C and O 1s AOs)
- σ*(1s): 2 electrons (from C and O 1s AOs)
- σ(2s): 2 electrons (from C and O 2s AOs)
- σ*(2s): 2 electrons (from C and O 2s AOs)
- π(2p): 4 electrons (from C and O 2p AOs, paired into two degenerate orbitals)
Step 4: Final MO Diagram
The filled MO diagram for CO would look like this (electrons are placed in the lowest available energy orbitals first):
- σ(1s) – 2 electrons
- σ*(1s) – 2 electrons
- σ(2s) – 2 electrons
- σ*(2s) – 2 electrons
- π(2p) – 4 electrons (2 in each degenerate π MO)
Explanation (300 Words)
In the CO molecule, the 10 valence electrons are distributed among the molecular orbitals formed by the atomic orbitals of carbon and oxygen. The energy levels of these molecular orbitals are determined by the ionization energies of the AOs and the relative overlap between them.
The 1s orbitals of both carbon and oxygen combine to form the lowest-energy bonding σ(1s) orbital and the antibonding σ(1s) orbital. Since these orbitals are filled first, we place 2 electrons in the σ(1s) bonding orbital and 2 electrons in the σ(1s) antibonding orbital, canceling out some of the bonding interaction.
The 2s orbitals from carbon and oxygen combine to form σ(2s) and σ(2s) MOs. These MOs are filled next, with 2 electrons in the bonding σ(2s) orbital and 2 electrons in the antibonding σ(2s) orbital.
Finally, the 2p orbitals from carbon and oxygen interact to form the π(2p) bonding orbitals (degenerate) and the π*(2p) antibonding orbitals (degenerate). In CO, 4 electrons are placed in the bonding π(2p) orbitals, filling them completely.
This distribution of electrons in the molecular orbitals leads to a stable bonding structure for the CO molecule, with a net bond order of 3 (as the number of bonding electrons minus the number of antibonding electrons divided by 2 results in a bond order of 3).