Source principles of general chemistry 3 rd edition

Source: Principles of General Chemistry, 3 rd edition. Silberberg, M.pdf p. 341
*The energies of the molecular orbitals are in the order σ
2p
, ᴨ
2 p
, ᴨ *
2 p
, σ *
2 p
. This is for the
second energy level. These diagrams (Figure 5.14) are for the homonuclear (same type of
atoms) diatomic molecules.

Figure 5.14. Energy Level Diagrams of Diatomic Molecules from H
2
to Ne
2
.
Source: Chemistry, 10 th edition. Whitten et al. pdf p. 334
TIPS on SOLVING for the Molecular Orbital Diagram.
Source: Chemistry, 10 th edition. Whitten et al. pdf p. 335
MO theory is often the best model to predict the bond order, bond stability, or
magnetic properties of a molecule or ion. The procedure is as follows:
1. Select (or draw) the appropriate MO energy level diagram.
2. Count the total number of electrons in the molecule or ion.
3. As each electron is placed in an available orbital having the lowest energy, the Pauli
Exclusion Principle and Hund’s Rule must be followed.
4. Calculate the bond order = ½ (bonding e - s - antibonding e - s)
5. Use the bond order to evaluate stability.
6. Look for the presence of unpaired electrons to determine if a species is paramagnetic
(with unpaired electrons) or diamagnetic (with paired electrons).

Heteronuclear Diatomic Molecule: NO
For diatomic molecules that are made up of different atoms, the molecular orbital
diagram is slightly different with the more electronegative atom skewed lower than the less
electronegative atom. This means that the more electronegative atom has lower energy
compared to the less electronegative one. Example is NO, nitric oxide (Fig. 5.15).
Figure 5.15. Energy Level Diagrams of Heteronuclear Diatomic
Molecule, NO. Source: Chemistry, 10 th edition. Whitten et al. pdf p. 339
The energy difference
between the molecular
orbitals is small signifying
that the molecule is
slightly polar. (remember
the difference in
electronegativities affects
the polarity of the whole
molecule).
Bond order = ½ (Bonding
MO e
-
s – Nonbonding
MO e - s)
= ½ (10 – 5)
= ½ (5)
= 2.5
Therefore, NO is quite a
stable molecule.

Heteronuclear Diatomic Molecule: HF
Figure 5.15. Energy Level Diagrams of Heteronuclear Diatomic
Molecule, NO. Source: Chemistry, 10 th edition. Whitten et al. pdf p. 341
Important Terms
Molecular orbital (MO) An orbital resulting from overlap and mixing of atomic orbitals
on different atoms. An MO belongs to the molecule as a whole.
Antibonding molecular orbital A molecular orbital higher in energy than any of the
atomic orbitals from which it is derived; when populated with electrons, lends instability
to a molecule or ion. Denoted with an asterisk (*) superscript on its symbol.
Bonding molecular orbital A molecular orbital lower in energy than any of the atomic
orbitals from which it is derived; when populated with electrons, lends stability to a
molecule or ion.
Bond order Half the number of electrons in bonding orbitals minus half the number of
electrons in antibonding orbitals.