Why is B2 paramagnetic while C2 is diamagnetic?

To understand why B2 is paramagnetic and C2 is diamagnetic, we need to delve into the concepts of molecular orbital theory and the electron configurations of these diatomic molecules. Let’s break it down step by step.

Molecular Orbital Theory Basics

Molecular orbital (MO) theory helps us understand the behavior of electrons in molecules. In this theory, atomic orbitals combine to form molecular orbitals, which can be occupied by electrons. These molecular orbitals are categorized as bonding or antibonding orbitals. Electrons tend to fill the lowest energy orbitals first, following the Pauli exclusion principle and Hund's rule.

Electron Configuration of B2

Boron (B) has an atomic number of 5, which means it has 5 electrons. When we consider the B2 molecule, we have a total of 10 electrons to distribute in the molecular orbitals. The molecular orbital filling for B2 is as follows:

• σ(1s)²
• σ*(1s)²
• σ(2s)²
• σ*(2s)²
• σ(2p)²
• π(2p)²

In B2, the relevant molecular orbitals for the valence electrons are the σ(2p) and π(2p) orbitals. The filling results in:

• σ(2p) has 2 electrons (bonding)
• π(2p) has 2 electrons (bonding)

However, the key point is that the π(2p) orbitals are filled with two electrons, but they are degenerate (equal in energy). This means that each π orbital can hold one electron before pairing occurs. In B2, we have two unpaired electrons in the π(2p) orbitals, which gives it a net magnetic moment, making B2 paramagnetic.

Electron Configuration of C2

Now, let’s look at carbon (C), which has an atomic number of 6. For the C2 molecule, we have a total of 12 electrons. The molecular orbital filling for C2 is:

• σ(1s)²
• σ*(1s)²
• σ(2s)²
• σ*(2s)²
• σ(2p)²
• π(2p)⁴

In C2, the relevant molecular orbitals for the valence electrons are again the σ(2p) and π(2p) orbitals. The filling results in:

• σ(2p) has 2 electrons (bonding)
• π(2p) has 4 electrons (2 pairs in two degenerate π orbitals)

In this case, all the electrons in the π(2p) orbitals are paired. Since there are no unpaired electrons in C2, it does not have a net magnetic moment, making it diamagnetic.

Summary of Differences

To summarize:

• B2 has two unpaired electrons in the π(2p) orbitals, making it paramagnetic.
• C2 has all paired electrons in its molecular orbitals, resulting in it being diamagnetic.

Understanding these differences in electron configurations and their implications on magnetic properties provides a clear insight into the behavior of these molecules. This is a fundamental concept in chemistry that illustrates the relationship between molecular structure and physical properties.