Question

The magnetic character of bulk matter is determined largely by electron spin magnetic moments, rather than by orbital dipole moments. (Nuclear contributions are negligible, as the proton's spin magnetic moment is about 658 times smaller than that of the electron.) If the atoms or molecules of a substance have unpaired electron spins, the associated magnetic moments give rise to paramagnetism or to ferromagnetism if the interactions between atoms or molecules are strong enough to align them in domains. If the atoms or molecules have no net unpaired spins, then magnetic perturbations of electrons' orbits give rise to diamagnetism. a) Molecular hydrogen gas \(\left(\mathrm{H}_{2}\right)\) is weakly diamagnetic. What does this imply about the spins of the two electrons in the hydrogen molecule? b) What would you expect the magnetic behavior of atomic hydrogen gas (H) to be?

Short answer

Answer: The weak diamagnetism of molecular hydrogen gas (H2) implies that its two electrons have paired spins, meaning that their spins are aligned in opposite directions and result in no net unpaired electron spins. On the other hand, atomic hydrogen gas (H) has only one electron with an unpaired spin, resulting in a paramagnetic behavior.

Step-by-step solution

Part a: Understanding the weak diamagnetism of molecular hydrogen gas

Molecular hydrogen gas (H2) is given to be weakly diamagnetic. Diamagnetism arises when there are no net unpaired electron spins in the substance. Therefore, the weak diamagnetism of molecular hydrogen gas implies that the two electrons in the hydrogen molecule have paired spins, which means their spins are aligned in opposite directions, resulting in no net unpaired electron spins.

Part b: Predicting the magnetic behavior of atomic hydrogen gas

In atomic hydrogen gas (H), there is only one electron present in the outermost shell. Since there is only one electron, it must have an unpaired spin. As a result, atomic hydrogen gas should exhibit paramagnetism due to the presence of this unpaired electron spin.

Key concepts

Diamagnetism

Diamagnetism refers to the property of an object to create an induced magnetic field in opposition to an externally applied magnetic field and, as a consequence, the object is repelled by the magnetic field. Materials that exhibit this behavior have no unpaired electrons and thus no net magnetic moment. The electrons, in essence, pair up with opposite spins, which cancels out their magnetic effects.

For example, molecular hydrogen gas \( \left(\mathrm{H}_{2}\right) \) is weakly diamagnetic, which means that the two electrons it has are paired with opposite spins. Therefore, when exposed to a magnetic field, molecular hydrogen gas will not be strongly attracted to the magnet and will exhibit a slight repulsion.

Paramagnetism

Paramagnetism occurs in materials that have at least one unpaired electron. Unpaired electrons have magnetic moments that can align with an external magnetic field, which causes the material to be weakly attracted to a magnet. This attraction is substantially weaker than ferromagnetism and is only noticeable when in the presence of a strong magnetic field.

Atomic hydrogen gas \(\text{H}\) exemplifies paramagnetism as it contains a single electron, which is inherently unpaired. This unpaired electron responds to magnetic fields, making atomic hydrogen paramagnetic. It's important to note that paramagnetic materials do not retain magnetization in the absence of a magnetic field, in contrast to ferromagnetic materials.

Electron Spin

Electron spin is a fundamental property of electrons, reflecting that they have an intrinsic form of angular momentum. Spin can be thought of as a small magnet due to the electron's charge and its spin. It is a quantum property, meaning the spin can only be in one of two states, commonly referred to as 'up' and 'down'.

When electrons in an atom have their spins paired, the atom as a whole does not have a net magnetic moment. An unpaired electron, on the other hand, gives rise to a magnetic moment, which may lead to paramagnetic or ferromagnetic behavior, depending on the interactions with other nearby atoms or ions.

Ferromagnetism

Ferromagnetism is a phenomenon where certain materials can become permanently magnetized. This is because the material's atoms or ions have magnetic moments that are capable of aligning together in the same direction, creating what are called magnetic domains. When a sufficient number of these domains are aligned, the material can produce a detectable macroscopic field.

Common examples of ferromagnetic materials include iron, cobalt, and nickel. These materials have unpaired electrons and the interaction between atoms in these materials is so strong that it creates a uniform alignment of the magnetic moments. Ferromagnetic materials not only respond to an external magnetic field but retain their magnetism after the external field is removed.