The magnetic behavior of a substance, whether weakly attracted or repelled by an external magnetic field, is determined by the arrangement of its electrons. This electronic structure dictates the material’s response to magnetism. The question of whether an ion, such as the nickel(II) ion (\(\text{Ni}^{2+}\)), is paramagnetic or diamagnetic depends on how its electrons are distributed within the available energy levels. By calculating the ion’s electron configuration and applying specific rules for electron filling, we can understand the magnetic properties of nickel compounds.
Understanding Paramagnetism and Diamagnetism
Materials exhibit two primary forms of magnetic behavior when placed near an external magnetic field: paramagnetism and diamagnetism. These behaviors are a direct consequence of the intrinsic magnetic moments generated by the electrons. The distinction between the two rests solely on the presence or absence of unpaired electrons.
Paramagnetic substances possess one or more unpaired electrons. Each electron has spin, which creates a tiny magnetic moment. When these magnetic moments are not canceled out by a partner electron, the atom or ion has a net magnetic moment. This net moment causes the substance to be weakly attracted to an externally applied magnetic field.
Diamagnetic substances have all of their electrons fully paired within their respective orbitals. The spin of one electron is canceled out by its partner, resulting in a net magnetic moment of zero. When a diamagnetic material is placed in a magnetic field, the field induces a very weak, opposing magnetic moment, causing the substance to be weakly repelled.
Deriving the Electron Configuration of the Nickel Ion
To determine the magnetic state of the nickel(II) ion, we must first establish its electron configuration, beginning with the neutral nickel atom. Nickel (\(\text{Ni}\)) is a transition metal with 28 electrons. The ground-state electron configuration of the neutral nickel atom is \([\text{Ar}] 4s^2 3d^8\). The argon symbol represents the stable inner core of 18 electrons.
The formation of the nickel(II) ion, \(\text{Ni}^{2+}\), requires the removal of two electrons. For transition metals, electrons are always removed from the orbital with the highest principal quantum number (\(n\)) first, because these electrons are spatially further from the nucleus.
The \(4s\) orbital (\(n=4\)) is higher than the \(3d\) orbital (\(n=3\)). Therefore, the two electrons are removed from the \(4s\) orbital. The resulting electron configuration for the \(\text{Ni}^{2+}\) ion is \([\text{Ar}] 3d^8\). This configuration is the foundation for determining the ion’s magnetic behavior.
Applying Hund’s Rule to Determine Magnetic State
The magnetic state of \(\text{Ni}^{2+}\) is determined by applying Hund’s rule to the \(3d^8\) configuration. The \(d\) subshell is composed of five separate orbitals, all of which are degenerate, meaning they possess the same energy level. Hund’s rule dictates the lowest-energy arrangement for electrons filling these orbitals.
This rule states that electrons will first occupy each orbital singly before any orbital is occupied by a second, paired electron. All singly occupied electrons must possess the same spin. In the \(\text{Ni}^{2+}\) ion, we have eight electrons to distribute across the five \(3d\) orbitals.
The first five electrons will each fill one of the \(d\) orbitals with parallel spin. The remaining three electrons must then pair up in three of the five orbitals. This distribution leaves two orbitals with only a single electron. Because the \(\text{Ni}^{2+}\) ion possesses two unpaired electrons in its \(3d\) subshell, it is classified as a paramagnetic species.
Observing Magnetic Behavior
The theoretical conclusion that \(\text{Ni}^{2+}\) is paramagnetic is consistently validated through experimental measurements of its magnetic susceptibility. Magnetic susceptibility is a measure of how much a material is affected by a magnetic field, and it is directly related to the number of unpaired electrons.
One of the most common laboratory methods used to measure this property is the Gouy balance. This sensitive instrument measures the apparent change in the mass of a sample when it is suspended in a powerful magnetic field. A paramagnetic sample, being attracted to the field, will appear to weigh more.
The measured magnetic susceptibility is used to calculate the magnetic moment, often expressed in units of Bohr magnetons (\(\mu_B\)). This experimental value is closely correlated with the theoretical “spin-only” magnetic moment, which is calculated directly from the number of unpaired electrons (\(n\)). For the \(\text{Ni}^{2+}\) ion with \(n=2\), the calculated spin-only magnetic moment is approximately \(2.83\) Bohr magnetons. The experimental measurement provides physical proof that the nickel(II) ion behaves as a paramagnetic species.