The question of whether a metal exists that cannot conduct electricity touches on the fundamental definition of what a metal is in physics and chemistry. By standard scientific classification, the answer is no, because the ability to conduct an electrical current is the very property that defines a material as a metal. However, some metallic elements exhibit such extremely low conductivity that they challenge the typical expectation of a good conductor. These materials represent the boundary where the definition of a metal starts to break down. They are technically metals but possess resistivities millions of times higher than common conductors like copper, placing them on the far edge of the conductive spectrum.
Defining Electrical Conductivity in Metals
The high electrical conductivity seen in metals stems from their unique form of chemical bonding, often described using the classical “electron sea” model. In this model, the outermost electrons, known as valence electrons, are not tethered to any single atom. Instead, they are delocalized and freely move throughout the entire metallic structure, forming a mobile “sea” of charge carriers surrounding a lattice of positively charged metal ions.
When an electrical voltage is applied across a piece of metal, these delocalized electrons are compelled to move in a coordinated direction, creating an electrical current. This easy movement of electrons is precisely why metals like silver and copper are such efficient conductors. Electrical conductivity measures a material’s ability to allow this charge flow, and it is the reciprocal of electrical resistivity, which measures the opposition to the flow.
Even the best conductors have some resistivity, which arises from electrons colliding with the vibrating metal ions in the lattice structure. Increasing the temperature causes the ions to vibrate more intensely, increasing the frequency of these collisions and thus raising the metal’s resistivity. Despite this inherent resistance, the high concentration and mobility of free electrons ensure that a metal’s conductivity remains vastly superior to other material classes.
The Role of Electron Energy Bands
Moving beyond the classical “electron sea” model requires a quantum mechanical perspective, which uses the concept of electron energy bands to explain differences in electrical behavior between materials. When atoms combine to form a solid, their discrete electron energy levels merge and broaden into continuous bands of allowed energy: the valence band and the conduction band.
The valence band contains the electrons involved in bonding, which are restricted in their movement. The conduction band is a higher energy level where electrons are free to move and sustain a current when an electric field is applied. The key difference between material types lies in the energy separation between these two bands, known as the band gap.
In a true metal, the valence band and the conduction band either overlap or the conduction band is only partially filled. This structural feature means that electrons require virtually no additional energy to jump into the mobile conduction band. Since charge carriers are readily available, metals exhibit inherently high conductivity regardless of temperature.
In contrast, materials classified as insulators possess a very large band gap, often exceeding 5 electron volts. This energy barrier is too great for electrons to jump across under normal conditions, meaning the conduction band remains empty and no current can flow. Semiconductors fall between these two extremes, featuring a small band gap that allows some electrons to jump into the conduction band when energy is added, such as from heat or light.
Exploring High-Resistivity and Borderline Materials
While no elemental metal is a true non-conductor, certain elements exhibit exceptionally high electrical resistivity within the metal category, making them the closest materials to challenging the definition. Bismuth, for example, is a post-transition metal with a resistivity of approximately \(1.29 \times 10^{-6}\) ohm-meters at room temperature. This is nearly 75 times higher than the resistivity of annealed copper, which is about \(1.72 \times 10^{-8}\) ohm-meters.
Manganese is another metallic element known for its comparatively high resistivity, measuring around \(1.6 \times 10^{-6}\) ohm-meters. This high internal resistance is attributed to complex crystal structures and electron scattering mechanisms that impede the free flow of charge carriers more than in simple metals. These elements are still fundamentally conductors, but their conductivity is poor enough to be remarkable.
To put this metallic resistivity into context, a true electrical insulator like Teflon has a resistivity in the range of \(10^{22}\) to \(10^{24}\) ohm-meters. The difference between a high-resistivity metal like Bismuth and a true insulator spans more than 16 orders of magnitude. This gap demonstrates that even the “worst” metallic conductor is still billions of times more conductive than a non-conducting material.
Borderline Materials
Materials can be engineered to exhibit non-metallic behavior in certain states, such as amorphous metals, where the disordered atomic structure can drastically increase resistivity. Metalloids like silicon or germanium are often mistaken for non-conducting metals because of their semiconductor properties. However, their electrical behavior is defined by that small band gap, which firmly classifies them outside the definition of a metal. The high-resistivity metals and unique material states demonstrate how close some conductors come to the non-conducting boundary.