Over the course of our chemistry studies, we learn how certain branches of physics—particularly quantum mechanics and thermodynamics—shape chemical behavior. Einstein's notoriously challenging theory of relativity, however, is often viewed as relevant only to black holes, GPS satellites, or particles moving near the speed of light, rather than to chemistry. Yet relativistic effects are essential for understanding the physicochemical properties of many elements—a connection that may be less counterintuitive than it initially appears. In the following lines, we provide a glimpse into relativistic chemistry^1-3 using the familiar examples of gold and mercury, as well as the inert pair effect in heavy p-block elements, before extending the discussion to superheavy elements and the prediction of elements that remain to be discovered. To set the stage, we first briefly discuss how relativistic effects arise within atoms.

Relativity and the Electrons Within Atoms

One of the central ideas of relativity is that the mass of a particle increases as its speed approaches the speed of light. This relates directly to the subatomic particles within atoms: as formulated in the relativistic Bohr model, explained elsewhere,³ the speed of an electron is proportional to the atomic number (Z) of the element. For larger Z, the electrons move increasingly faster, and in the heaviest atoms they reach a significant fraction of the speed of light. Under these circumstances, the electrons behave as if they have greater mass, hence influencing the chemical and physical properties of such atoms, as understood by the combined effects of the classic Bohr model and electrostatic Coulomb attraction.

The true impact of these relativistic effects emerges when examining specific elements, where they reveal subtle connections between atomic motion and macroscopic properties, as illustrated below.

The Hidden Relativity of Gold

Gold provides one of the most striking examples of relativistic chemistry. For thousands of years, it has been prized for its brilliant yellow color and remarkable resistance to corrosion. Unlike most metals, gold remains shiny even after centuries of exposure to air and moisture.

On closer thought, this behavior may seem surprising. Gold and cesium both belong to the sixth period of the periodic table, and each has a single outermost 6s electron, suggesting that they should exhibit similar chemistry. Yet cesium, a highly electropositive alkali metal, readily loses this electron and reacts violently with air or water, whereas gold is exceptionally resistant to oxidation. The explanation of this divergence goes further than the general trend of decreasing atomic size across the period and is rooted in relativity.

Gold has a much higher atomic number than cesium (79 versus 55), so its electrons move at speeds approaching a significant fraction of the speed of light. At these velocities, relativistic effects cause the 6s orbital to contract due to the increase in electron mass (r ∝ m⁻¹ in the classic Bohr model)—a phenomenon known as relativistic contraction. The reduced orbital size strengthens the electrostatic Coulomb attraction between the electron and the nucleus, making it more tightly bound and less readily removed from the atom. This relativistic stabilization of the valence electron is one of the primary reasons for gold's chemical inertness and its ability to retain its luster over ages (Figure 1).

Figure 1 - Everlasting "relativistic" gold on rusted Mycenaean swords (1400-1300 BC). (Photograph by A. Giumlia-Mair CC BY-NC-ND 4.0, Heraklion Archaeological Museum, Crete)

Gold is not the only metal whose chemistry is shaped by relativity. Platinum, with atomic number 78, is the next most oxidation-resistant metal and sits immediately to the left of gold in the periodic table. Like gold, platinum possesses a sufficiently high atomic number for relativistic effects to stabilize its valence 6s electron through orbital contraction, strengthening the electrostatic Coulomb attraction between the electron and the nucleus and contributing to its high resistance to oxidation.

That is why gold and platinum are called noble metals: they show low chemical reactivity, like noble gases. The important nuance is that the latter are chemically inert because their electron shells are completely filled. In contrast, noble metals achieve their resistance to chemical reactions through the relativistic stabilization of their valence electrons. Thus, while a closed-shell configuration represents one pathway to chemical inertness, relativistic effects provide another mechanism by which atoms can become resistant to chemical transformation.

We have seen that gold is resistant to oxidation, but what about reduction? Interestingly, its reduction behavior is again linked to relativistic effects. The relativistic contraction of gold's 6s orbital stabilizes the valence electron, giving gold a high tendency to attract and accept an additional electron—almost behaving like an electronegative element despite being a metal—which leads to the formation of the auride anion, Au⁻ (6s²).

Where does this electron come from? The answer is the most electropositive element in the periodic table: cesium. Cesium readily loses its 6s electron to form Cs⁺, while gold accepts that electron to form Au⁻. The two ions then combine to produce the ionic compound cesium auride, Cs⁺Au⁻, often described as a relativistic salt.

This unusual chemistry extends further: CsAu can be synthesized through the direct reaction of cesium and gold in a 1:1 stoichiometric ratio under controlled conditions, transforming two metallic elements into an ionic compound (Figure 2, right)—a process that defies the conventional metal–nonmetal pattern of ionic bonding. What's more, the relativistic salt dissolves much like sodium chloride (NaCl), dissociating into Cs⁺ and Au⁻ ions in a suitable solvent (Figure 2, left).

Figure 2 - CsAu: an unusual, soluble relativistic salt forming golden-yellow crystals. (Reproduced from Martin Jansen CC BY-SA 4.0, via Wikimedia Commons)

Relativity influences not only the chemical behavior of heavier elements but also their physical properties, including their optical ones. Have you ever wondered why gold is one of the few metals in the periodic table that does not have the white silvery appearance of most metals?

The distinction lies in the way relativistic effects modify gold's electronic structure. The color of metals is determined by the interaction between incoming light and the movement of electrons between different energy levels. In silver, for example, which belongs to the same group as gold in the periodic table, the energy difference between the filled d orbitals and the higher-energy s orbitals corresponds to an ultraviolet electronic transition responsible for its optical properties. Because ultraviolet light is invisible to our eyes, silver does not absorb visible wavelengths; instead, it reflects most visible light, giving it the characteristic shiny, silvery appearance common to many metals.

In gold, on the other hand, relativistic contraction reduces the energy gap between the d and s orbitals, shifting the electronic transition from the ultraviolet into the visible range. This shift causes gold to selectively absorb blue light while reflecting the complementary colors, giving the metal its bright yellow appearance.

When Relativity Turns Mercury into a Liquid

Beyond noble metals, mercury offers another dramatic demonstration of relativistic chemistry: it is the only metal that is liquid at room temperature. With an atomic number of 80, mercury is adjacent to gold in the periodic table and experiences significant, though slightly weaker, relativistic effects.

In mercury, these effects are expressed not primarily through chemical inertness, but through its metallic bonding behavior. The relativistic contraction of the 6s orbital stabilizes the two electrons occupying the orbital, holding them more tightly near the nucleus. Because these electrons are less available for metallic bonding, the interactions between mercury atoms become weaker. Consequently, the metal has an unusually low melting point of −39 °C, allowing it to remain liquid under ambient conditions. In fact, without relativity, mercury would likely lose its most familiar feature—it would no longer be a liquid metal at room temperature, but a solid.

Worth noting, gold and mercury—two “relativistic” elements—interact readily, with gold dissolving in liquid mercury to form an amalgam. This well-known property was exploited by early gold seekers, who used liquid mercury to extract the precious metal from rocks and ores. The mercury selectively combined with the gold, enabling its separation from the surrounding material. Subsequent heating evaporated the mercury, leaving behind the recovered metal.

Inert Pair Effect

Relativistic chemistry manifests itself in the inert pair effect as well, observed among the heavy p-block elements thallium, lead, and bismuth, which lie close to mercury in the periodic table. As the nuclear charge increases, the outermost ns electrons experience relativistic contraction, bringing them closer to the nucleus. This makes these electrons less available for chemical bonding, causing them to behave as an "inert pair". As a result, the lower oxidation states—Tl(I), Pb(II), and Bi(III)—are significantly more stable than the higher oxidation states, Tl(III), Pb(IV), and Bi(V), that would otherwise be expected from simple periodic trends. It follows that relativity also accounts for the characteristic oxidation-state preferences of neighboring heavy elements, demonstrating that Einstein's theory subtly shapes the chemistry of the periodic table.

Relativistic Effects in Superheavy Elements

The story continues into the realm of superheavy elements, where relativistic effects become even more pronounced. One notable example is flerovium (Fl, atomic number 114), located in group 14 of the periodic table, directly below lead. Because only a few atoms of flerovium have ever been produced, its chemical properties are extremely difficult to study experimentally. Nevertheless, early theoretical calculations suggested that relativistic stabilization of its valence electrons could make flerovium inert, with a chemical behavior approaching that of a noble gas rather than a typical member of the carbon group. A similar trend is predicted for copernicium (Cn, atomic number 112), whose strongly contracted electrons give it a noble-gas-like character despite its position below mercury in group 12 of the periodic table.

In the context of superheavy elements, relativity may even impose a numerical limit on how far the periodic table can extend. As mentioned earlier, in the relativistic Bohr model,³ the velocity of electrons increases with increasing atomic number (Z). Still, nature ultimately restricts this progression: no electron can move faster than the speed of light. Applying this constraint leads to a maximum atomic number of Z = 137, which corresponds to the inverse of the fine-structure constant (α ≈ 1/137).

This intriguing relativistic limit became widely known through discussions by the famous physicist Richard Feynman, who emphasized its significance for atomic structure. Accordingly, the hypothetical element with Z = 137 is sometimes informally referred to as feynmanium (Fy) in his honor, although this name has not been officially recognized by IUPAC.

A Final Perspective

All the examples discussed above highlight a fascinating lesson: chemistry is deeply connected to physics. Many unexpected chemical behaviors can be grasped when fundamental physical principles, like relativity, are applied. Relativistic effects are not limited to unusual elements; they also play a role in familiar technologies, from lead-acid batteries used in automobiles to advanced materials containing heavy elements. A basic understanding of physics can therefore open the door to new concepts and a deeper appreciation of the underlying principles that govern matter. After all, in chemistry, too, everything is relative...

Acknowledgment

An AI-based language tool was used solely for improving language clarity and readability. The author takes full responsibility for the content and integrity of the manuscript.

Further Reading

1.    Pyykkö, P. Relativistic Effects in Chemistry: More Common Than You Thought. Annu. Rev. Phys. Chem. 2012, 63, 45–64.

2.    Das, A.; Das, U.; Das, R.; Das, A. K. Relativistic Effects on the Chemistry of Heavier Elements: Why Not Given Proper Importance in Chemistry Education at the Undergraduate and Postgraduate Level? Chem. Teach. Int. 2023, 5, 365–378.

3.         Kuntzleman, T. Search for the Final Element. ChemEd X 2016. https://www.chemedx.org/blog/search-final-element