Like any individual chemical element, gold has unique characteristics that make it a bright yellow metal. Thanks to these, it is also very difficult to counterfeit. Gold has a specific gravity of 19.3 grams per cubic centimeter, a property not possessed by any other metal.
This property was discovered by Archimedes in Antiquity. Although it is unlikely that the great mathematician discovered its exact density, he nevertheless proved to the ruler of Syracuse by his experiments that his crown was not made of pure gold.
In today’s article, however, we will consider other properties of gold – the reason for its luster and yellow color. They are what make gold visually appealing. This, combined with its difficulty in being mined and forged, made gold a desired metal for making jewelry and eventually money.
Moreover, the precious metal has not changed its status as a sought-after product since the dawn of human history. Mankind has been interested in gold for millennia, and the excitement it generates doesn’t seem to be waning. It’s also interesting that it seems that the older the gold, the more sought after it is.
In metals, loosely bound negatively charged electrons revolve around a positively charged nucleus. When metals are formed, the accumulation of the many electrons surrounding atomic nuclei (consisting of positive protons and negative neutrons) creates what is scientifically called an ” electron sea “. Within it, electrons are not directly bound to any atom.
The atoms that make up the metal can move relative to other atoms without breaking the bond between them. The metal is thus not destroyed by their movement. The bond is not molecular, and most metals have similar characteristics. These include high electrical conductivity, high melting point, hardness and malleability.
Light is an electromagnetic wave. When it collides with the surface of a metal, its electromagnetic field creates a “ripple effect” in the sea of electrons. They absorb the energy of the wave, which is normally in the ultraviolet light spectrum, and begin to vibrate.
As they vibrate, the negatively charged electrons in the “sea” interact with each other. Thus an electric field is created. But this is not the only phenomenon that occurs. If that were the case, every time the light hit a metal and a person touched it, they would get a slight electric shock.
The electrons actually create a second wave of light, having 2 effects. First, the total electric field of the metal remains 0. Therefore, metals do not cause electric shocks. Second, this wave is visible. This creates the metallic luster we know and defines the “color” of the metal.
The second wave created by the wave of electrons is actually a combination of waves with very different wavelengths in the visible spectrum. They are not equal in proportion, but since they include the entire spectrum, most metals have a gray-white appearance.
The periodic number of gold (Au) in Mendeleev’s table is 79, respectively in the nucleus of a gold atom there are 79 neutrons and 79 protons. For this reason, it is a relatively dense (as mentioned at the beginning) and heavy yellow metal. This also means that the electrons have to work hard to avoid hitting the nucleus, attracted by the electrostatic field.
According to Niels Bohr’s atomic model, electrons orbit around the nucleus. They retain a certain amount of kinetic energy to avoid being attracted to the nucleus. Imagine what would happen if you tied a rock to a string and started spinning it. The stone would maintain a certain distance from the center as it revolved around the stick in an orbit. When you stopped the rotation, the stone would fall in a spiral motion.
The “hard work” in question means that the electrons have to move very fast. In fact, the electrons in the gold atom orbit the nucleus at about half the speed of light. As we know from Einstein’s Special Theory of Relativity (E=mc2), the closer a particle’s speed is to the speed of light, the heavier it becomes. Because of this, the mass of the electrons orbiting around the gold atomic nucleus is increased. Calculations show that their speed causes the weight to increase by about a fifth.
But since they are already heavier, the radius along which they move around the nucleus decreases. In physics, this is called the Bohr ray. Due to its reduction and as a result of the increased electron mass, gold has a characteristic yellow glow.
However, the logic presented is not enough. If the only reason gold was a uniquely yellow metal was its periodic number, shouldn’t lead, which has 82 protons and 82 neutrons in its nucleus, be even more yellow?
This is where quantum physics comes in, according to which electrons have the nature of quanta and waves and circulate in a probabilistic cloud. This behavior is described by the atomic orbital . Depending on the layer in which the electrons are located, depending on the orbital, they have a different shape. The spherical S orbitals located in the outermost electron shell are the ones that feel the strongest attraction from the positively charged protons in the atomic nucleus.
When they have a stronger attraction to the atomic nucleus, the force with which the electrons in the other layers are attracted to the nucleus decreases. Therefore, the distance between the last (that of the s orbitals) and the penultimate (that of the d orbitals) electron shell decreases.
Let’s go back to the description above. Light falls on gold and is absorbed. But since the distance between the orbitals of the different layers is smaller because of the different forces they exert on the nucleus, it takes less energy for it to jump from the d orbitals to the s orbitals.
This makes gold unique compared to most metals and therefore does not have the typical grey-white sheen. Because of the lower energy that gold absorbs, it is no longer in the ultraviolet spectrum, but in the blue-violet spectrum. Therefore, the second wave of light that gold emits that we see contains all the colors in the visible spectrum except blue and violet.
In other words, the waves we see are in the red-green spectrum. As we know from art class, the combination of red and green results in yellow. This is why gold is a yellow metal.