If you ask a person, what color is the most expensive? Nine out of ten people will say it is gold, because gold is the color of gold. Gold, a rare metal, is not only expensive, but also a favorite of the royal families of all countries in the world. It is often made into various beautiful jewelry that symbolizes power and status.
Indeed, in a metal family with many members, silver-white aluminum, silver, magnesium, red copper, black iron powder and manganese, none of them are as bright as the color of gold. Gold is like hanging. The dazzling sun in the sky.
So why does gold appear golden yellow, while other metals such as silver appear silvery white?
To understand the special features of gold, you must first understand the general principles of metal coloration.
Energy level transition
We know that the internal structure of a metal atom is like this: a positively charged nucleus in the middle is surrounded by a large number of negatively charged electrons, which are distributed in the electron shell around the nucleus according to certain rules. The energy of each layer of electrons is different, high and low. When an electron at a low energy level absorbs light energy in a certain wavelength, it will "run" to the electron layer with a higher energy. This is called energy. Level jump. Which wavelength band of light energy an electron can absorb depends on the energy level difference between the two layers. Only when the energy of the external light wave and the energy level difference are the same, the electron can successfully transition from one low energy level to another high energy level. In other words, if the energy level difference is large, the electrons will absorb light in the higher energy band, and if the energy level difference is small, the electrons will absorb light in the lower energy band. All other light is reflected and received by the eyes.
Therefore, the light reflected by metal into human eyes determines the kind of colors we see, and the energy level transition of electrons determines what kind of light reflects into our eyes. If the light wave absorbed by the electron happens to be in the visible light region, then the metal will show a complementary color that absorbs light, and the light that is not absorbed will be reflected into our eyes. For example, an object that has absorbed yellow light will appear blue, and an object that has absorbed orange light will appear cyan. However, because not all complementary colors can be seen by the human eye, what we can see is only a small band of visible light. Therefore, if the complementary color of the incident light absorbed by the metal is not in the visible light range and is a kind of light that we cannot see, then the metal tends to appear white.
Of course, the golden yellow color of gold is also the result of the electronic energy level transition, but it is a more unique energy level transition.
Inside the metal, due to the relativistic effect, the energy of the inner orbital will show a downward trend, which means that the inner orbital will be closer to the nucleus, and the attraction of the nucleus to the inner electron orbit will increase. This is called relativistic contraction. The relativistic contraction in turn makes the electron cloud near the nucleus more densely gathered around it, and the shielding effect increases, making the nucleus’s attraction to the outer electrons weaker, leading to a trend of outward expansion of the outer electron orbits, which is called relativistic expansion.
Compared with light and heavy atoms, the relativistic effect of heavy atoms is more significant because of the larger mass of heavy atoms. In other words, compared with metals such as silver and iron, the relativistic effects of gold, shrinkage and expansion are more significant. This reduces the energy level difference between the two outermost layers of gold, making it choose to absorb visible light with a longer wavelength and lower energy, which is equivalent to 539nm blue-violet light, showing a complementary color-bright red and yellow. Compared with gold, the relativistic effect of silver is not so obvious, so the energy level difference between its outermost two layers is larger, which allows silver to choose to absorb higher-energy photons and ultraviolet light, thus showing a silvery white color.
Now that we know the source of gold, we have the opportunity to change its color. For example, some special methods can be used to grind gold into nano-scale ultra-fine gold powder, and then put them in water to form a colloid. At this time, they will show different shades due to the change in the absorption light band. Red. Moreover, if you further control the shape of the gold particles, you can also make them into a variety of colors, red, orange, yellow, green, blue, indigo, purple, any color you like.