Relativity phenomenon : Why is mercury liquid and gold soft?

One of many reasons why I like chemistry is it can explain many phenomenon clearly. We may be ever ask, “Why is mercury liquid besides it is a metal?” After I have tried to find this answer for long time, I earned this answer when I studied Inorganic chemistry 3 several days ago.

Tetesan raksa

If in the last articles, we discussed “life at molecular level”, we must look world at smaller level that’s “life at atomic level” and little knowledge about modern physic to answer this phenomenon. Electron rotates around nuclei at certain orbital that have appropriate energy with electron’s energy. Orbital is a space or region where have maximum probability to find electron in there and in classical physic it is called orbit. For movement of electron, there is a pull force between electron and proton (nuclei) because they have opposite charge. This force is known as Coulomb force. Its magnitude is proportional with number of protons in the nuclei.

From periodic table we know that electron configuration of mercury is [Kr]4f¹⁴5d¹⁰6s². Furthermore, we also know metal bond is produced from sharing all valence electrons in a material. The strength of this bond rises if the number of valence increase. Number of valence electrons in mercury can determine easily from electron configuration that is two electrons which same with barium [Kr]6s². But, their melting point is very different. Barium melts at 727 ⁰C and mercury -38,7⁰C.

This phenomenon is caused by relativity effect. For people which ever studied relativity in modern physic or elementary physic at TPB and still remember it until now will understand this effect easily, but don’t worry if we didn’t ever study it or we ever study but forgot it, I will give short explanation. Concept of relativity is postulated by Albert Einstein. Reality, there are two kinds of relativity that are general and special theory relativity, but we only will use special theory relativity to explain that phenomenon. There are two basics of this concept. First, the laws of physics must be the same in all inertial reference frames. Second, velocity of light must have same value at all reference frames, regardless of velocity movement of observer or object. Its concept cause many new phenomenon such us length contraction, mass of thing be more heavy, and time be longer. It’s very interesting, isn’t it?. But, we only can see these phenomenons if we move with high velocity that our relative velocity close to light speed

We back to mercury phenomenon. From periodic table, we know mercury have number of proton in nuclei greater than barium. Consequently, Coulomb force between 6s electron and nuclei in mercury is stronger than barium hence cause 6s electron must rotate around nuclei faster in order to don’t fall to nuclei. In mercury, the electron velocity close to light speed so cause mass of 6s electron be more heavy. Since its weight increased, its energy is lower ( the magnitude of its energy increase but it have negative sign). It cause distance between 6s electron with nuclei become shorter. Consequently, electron in 6s orbital that ought to be valence electron is attracted by nuclei even it become inner electron and 5d electron be outer one. Since valence electrons become inner electron and 5d orbital of mercury is fully occupied, metal bond in the mercury become very weak. Hence, it is liquid at room condition.

Gold

We also can explain why gold soft with using same concept. We have known from periodic table that electron configuration of gold is [Kr]4f¹⁴5d¹⁰6s¹. Movement of electron in 6s orbital of gold also is affected by relativistic phenomenon. 6s orbital in gold become inner orbital and 5d outer orbital, but when relativistic phenomenons occur, other phenomenon happen that there is transition an electron from 5d to 6s orbital hence electron configuration become [Kr]4f¹⁴6s²5d⁹. It effect although valence electron of gold become core electron like mercury, orbital 5d has a non pairing electron that can use for metal bond despite in spite of their bond is weaker than general metal. Consequently, dislike mercury despite gold is soft, it is still solid.

Actually, Relativity phenomenon is not only mercury and gold but also any weight element that have large number of protons that doubly have f orbital such us cerium, uranium, etc. Furthermore, overall of these effect make new phenomenons that are very familiar in the chemistry, they are lanthanide and actinide contraction.

If you want discuss about other phenomenon at molecular level, you can send email to me or comment this article. I am very glad if you also give a comment about my English.

Thank you.

Reference:

• Lecture Inorganic Chemistry III by Bambang Prijamboedi
• http://www.webelements.com