Mercury has been one of the most mysterious elements (well, atleast for me). Not only is it a bad conductor of electricity, has low melting and boiling points, and extremely unreactive, unlike most metals, it is the only liquid metal. Today, in this article, we shall see as to why mercury is a liquid.

Predominantly, we shall be focusing on the fact that mercury is a liquid whereas all the other elements surrounding it (Cd- Cadmium, Ag- Silver, Au- Gold, In- Indium, and Tl- Thallium) are all usually found in their solid states. Moreover, the elements belonging to the same group (column) as mercury, which include zinc and cadmium, should have similar properties to mercury. However, they differ in their state there. So, what is the cause of this difference?

First, we have to inspect the electronic configuration of mercury. The electronic configuration refers to how the electrons in an atom are arranged in different levels (called shells) in an atom. Each shell can hold a specific amount of electrons. According to that, mercury’s electronic configuration would be as follows:

2,8,18,32,18,2

I will not be going over how this arrangement came into existence as it requires some advanced knowledge of Chemistry (however, if you are curious, check out the concepts of Bohr’s Atomic Model). Now, we shall try to establish a link between the way electrons in mercury are arranged and the state of mercury.

Now, if we go deeper into mercury’s electronic configuration, we get:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 

You don’t need to bother about how this came into existence too (however, if you are curious, check out the concepts of atomic orbitals, quantum numbers, and filling of orbitals.)

Now, if you observe the configuration I wrote, the highest number you see is in 6s² . This highest number indicates that electrons have been filled in 6s at the last. Now, ‘s’ indicates a small area inside a shell, which can hold atmost two electrons. That means that 6s has been filled, as it has two electrons, evident from the fact that 2 is written above it. This is where the reasoning comes.

There are now several reasons to mercury’s nature, I will go over them one at a time.

Firstly, as I have already told you that 6s is filled with 2 electrons, all the others ‘ones’ like 4f, 5s, 5d etc. are also completely filled. This gives mercury an unusually high stability. Now, the mantra of most atoms in chemistry is to combine and become stable. However, mercury is already stable. So, the atoms of mercury don’t form any ‘bonds’ (Bonds are attractive forces between atoms which are formed to gain stability) with other adjacent mercury atoms. So, the atoms in the structure tend to remain loose and make mercury liquid in state.

The previous paragraph was probably the best way to explain this phenomenon. Secondly, certain relativistic effects give mercury its characteristic nature. To put this effect in simple words, let us visualize an atom:

Now, in this image, the green circles are the electrons and the blue-pink region is the nucleus. The electrons have a negative charge and the nucleus has a positive charge. Now, it is natural for the nucleus to attract the electrons. Moreover, the 6s area I talked about earlier is very far away from the nucleus (The 6 in 6s indicates that it is 6 levels away from the nucleus. That distance is very far for an atom.).

However, due to high speed revolution, it tends to come closer to the nucleus quite often, which also increases its mass due to high attractive forces. So, this 6s area tends to be bonded closely to the nucleus, which is known as the ‘relativistic contraction’, as the 6s area has contracted by coming closer to the nucleus as compared to other ‘areas’ like 4f, 4d etc..

Now, unfortunately (Or fortunately, however you would rather take it), the two electrons in the 6s area are the ones which mercury uses to form bonds. Now, if those electrons are held tightly by the nucleus, there is no chance for them to form bonds. So, the atoms remain non-bonded and distant from each other, Oh, and if you were curious as to what the ‘area’ in ‘6s area’ refers to, it refers to the ‘6s orbital’, which is just a 3D space where electron are present in an atom.

Finally, an effect called the screening/shielding effect. Now, considering the below image, where I have (tried and) shaded an electron in red for better understanding.

We know that the nucleus will attract the red electron as unlike charges attract (If you didn’t know this, time to go back to school for you!). However, the adjacent electrons will start to repel the red electron as like charges repel each other. Now, due to sheer abundance of electrons, the force the nucleus exerts on the red electron will be reduced considerably due to the adjacent electrons’ force cancelling some of it out. This effect is known as the screening/shielding effect. It is usually used to identify exceptions in the usual trend of elements.

Now, contrary to this, the orbitals in which the electrons are in do not show this effect to a great extent for mercury. Due to this, the electron are bounded to the nucleus and are not allowed to react. This too makes the structure loose and liquid.

I suppose this article introduced you to a lot of different terms of what we call ‘quantum mechanics’. Don’t worry if you didn’t get some parts of it (or any part of it, for that matter), as a famous physicist, Richard P. Feynman, once said – “If you think you understand quantum mechanics, then you don’t understand quantum mechanics.” Quite contrary, right? I’ll leave you there, until my next article. Until then, Stay Home, Stay Safe.

Author: Venkata Bhamidipati