[melaniejs (OP)]

V has a question:

In mercury the two 6 s valence electrons are tightly held to the nucleus because of relativistic effect. As such the two electrons are not free to float around the nucleus and follow electron sea model like in other metals. Mercury behaves more or less like an inert gas.

In these circumstances how can mercury be classified as a metal?

Anyone here knows?

[Kryptid]

Mercury is much more reactive than the noble gases, as it forms quite a few chemical compounds and does not require any special circumstances to do so. As far as I know, none of the noble gases react with sulfur or carbon, but mercury does. Since it also conducts electricity, its valence electrons must be free enough to flow around.

[chiralSPO]

It is true that mercury holds its electrons a little closer in than other transition metals, and that is reflected by its relatively low boiling point (high vapor pressure) compared to other transition metals (about 360 °C for Hg, vs 770 °C for Cd, which is just above Hg on the periodic table). But this is a far cry from those of noble gasses (roughly –100 °C for Xe).

And as pointed out by Kryptid, mercury also conducts electricity like metals do (increasing conductivity with decreasing temperature).

So clearly mercury is NOT a noble gas, and at STP it is a metal. Why is this? The electrons in mercury actually ARE able to interact with neighboring atoms to form a sea of electrons, despite the claims of the OP. I also wouldn't be surprised if there were also some contributions from the empty 6p orbitals.

https://iopscience.iop.org/article/10.1088/0953-8984/11/24/303/meta
This article itself is behind a paywall, but the abstract says:

The reason for the lowering of the DOS at the Fermi level below the free electron value is the hybridization of the narrow d band with the much broader sp band. This opens the so-called Fano gap in the sp density of states, pushing states from the middle of the sp band to the edges. However, the overall (sp+d) density of states has no deep or pronounced local minimum at the Fermi energy.