About giant metallic structures

Generally speaking, the statement is correct with one exception: Mercury
Note that mercury is a liquid under room conditions although it is a metal.
The reason for explanation can be explained by the "Inert pair effect" of the electrons in mercury atoms.
First of all, the electronic configuration of mercury is:
[Xe] 4f14 5d10 6s²
And from the configuration, we can see that all sub-shells are fully filled, thus gaining extra stability and therefore the electrons are more difficult to be excited out than expected.
Moreover, another reason is the screening effect of the 6s electrons that, the effective nuclear charge on it by the nucleus is unexpectedly high. Hence the difficulty in exciting the electron out is further increased.
Therefore, integrating the above two reasons, the "sea" of electrons contains much less free electrons than expected and hence the binding between the mercury ataoms is abnoamally weak.
Finally, mercury is a liquid under room conditions and it is NOT a giant metallic structure.