Are all metals solid?

All the metals, except for mercury (Hg) are solid at room temperature. This is one of the characteristics of metals. Other notable characteristics include electrical and thermal conductivity, malleability, ductility, lustre, etc. 

The liquid state of mercury at room temperature is one of the reasons it is used in thermometers.

The reason for the solid state of metals at room temperature is the sharing of electrons between different atoms. Metals have low ionization energies and find it easy to lose valence electrons. One can think of metals as a large network formed by interconnected metal atoms in a sea of valence electrons. The easy sharing of these valence electrons results in a very strong bonding of metal atoms and this gives rise to a high melting point. This makes the metals solid at room temperature.

Hope this helps.