Here are the list of the melting points of the group I metals.
Metal Melting point (in Kelvin) Boiling point (in Kelvin)
Li 454 1615
Na 371 1156
K 337 1032
Rb 312 961
Cs 302 944
Fr* 900 950
Generally, the melting points of the group I metals are lower compared to other metals in the periodic table. There are certain reasons that can account to this phenomenon. First, group I metals or the alkali metals have only 1 electron in the outer shell that contributes to the "sea of metals" therefore the metallic bonding is weak. Secondly, the group I metals are not as closely packed as compared to the other metals. Consequently, they will have smaller densities and are generally softer. Since the attraction of the metal to each other is weak, not much energy is required to break them thus having a lower melting point.
Melting points of elements, along with boiling points as well, rely on the strength of of the particles that make these elements up.
Because melting points show periodic properties, we can say that elements' melting points vary based on their position in the periodic table.
Group 1 elements are grouped in such a way that they descend, or go down, on the periodic table. The reason they go down, or descend, like this is because the forces of attraction between the atoms is gradually getting more weak. Therefore, because their attraction between the atoms is weaker, the melting point decreases also.
The attraction between the atoms in Aluminum and Iron are much stronger and therefore have much higher melting points to break these bonds.