Question

why does gallium and caesium have low melting and boiling points

Answer 1

Because they have very low melting points compared to most other metals (except for mercury). The melting point of gallium is 29.77°C (85.59°F), which is low enough that the temperature of your skin may be warm enough to melt it. The melting point of cesium is 28.44°C (83.19°F), also within range of normal skin temperature.

Now, if you want to know WHY gallium and cesium have low melting points, read on:

In the solid state, gallium atoms pack differently than atoms of other metals. Whereas most metals pack together in a fairly even pattern, gallium atoms pack together with one close neighbor and six more distant neighbors. This causes gallium to behave more like a diatomic molecule than a metal, which accounts for its very low melting temperature.

The alkali metals (of which cesium is one) have melting points that decrease as you move down the group. Theoretically, francium's melting point is even lower than gallium's melting point, but francium is so incredibly rare on Earth that you'd be hard pressed to get enough of it together in one place to witness its melting point (and then it would be so radioactive that it would generate enough heat to melt itself).

Anyway...back to cesium. As you move down the list of alkali metals, the atoms become progressively larger. The melting point of a metal depends on the attraction between metal cations and the "sea of electrons" - a movement of "loose" valence electrons - that surrounds them. As the alkali metal cations get larger, the attractive force is spread out over a wider volume and therefore diluted somewhat. As a result, the alkali metals at the bottom of the table have the weakest metallic bonding and therefore the lowest melting points.