Video Transcript
Which of the following alkali metals has the lowest melting point? (A) Cesium, (B) potassium, (C) sodium, (D) lithium, or (E) rubidium.
The melting point of a metal depends on the strength of metallic bonding. Metallic bonding is a strong electrostatic attraction that exists between positively charged metal cations and delocalized electrons. This electrostatic attraction is stronger when the positively charged cations and negatively charged electrons are packed into a smaller volume and there is less average distance between them.
The oppositely charged particles can be packed closer together when the metal ions are small and do not take up much space. So it takes more energy to break apart a metal lattice that is made up of smaller particles. We have determined that the metallic bonding for smaller ions is stronger. And we’ve already discussed that the melting point depends on the strength of the metallic bonding. So, for smaller ions, where metallic bonding is stronger, the melting point is higher. So we now need to determine which of the listed alkali metals is the smallest.
The alkali metals can be found in group one of the periodic table. Other than the elements listed, hydrogen and francium are also in group one. Francium is also considered to be an alkali metal, but hydrogen is not. As we descend group one, the atomic number increases, which means the number of electrons increases. Thus, the number of electron shells increases. So, as we descend the group, the size increases.
As we are looking for the alkali metal with the lowest melting point, we are looking for the metal with the weakest metallic bonding. This belongs to the largest metal. As size increases as we descend the group, the largest metal can be found at the bottom. So, the largest metal which would have the lowest melting point is Cs, cesium.
So, the answer to the question “Which of the following alkali metals has the lowest melting point?” is (A) cesium.
