Question

Which metal from the following alkali metals has the lowest melting point?
A. Hg
B. K
C. Rb
D. Cs

Answer 1

Hint: If we observe the options carefully, we find out that out of four options three are from the same group and one option is the only metal that is liquid at room temperature. And this metal is used in thermometers.

Step by step answers:
So first of all, we will see that out of the four options that are Mercury, Potassium, Rubidium and Caesium, we find out that Potassium, Rubidium and Caesium are from the same group.
Potassium atomic number = 19
Rubidium atomic number = 37
And Caesium atomic number = 55
So according to the trends that we follow in periodic tables we find out that as we go down the group melting point decreases. This we can explain by when any of the Group 1 metals is melted, the metallic bond is weakened enough for the atoms to move more freely, and is broken completely when the boiling point is reached. The decrease in melting and boiling points reflects the decrease in the strength of each metallic bond. As we know that the atoms in a metal are held together by the attraction of the nuclei to electrons. As the atoms increase in size, the distance between the nuclei and these electrons increases; therefore, attractions fall. The atoms are more easily pulled apart to form a liquid, and then a gas. The increased charge on the nucleus down the group is offset by additional levels of screening electrons. So melting point is determined by the distance between the nucleus and the bonding electrons.
So according to the above concept, Caesium should have the lowest melting point among group 1 elements.
As we know that Mercury is a chemical element with the symbol Hg and atomic number 80. It is the only metal which is liquid at room temperature. So its melting point should be very low than Caesium. So we can say that mercury has the lowest melting point.
Our correct option is A.

Note: We should know about Rubidium because its melting point is so low you could melt it in your hand if you had a fever (39°C). But that would not be a good idea because it would react violently with the moisture in your skin.
Caesium is so reactive that it will even explode on contact with ice! It has been used as a "getter" in the manufacture of vacuum tubes (i.e., it helps remove trace quantities of remaining gases). An isotope of caesium is used in the atomic clocks.
We should know that potassium has a white-sliver colour but it quickly oxidizes upon exposure to air, tarnishing in minutes if it is not stored under oil or grease. Potassium is essential to several aspects of plant, animal, and human life and is thus mined, manufactured, and consumed in huge quantities around the world.