Which of the following has lowest melting point? A. Li B. Na C. K D. Cs
The Correct Answer and Explanation is:
Correct Answer: D. Cs (Cesium)
Explanation:
The melting point of an element refers to the temperature at which it changes from a solid to a liquid. Among the given options — lithium (Li), sodium (Na), potassium (K), and cesium (Cs) — all are members of the alkali metals group (Group 1) of the periodic table. Alkali metals are known for their low melting points compared to other metals, and their melting points decrease as you move down the group in the periodic table.
Here’s a quick look at the approximate melting points of these elements:
- Li (Lithium): 180.5°C
- Na (Sodium): 97.8°C
- K (Potassium): 63.5°C
- Cs (Cesium): 28.5°C
As you move down the group from lithium to cesium, the atomic size increases. This increase in atomic radius causes a weaker metallic bond between the metal atoms because the outermost electrons are further from the nucleus and less tightly held. Since metallic bonding becomes weaker, less thermal energy is required to overcome these bonds, resulting in a lower melting point.
Cesium, being one of the largest alkali metals with a highly reactive and loosely held outer electron, has the weakest metallic bonding among the options. This explains why it melts at just 28.5°C — which is so low that cesium can even melt in the palm of your hand in a warm environment.
In summary, cesium has the lowest melting point among the listed alkali metals because of its large atomic radius and weak metallic bonds, making option D the correct choice.
