Question

A metal 'M' which is one of the best conductors of heat and electricity and is used in making electric wires is found in nature as sulphide ore \[{{M}_{\mathbf{2}}}S\], name the metal 'M'.
a.) Cu
b.) Al
c.) Fe
d.) Ag

Answer 1

Hint: While Ag is the best conductor of heat and electricity bar none and is found in nature as acanthite ($A{{g}_{2}}S$) occurs in mineralogy under temperatures of 173 – 178 $^{0}C$, it is not used in making electric wires due it being ridiculously expensive. With this in mind, let us solve the given question.

Step-by-Step Solution:
A good look at the options shows us that two of the four elements are excellent conductors of electricity i.e. Copper and Silver. While Aluminium and Iron can also potentially conduct electricity, they are not as effective in conduction when compared to the other two elements due to the lack of free electrons present in their structure.
It is well known that Silver is a better conductor of heat and electricity than any other element in the periodic table due to it having a large number of free electrons, with Copper being a close second when it comes to electrical and thermal conductivity with its specific resistance being only slightly greater than Ag’s and lower than every other elements’.
The next clue about them existing in their natural sulphide ores also doesn’t help us point out one of the two since both of these elements exist naturally in their sulphide ores, Copper in Chalcocite ($C{{u}_{2}}S$) and Silver in both Acanthite for under temperatures of 173 – 178 $^{0}C$and Argentite for temperatures above 178 $^{0}C$ ($A{{g}_{2}}S$). Therefore, this does not help us differentiate between the two either.
However, it is in the making of wires where we can eliminate Ag as a potential answer as it isn’t used for wire making due to its excessive cost and the subsequent economic loss, therefore the answer to this question is Cu.
Therefore, the answer to this question is a) Cu.

Note: Argentite is cubic silver sulphide ($A{{g}_{2}}S$), which can only exist at temperatures above 173 °C, 177 °C or 179 °C. When it cools to ordinary temperatures it turns into its monoclinic polymorph, acanthite. Remember this transition of silver sulphide ores as it is critical to answer this question.