Which of the following metals is the least reactive?

Analysis

The question asks us to identify the least reactive metal among the given options. Reactivity in metals is determined by their ability to lose electrons to form positive ions (cations). The more easily a metal loses electrons, the more reactive it is.

Concept: The Reactivity Series

In Chemistry, the Reactivity Series (or Activity Series) is a list of metals arranged in descending order of their reactivity. Metals at the top of the series are highly reactive (e.g., Potassium, Sodium), while those at the bottom are very unreactive (noble metals).

The order of the metals provided in the options, from most reactive to least reactive, is:

1. Sn (Tin)
2. Pb (Lead)
3. Hg (Mercury)
4. Au (Gold)

Step-by-Step Solution

1. Evaluate Sn (Tin): Tin is a moderately reactive metal that reacts slowly with acids and is often used in plating to prevent corrosion.
2. Evaluate Pb (Lead): Lead is less reactive than Tin. It sits below Tin in the reactivity series.
3. Evaluate Hg (Mercury): Mercury is a transition metal that is significantly less reactive than both Lead and Tin. It does not react with dilute acids to displace hydrogen.
4. Evaluate Au (Gold): Gold is a noble metal. It is extremely resistant to chemical attack, oxidation, and corrosion. It is found at the very bottom of the reactivity series, making it the least reactive among the options provided.

Conclusion

Comparing the positions of these metals in the reactivity series, Gold (Au) is the least reactive. It is so unreactive that it is often found in its native (pure) state in nature.

Exam Tip: Remember the mnemonic for the reactivity series: "Please Stop Calling Me A Careless Zebra Instead Try Learning How Copper Saves Gold." (Potassium, Sodium, Calcium, Magnesium, Aluminium, Carbon, Zinc, Iron, Tin, Lead, Hydrogen, Copper, Silver, Gold). The further to the right (or bottom) the metal is, the less reactive it becomes.