Question

Which element from following rapidly loses its its luster in air and tarnishes?

• A. Ba
• B. Be
• C. K
• D. Mg

Hint:

Alkali metals (Group 1) are highly reactive and readily oxidize in air, losing their luster. Their reactivity generally increases down the group. Alkaline earth metals (Group 2) are also reactive but often form a stable protective oxide layer. Transition metals vary in reactivity, with some being quite noble (resistant to tarnishing) and others forming stable oxide layers.

Answer 1

The Correct Option is A

Concept:
Metals lose their metallic luster when they react with substances present in air such as:

• Oxygen
• Moisture
• Carbon dioxide

This forms a dull surface layer of oxide, hydroxide, or carbonate called tarnish. More reactive metals tarnish faster.
Step 1: Compare the given elements
Options are:

• Barium (Ba)
• Beryllium (Be)
• Potassium (K)
• Magnesium (Mg)

Step 2: Check reactivity of each metal

• Be: Least reactive among these and forms protective oxide layer.
• Mg: Reacts slowly and forms thin oxide coating.
• Ba: Reactive alkaline earth metal, but less reactive than potassium.
• K: Alkali metal, extremely reactive, reacts immediately with air and moisture.

Step 3: Reason for rapid tarnishing
Potassium reacts quickly with oxygen: \[ 4K + O_2 \rightarrow 2K_2O \] It may also form peroxide or superoxide in air. Hence its shiny surface disappears very rapidly.
Step 4: Special note
Because of very high reactivity, potassium is stored under kerosene or mineral oil to prevent contact with air.
Step 5: Final Answer
The metal which rapidly loses luster in air is: \[ \boxed{\text{K (Potassium) \] Quick Tip:
Alkali metals (Group 1) are more reactive than alkaline earth metals (Group 2), so they tarnish faster.