Understanding Group 1 Soft Metals and Reactivity

Group 1 of the periodic table contains the alkali metals. These elements are known for being soft metals, and they are highly reactive. Their reactivity is due to having just one electron in their outermost shell, which they readily lose to form a positive ion.

When exposed to air, these reactive metals can react with various components, including oxygen, moisture (water vapor), and carbon dioxide. This reaction often leads to the formation of a layer on the metal's surface, which we observe as tarnishing.

Analyzing the Tarnishing of Group 1 Metals

The question asks about a soft metal in Group 1 that tarnishes within a few seconds of exposure to air. Let's consider the common characteristics of Group 1 metals when exposed to air:

• They are very reactive.
• They react with oxygen to form oxides (e.g., \(4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}\)).
• These oxides can then react with moisture to form hydroxides (e.g., \(\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}\)).
• Hydroxides and oxides can also react with carbon dioxide from the air to form carbonates (e.g., \(2\text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O}\)).
• This combination of reactions causes the shiny metallic surface to dull rapidly, which is the process of tarnishing.

While all alkali metals are reactive, Sodium is a common example that visibly and rapidly tarnishes upon cutting or exposure to fresh air. Its reactivity is significant enough that the tarnishing effect is observed almost immediately, within seconds.

Evaluating the Options

Let's look at the provided options:

• Barium: Barium is in Group 2 of the periodic table (alkaline earth metals). While reactive, it is not a Group 1 metal.
• Sodium: Sodium (Na) is a soft metal located in Group 1. It is known to be highly reactive with air and moisture, tarnishing very quickly.
• Strontium: Strontium is also in Group 2 of the periodic table. It is not a Group 1 metal.
• Francium: Francium (Fr) is in Group 1, but it is extremely rare and highly radioactive. While it would be expected to be even more reactive than Sodium (reactivity increases down the group), it is not a commonly handled metal, and its properties are studied differently due to its instability.

Based on the common properties and handling of these elements, Sodium fits the description of a soft metal in Group 1 that visibly tarnishes within a few seconds of exposure to air.

Conclusion

Sodium is a highly reactive alkali metal that readily reacts with oxygen, water vapor, and carbon dioxide in the air, causing its bright, shiny surface to dull or tarnish very quickly, often within seconds of being cut or exposed.

Revision Table: Reactivity of Group 1 & 2 Metals

Additional Information on Alkali Metal Reactivity

Alkali metals like Sodium are typically stored under inert substances such as mineral oil or kerosene. This is to prevent them from reacting with the air and moisture, preserving their metallic state. The reactivity of alkali metals generally increases as you move down the group due to the increasing atomic size and decreasing ionization energy, making it easier to lose the valence electron.

For instance, Potassium is even more reactive than Sodium and ignites spontaneously in air. However, among the commonly handled alkali metals, Sodium's rapid tarnishing within seconds upon cutting is a very characteristic and easily observable property.