Understanding Dobereiner's Triads and Element Classification

This question asks about Dobereiner's Law of Triads, an early attempt to classify elements based on their properties. Johann Wolfgang Dobereiner, in 1829, observed that groups of three elements (called triads) with similar chemical properties could be identified, where the atomic weight of the middle element was approximately the average of the atomic weights of the other two members.

Identifying the Lithium-Sodium Triad

The question specifically mentions a Dobereiner triad that includes Lithium (Li) and Sodium (Na). We need to find the third element in this specific triad.

Dobereiner identified several triads. One prominent triad consists of alkali metals:

• Lithium (Li)
• Sodium (Na)
• Potassium (K)

Let's look at the approximate atomic masses of these elements:

• Lithium (Li): Approximately ${6.94}$
• Sodium (Na): Approximately ${22.99}$
• Potassium (K): Approximately ${39.10}$

According to Dobereiner's law, the atomic mass of the middle element (Sodium) should be close to the average of the other two (Lithium and Potassium). Let's calculate the average:

Average atomic mass ${= \frac{\text{Atomic mass of Li} + \text{Atomic mass of K}}{2}}$

Average atomic mass ${= \frac{6.94 + 39.10}{2}}$

Average atomic mass ${= \frac{46.04}{2}}$

Average atomic mass ${= 23.02}$

The calculated average atomic mass ($23.02$) is very close to the actual atomic mass of Sodium ($22.99$). This confirms that Lithium, Sodium, and Potassium form a Dobereiner triad.

Determining the Third Member

In the triad consisting of Lithium, Sodium, and Potassium, if Lithium is considered the first member and Sodium the second, then Potassium is clearly the third member.

Analyzing the Options

Let's consider the given options:

1. Potassium: As discussed, Potassium is the third member of the Li-Na-K triad and fits Dobereiner's criteria for this group.
2. Boron: Boron (B) is in a different group (Group 13) and has very different chemical properties compared to alkali metals like Lithium and Sodium. It does not form a triad with them.
3. Barium: Barium (Ba) is an alkaline earth metal (Group 2) and also has different properties from alkali metals. It is not part of the Li-Na-K triad.
4. Hydrogen: Hydrogen (H) is unique and often placed separately or in Group 1, but its properties are significantly different from the alkali metals, and it does not form a Dobereiner triad with Lithium and Sodium.

Based on Dobereiner's Law of Triads and the known triads, Potassium is the correct third member of the triad featuring Lithium and Sodium.

Conclusion

The third member of the Dobereiner triad that also features Lithium and Sodium is Potassium.

Revision Table: Key Concepts

Additional Information: Limitations of Dobereiner's Triads

While Dobereiner's Triads were an important step in the classification of elements, they had limitations:

• Not all known elements could be arranged into triads.
• For some triads, the atomic masses did not fit the average rule perfectly.
• Later developments, like Newlands' Law of Octaves and Mendeleev's Periodic Table, provided more comprehensive and accurate ways to classify all known elements.

Other examples of Dobereiner's Triads include:

• Halogens: Chlorine (Cl), Bromine (Br), Iodine (I)
• Alkaline Earth Metals: Calcium (Ca), Strontium (Sr), Barium (Ba)
• Chalcogens: Sulfur (S), Selenium (Se), Tellurium (Te)