Elements A, B and C occur as Dobereiner’s triads. If the atomic mass of A is 40 and that of C is 137, what will be the atomic mass of B?
88
Understanding Dobereiner's Triads
Dobereiner's Triads were an early attempt to classify elements based on their properties. Johann Wolfgang Dobereiner observed that certain groups of three elements (triads) had similar chemical properties. He also noticed a relationship between their atomic masses.
According to Dobereiner's observation, in a triad arranged in increasing order of atomic mass, the atomic mass of the middle element was approximately the arithmetic mean of the atomic masses of the other two elements.
Calculating Atomic Mass in a Dobereiner's Triad
The question provides a Dobereiner's triad consisting of elements A, B, and C. Element B is the middle element. We are given the atomic masses of elements A and C.
- Atomic mass of A = 40
- Atomic mass of C = 137
The rule for Dobereiner's triads states:
\text{Atomic mass of B} \approx \frac{\text{Atomic mass of A} + \text{Atomic mass of C}}{2}
Let's substitute the given values into the formula:
\text{Atomic mass of B} \approx \frac{40 + 137}{2}
\text{Atomic mass of B} \approx \frac{177}{2}
\text{Atomic mass of B} \approx 88.5
Finding the Closest Atomic Mass for Element B
The calculated atomic mass for element B is approximately 88.5. Now, let's look at the given options:
- 35
- 120
- 88
- 74
Comparing the calculated value (88.5) with the options, the closest value is 88.
Therefore, based on Dobereiner's law of triads, the atomic mass of element B is approximately 88.
Revision Table: Key Concepts of Dobereiner's Triads
| Concept | Description |
|---|---|
| Definition | A group of three elements with similar chemical properties. |
| Atomic Mass Relationship | Atomic mass of the middle element is approximately the average of the atomic masses of the other two. |
| Examples | Li, Na, K; Ca, Sr, Ba; Cl, Br, I. |
| Significance | One of the earliest attempts to find a logical relationship between the properties and atomic masses of elements. |
| Limitation | Could identify only a few triads; did not encompass all known elements at the time. |
Additional Information on Atomic Mass and Classification
Atomic mass is a fundamental property of an element, representing the average mass of atoms of an element, calculated using the relative abundance of isotopes. Early classification systems like Dobereiner's triads paved the way for more comprehensive systems such as Newlands' Law of Octaves and eventually the modern Periodic Table developed by Mendeleev.
The periodic table arranges elements based on increasing atomic number (number of protons), which is a more fundamental property than atomic mass. While atomic mass generally increases with atomic number, there are exceptions.
Dobereiner's work highlighted the importance of atomic mass in understanding the relationships between elements and grouping them based on similar properties, a concept that was crucial for the development of chemistry.
Important Questions from Organic Chemistry
Which of the hydrocarbons are arranged as per the increasing order of their boiling points?
Which one of the following statements is NOT correct?
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1. Carbon forms the basis for all living organisms and many things we use
2. Carbon shows tetra-valency and the property of catenation
3. Carbon forms covalent bonds with itself and other elements
4. Carbon forms compounds containing triple and tetra bonds between carbon atoms
Select the correct answer using the code given below:
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