All questions of Periodic Classification of Elements for UPSC CSE Exam

Understanding the Center of the Periodic Table
The periodic table is organized based on the atomic number, which reflects the number of protons in an atom's nucleus. The concept of a "center" can be interpreted in various ways, but in terms of atomic structure and properties, iron (Fe) is often considered the central element.
Why Iron is Considered Central
- Atomic Number: Iron has an atomic number of 26, which places it in the middle of the first 40 elements in the periodic table.
- Transition Metals: Iron is one of the first transition metals and plays a crucial role in the transition from metals to metalloids and nonmetals.
- Stability and Abundance: Iron is one of the most abundant elements in the universe and is essential for life. Its properties make it a significant component in various biological and geological processes.
Comparative Positioning
- Hydrogen (1): The lightest element, located in the top left corner.
- Helium (2): The second lightest, situated in the top right corner.
- Carbon (6): Found in the second row, but still far from the center.
Conclusion
In summary, while hydrogen, helium, and carbon are critical elements, iron's position and characteristics make it central to understanding the periodic table's structure and the elements' relationships. Its role in chemistry and biology further cements its importance as a central element.

Development of the Periodic Table
The development of the periodic table is credited to Mendeleev, a Russian chemist. He is known for creating the first widely recognized periodic table of elements in 1869, which laid the foundation for the modern periodic table we use today.

Mendeleev's Contribution
- Mendeleev arranged the known elements based on their atomic weights and properties, noticing a periodicity in their chemical behavior.
- He left gaps in his table for undiscovered elements, predicting their properties and even their atomic weights accurately.
- Mendeleev's periodic table was groundbreaking as it allowed for the prediction of the properties of unknown elements, leading to the discovery of new elements like gallium and germanium.

Importance of Mendeleev's Periodic Table
- Mendeleev's periodic table provided a systematic way to organize the elements, making it easier to study and understand their properties.
- It helped in identifying patterns and relationships between elements, leading to the development of the periodic law and the modern periodic table we use today.
- The periodic table is now a fundamental tool in chemistry, used to predict the behavior and properties of elements and compounds.
In conclusion, Mendeleev is rightfully credited with the development of the periodic table due to his innovative approach to organizing the elements and his contributions to the field of chemistry.

Understanding Electron Shells
The arrangement of electrons in an atom is defined by shells, which are designated by principal quantum numbers (n). Each shell can hold a specific maximum number of electrons.
Electron Capacity Formula
The maximum number of electrons that can occupy a shell is given by the formula:
- Maximum electrons = 2n²
Where 'n' is the principal quantum number of the shell.
Applying the Formula to the Third Shell
For the third shell, we have:
- n = 3
Now, substituting into the formula:
- Maximum electrons = 2(3)²
- Maximum electrons = 2(9)
- Maximum electrons = 18
Thus, the third shell can accommodate a maximum of 18 electrons.
Shell Arrangement
The electron configuration in shells is as follows:
- First Shell (n=1): Maximum 2 electrons
- Second Shell (n=2): Maximum 8 electrons
- Third Shell (n=3): Maximum 18 electrons
- Fourth Shell (n=4): Maximum 32 electrons
Conclusion
Therefore, the correct answer is indeed option 'C', as the third shell can hold up to 18 electrons. This fundamental concept is essential in understanding atomic structure and behavior in chemistry.

Location of Elements in the Periodic Table
The periodic table is organized in a way that reflects the properties of elements based on their atomic number. The bottom right corner of the periodic table typically contains the heaviest noble gases, which are characterized by their full valence electron shells that make them largely inert.

Understanding the Noble Gases
- **Noble Gases**: The elements in this group are known for their lack of reactivity due to their complete outer electron shells. This group includes Helium, Neon, Argon, Krypton, Xenon, and Radon.


Radon - The Element in Question
- **Radon (Rn)**: Radon is located at the bottom right corner of the periodic table in Group 18 (Noble Gases). It has the atomic number 86 and is the heaviest noble gas.
- **Properties**: As a colorless, odorless, and tasteless gas, Radon is radioactive and is produced naturally from the decay of uranium in soil and rocks. Its radioactivity makes it significant in both environmental studies and health assessments.

Why the Other Options Are Incorrect
- **Helium (He)**: Located at the top right corner, Helium has the atomic number 2 and is much lighter than Radon.
- **Neon (Ne)**: Found above Radon, Neon has the atomic number 10 and is also a lighter noble gas.
- **Argon (Ar)**: Positioned above Radon with the atomic number 18, Argon is heavier than Helium and Neon but lighter than Radon.
In conclusion, the correct answer to the question is indeed option 'D', Radon, as it is the element located at the bottom right corner of the periodic table.