Which one of the following inert gases is placed in period 4 of the modern periodic table?

Introduction / Context:
The noble or inert gases occupy group 18 of the modern periodic table and are placed at the far right of each period. Each noble gas belongs to a specific period corresponding to its highest occupied electron shell. Recognising which noble gas belongs to a particular period helps in understanding periodic trends and electron configurations. This question asks you to identify the inert gas located in period 4.

Given Data / Assumptions:

• The focus is on noble gases and their positions in the periodic table.
• Options list krypton, radon, xenon, argon, and neon.
• We assume the standard long form periodic table as taught in school chemistry.
• We recall that periods are horizontal rows numbered from 1 to 7.

Concept / Approach:
Each noble gas corresponds to the end of a period. Helium ends period 1, neon ends period 2, argon ends period 3, krypton ends period 4, xenon ends period 5, and radon ends period 6. Period number is linked to the principal quantum number of the valence shell. Therefore, the noble gas in period 4 has its valence shell with principal quantum number 4 and is krypton. Other noble gases occupy different periods.

Step-by-Step Solution:
Step 1: List noble gases in order of increasing atomic number: helium, neon, argon, krypton, xenon, and radon. Step 2: Associate each noble gas with the period it ends. Helium ends period 1, neon ends period 2, argon ends period 3. Step 3: Krypton is the next noble gas and is placed at the end of period 4. Step 4: Xenon appears in the next row and ends period 5, while radon ends period 6. Step 5: The question specifically asks for the inert gas in period 4, which is krypton. Step 6: Therefore, krypton (Kr) is the correct answer.

Verification / Alternative check:
You can verify by looking at the electron configuration. Krypton has the configuration [Ar]3d^10 4s^2 4p^6, showing that the valence shell has principal quantum number 4. Hence, it belongs to period 4. Argon has its valence electrons in the third shell and is in period 3, while xenon has valence electrons in the fifth shell and is in period 5. Periodic tables printed in textbooks also clearly place krypton at the end of the fourth row, confirming its position in period 4.

Why Other Options Are Wrong:
Radon (Rn): A noble gas in period 6, associated with heavy radioactive elements.
Xenon (Xe): A noble gas at the end of period 5, not period 4.
Argon (Ar): Ends period 3 and is placed one period above krypton.
Neon (Ne): Ends period 2 and is much lighter than krypton, occupying an earlier period.

Common Pitfalls:
Students sometimes confuse the order of noble gases or miscount the periods from memory. Another common mistake is to mix up krypton and xenon because both are heavier noble gases and their symbols look similar. To avoid this, remember the sequence of noble gases and associate each with its period: helium (1), neon (2), argon (3), krypton (4), xenon (5), radon (6). Visualising the periodic table grid and linking valence shell number to period number can also help.

Final Answer:
Krypton (Kr) is the inert gas placed in period 4 of the periodic table.