Periodic Properties

Step 1: Concept
Ionisation potential (IP) is the energy required to remove an electron. IP decreases as you move down a group because the atomic radius increases and shielding increases.
Step 2: Analysis
Li, K, and Cs are all Alkali metals (Group 1). Their order in the group is: Li (top) Na K Rb Cs (bottom).
Step 3: Conclusion
IP follows the trend: .
Final Answer: (C)

Step 1: Concept
and are isoelectronic species (same number of electrons).
Step 2: Analysis
For isoelectronic ions, as the negative charge increases, the ionic radius increases.
Step 3: Conclusion
The electrons are held less tightly by the nucleus in , making its radius greater than .
Final Answer: (B)

Step 1: Understanding the Concept:
Ionic radius increases down a group.

Step 2: Detailed Explanation:
All are alkali metal ions (Group 1). As we go down from Li to Cs, the number of shells increases, so ionic radius increases. Cs has the largest radius.

Step 3: Final Answer:

Step 1: Understanding the Concept:
Ionisation enthalpy generally increases across a period, but N has higher IE than O due to half-filled stability.

Step 2: Detailed Explanation:
(A) Correct: Atomic radius increases down Group 1.
(B) Correct: Electron gain enthalpy becomes more negative from I to Cl; F is an exception.
(C) Incorrect: Correct order is B ) has higher IE than O (2p ).
(D) Correct: For isoelectronic species, size decreases as nuclear charge increases.

Step 3: Final Answer:
Option (C) is not according to the property.

Step 1: Understanding the Concept:
Electron affinity is the energy released when an electron is added to a neutral atom in the gaseous state. Anions have a negative electron affinity (energy is required to add an electron).

Step 2: Detailed Explanation:
• K: A metal with low electron affinity. • O : Adding an electron to an anion is unfavorable (positive electron affinity value). • F : Adding an electron to F is highly unfavorable. • O: Oxygen has a high electron affinity (-141 kJ/mol) due to its small size and high nuclear charge. It is second only to chlorine in the periodic table. Neutral oxygen has a higher tendency to accept an electron compared to the others listed.

Step 3: Final Answer:
Oxygen (O) has the highest electron affinity among the given options, which corresponds to option (D).

Step 1: Understanding the Concept:
Electron affinity depends on electronic configuration.
Step 2: Detailed Explanation:
Beryllium (Be) has a stable electronic configuration ( ) and does not readily accept an electron. Similarly, neon (Ne) has a completely filled shell and low electron affinity.
Step 3: Final Answer:
The electron affinity of Be is similar to that of Ne.

Step 1: Understanding the Concept:
Second electron affinity is always endothermic.
Step 2: Detailed Explanation:
After gaining one electron, has a stable configuration and resists adding another electron due to electron-electron repulsion.
Step 3: Final Answer:
The ion resists addition of another electron.