Question

The correct order of variation of first ionisation enthalpies is:

• A. Ne<Xe>Li>K<Cs
• B. Xe<Li<K<Cs<Ne
• C. Cs>K>Li>Xe>Ne
• D. Li>K>Cs>Ne>Xe
• E. Ne>Xe>Li>K>Cs

Answer 1

Answer: (E)

Ionization enthalpy trends:

• Neon (Ne): Highest (noble gas configuration)
• Xenon (Xe): Lower than Ne but higher than metals (larger size)
• Alkali metals: Decreases down group (Li > K > Cs)

 

Correct order: Ne > Xe > Li > K > Cs

Thus, the correct option is (E): \( \text{Ne} > \text{Xe} > \text{Li} > \text{K} > \text{Cs} \).

Answer 2

1. Understanding ionisation enthalpy:

The first ionisation enthalpy is the energy required to remove the outermost electron from a neutral gaseous atom in its ground state. It generally depends on:

• Atomic size: Larger atoms have lower ionisation enthalpies because the outer electrons are farther from the nucleus and are more shielded by inner electrons.
• Nuclear charge: Higher nuclear charge increases the attraction between the nucleus and electrons, leading to higher ionisation enthalpies.
• Electronic configuration: Stable electronic configurations (e.g., noble gas configurations) result in higher ionisation enthalpies due to increased stability.

2. Analyze the elements given:

• Neon (Ne): A noble gas with a stable octet configuration. It has the highest ionisation enthalpy among the given elements.
• Xenon (Xe): Another noble gas, but it is much larger than neon. Its ionisation enthalpy is lower than that of neon due to reduced nuclear attraction caused by increased atomic size.
• Lithium (Li): An alkali metal with one valence electron. It has a relatively low ionisation enthalpy compared to noble gases, as it readily loses its outer electron.
• Potassium (K): Another alkali metal, larger than lithium. Its ionisation enthalpy is lower than that of lithium due to increased atomic size and shielding.
• Cesium (Cs): The largest alkali metal among the given elements. It has the lowest ionisation enthalpy due to its large size and weak nuclear attraction for the outermost electron.

3. Arrange the elements in order of decreasing ionisation enthalpy:

• The noble gas Neon (Ne) has the highest ionisation enthalpy.
• Xenon (Xe) follows, as it is also a noble gas but has a lower ionisation enthalpy due to its larger size.
• Among the alkali metals, Lithium (Li) has the highest ionisation enthalpy, followed by Potassium (K), and finally Cesium (Cs), which has the lowest ionisation enthalpy.

The correct order is: Ne > Xe > Li > K > Cs.

4. Final answer:

The correct option is (E) Ne > Xe > Li > K > Cs.