1. Which metal has the lowest ionization enthalpy?
+
Correct Answer
Cesium (Cs)
Detailed Explanation
Among the naturally occurring stable metals, Cesium (Cs) has
the lowest first ionization enthalpy.
The first ionization enthalpy is the energy required to remove the most loosely
held electron from an isolated gaseous atom:
M(g) → M+(g) + e−
Ionization enthalpy generally:
Decreases down a group → Increases across a period
Cesium is located near the bottom of Group 1. Its valence electron is in the
6s orbital, far from the nucleus and strongly shielded by many
inner-shell electrons.
Therefore, this electron experiences relatively low effective nuclear
attraction and is easily removed.
Large atomic size + strong shielding → low IE
Hence, Cs has a very low first ionization enthalpy.
Why Not Francium?
At first glance, Francium (Fr) should have the lowest
ionization enthalpy because it lies below Cs in Group 1.
However, due to relativistic effects and the contraction/stabilisation of the
7s electron, the experimentally relevant first ionization enthalpy of Cs is
slightly lower than that of Fr.
Therefore, for standard periodic-table questions:
Cs → Lowest First Ionization Enthalpy among the elements commonly
considered
Periodic Trend
For Group 1:
Li > Na > K > Rb > Cs
This represents the general decrease in first ionization enthalpy down the
group.
The outer electron becomes farther from the nucleus and experiences greater
shielding.
JEE / NEET Shortcut
If asked:
“Which alkali metal has the lowest ionization enthalpy?”
→ Cs
If the options contain Li, Na, K, Rb, Cs:
• Larger atomic radius
• Greater shielding
• Lower effective nuclear attraction
• Greater distance of valence electron from nucleus
Cs is the answer.
Remember:
Low ionization enthalpy = electron is easy to remove.
This is favoured by:
• Larger atomic radius
• Greater shielding
• Lower effective nuclear attraction
• Greater distance of valence electron from nucleus
30 Related MCQs with Solutions
1. Which alkali metal has the lowest first ionization enthalpy?
A. Li
B. Na
C. K
D. Cs
Answer: D. Cs
2. Ionization enthalpy is the energy required to:
A. Add an electron
B. Remove an electron from a gaseous atom
C. Remove a proton
D. Add a neutron
Answer: B
3. Which has lower first IE?
A. Li
B. Cs
C. Both equal
D. H
Answer: B. Cs
4. Ionization enthalpy generally decreases down a group because:
A. Atomic size decreases
B. Shielding increases
C. Nuclear charge decreases
D. Electrons disappear
Answer: B
5. Which Group 1 element has the largest atomic radius among Li, Na, K, Rb and Cs?
A. Li
B. Na
C. K
D. Cs
Answer: D. Cs
6. The valence electron of Cs is present in:
A. 5s
B. 5p
C. 6s
D. 6p
Answer: C. 6s
7. Which factor lowers ionization enthalpy?
A. Greater effective nuclear charge
B. Greater atomic radius
C. Smaller shielding
D. Smaller distance from nucleus
Answer: B
8. Correct order of IE in Group 1 is:
A. Li < Na < K < Rb < Cs
B. Li > Na > K > Rb > Cs
C. Cs > Rb > K > Na > Li
D. All equal
Answer: B
9. Which factor makes the outer electron of Cs weakly bound?
A. Small size
B. Strong shielding
C. High electronegativity
D. High electron affinity
Answer: B
10. Low ionization enthalpy indicates that an atom:
A. Easily loses an electron
B. Easily gains a proton
C. Cannot form ions
D. Has no valence electron
Answer: A
11. Which is generally easiest to ionize?
A. Cs
B. F
C. Ne
D. Cl
Answer: A. Cs
12. Which property increases down Group 1?
A. First IE
B. Atomic radius
C. Electronegativity
D. Ionization energy only
Answer: B
13. The lowest first IE among Li and Rb is:
A. Li
B. Rb
C. Equal
D. H
Answer: B. Rb
14. Which factor increases ionization enthalpy?
A. Larger atomic radius
B. Greater shielding
C. Greater effective nuclear charge
D. Greater principal shell number
Answer: C
15. Which Group 1 element has its valence electron farthest from the nucleus?
A. Li
B. Na
C. K
D. Cs
Answer: D. Cs
16. The first ionization process is represented by:
A. M(g) → M+(g) + e−
B. M(g) + e− → M−(g)
C. M+ + e− → M
D. M(s) → M(g)
Answer: A
17. Which metal is commonly considered to have the lowest first ionization enthalpy?
A. Cs
B. Al
C. Mg
D. Fe
Answer: A. Cs
18. Greater shielding causes:
A. Greater attraction on valence electron
B. Lower effective nuclear attraction
C. Smaller atomic radius
D. Higher IE always
Answer: B
19. Which has the lowest IE among K and Cs?
A. K
B. Cs
C. Equal
D. Cannot say
Answer: B. Cs
20. Down a group, the valence electron generally becomes:
A. Closer to nucleus
B. Farther from nucleus
C. A proton
D. A neutron
Answer: B
21. Which has greater first IE?
A. Cs
B. Na
C. Both equal
D. K
Answer: B. Na
22. Which combination favours minimum IE?
A. Small radius + high effective nuclear charge
B. Large radius + high shielding
C. Small radius + low shielding
D. High electronegativity
Answer: B
23. Which is more electropositive?
A. Cs
B. F
C. O
D. Cl
Answer: A. Cs
24. Low IE is generally associated with:
A. Metallic character
B. Non-metallic character
C. Noble gas character
D. Inertness only
Answer: A
25. Which Group 1 element is the most difficult to ionize among these?
A. Li
B. Na
C. K
D. Cs
Answer: A. Li
26. First IE generally increases:
A. Down a group
B. Across a period from left to right
C. With atomic radius
D. With shielding
Answer: B
27. Which electron is easiest to remove?
A. One close to nucleus
B. One far from nucleus with strong shielding
C. Core electron
D. Electron in a noble gas shell
Answer: B
28. The low IE of alkali metals explains their:
A. High reactivity
B. Low electropositivity
C. Inertness
D. High electronegativity
Answer: A
29. Among Cs, Na and Li, the easiest electron removal occurs from:
A. Li
B. Na
C. Cs
D. All equally
Answer: C. Cs
30. The best reason for low IE of Cs is:
A. Small size and low shielding
B. Large size and high shielding
C. High electronegativity
D. Complete octet
Answer: B
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