📘 Answer
Ionization enthalpy (IE) is the minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state.
The main factors affecting ionization enthalpy are:
- Atomic size
- Effective nuclear charge
- Shielding or screening effect
- Penetration of orbital
- Electronic configuration
- Charge on the atom/ion
🔹 1. Atomic Size
As the atomic size increases, the distance between the nucleus and the valence electron increases. Therefore, the nuclear attraction on the valence electron decreases and less energy is required to remove it.
For example, ionization enthalpy generally decreases down a group because atomic size increases.
🔹 2. Effective Nuclear Charge
Effective nuclear charge is the net positive charge experienced by an electron after considering the shielding effect of other electrons.
A higher effective nuclear charge attracts the valence electron more strongly, making its removal difficult.
🔹 3. Shielding / Screening Effect
Inner-shell electrons reduce the attractive force between the nucleus and the outermost electron. This is called the shielding effect.
Greater shielding makes the valence electron easier to remove.
🔹 4. Penetration of Orbital
The ability of an orbital to approach the nucleus is called penetration.
For orbitals in the same shell, the order of penetration is:
An electron in a more penetrating orbital is held more strongly by the nucleus and is therefore harder to remove.
🔹 5. Electronic Configuration
The stability of an electronic configuration affects ionization enthalpy. Completely filled and half-filled subshells are relatively stable and require more energy for electron removal.
| Configuration | Stability | Effect on IE |
|---|---|---|
| Filled subshell | High | IE increases |
| Half-filled subshell | Relatively high | IE increases |
| Partially filled subshell | Comparatively lower | IE may be lower |
For example, Be has a stable 2s² configuration, so its first IE is higher than that of B, which has a 2p¹ electron.
🔹 6. Charge on Atom or Ion
A positively charged ion generally holds its electrons more strongly than the neutral atom. Therefore, ionization enthalpy increases with increasing positive charge.
The second ionization enthalpy is also greater than the first ionization enthalpy because an electron is removed from an already positively charged ion.
📊 Summary Table
| Factor | Increase in Factor | Effect on IE |
|---|---|---|
| Atomic size | ↑ | ↓ |
| Effective nuclear charge | ↑ | ↑ |
| Shielding effect | ↑ | ↓ |
| Orbital penetration | ↑ | ↑ |
| Electronic stability | ↑ | ↑ |
| Positive charge | ↑ | ↑ |
✅ Final Answer
The important factors affecting ionization enthalpy are atomic size, effective nuclear charge, shielding effect, orbital penetration, electronic configuration and charge on the atom/ion.
20 MCQs with Answers
1. Ionization enthalpy is the energy required to:
✅ Answer
B
2. Increase in atomic size generally causes IE to:
✅ Answer
B) Decrease
3. Effective nuclear charge increases the:
✅ Answer
B
4. Greater shielding effect generally causes:
✅ Answer
B
5. Which orbital has the greatest penetration?
✅ Answer
A) s
6. Correct order of penetration is:
✅ Answer
A
7. Higher effective nuclear charge generally means:
✅ Answer
B
8. Which configuration is relatively more stable?
✅ Answer
A
9. Half-filled subshells are generally:
✅ Answer
A
10. Which has higher IE generally?
✅ Answer
B) Smaller atom
11. Down a group, atomic size generally:
✅ Answer
B
12. Down a group, IE generally:
✅ Answer
B
13. Across a period, effective nuclear charge generally:
✅ Answer
A
14. Across a period, IE generally:
✅ Answer
B
15. Which is a major cause of lower IE down a group?
✅ Answer
B
16. Be has higher IE than B mainly because:
✅ Answer
B
17. Which factor makes an electron easier to remove?
✅ Answer
B
18. A highly positive ion generally has:
✅ Answer
B
19. Which factor is directly related to the distance of the valence electron from nucleus?
✅ Answer
A
20. Which combination generally gives lower IE?
✅ Answer
B
Q. Name any two factors affecting ionization enthalpy.
Solution:
- Atomic size: Larger atomic size generally lowers IE.
- Effective nuclear charge: Higher effective nuclear charge generally increases IE.
Q. Explain the effect of atomic size, shielding effect and effective nuclear charge on ionization enthalpy.
Solution:
- Atomic size ↑ → distance of valence electron from nucleus ↑ → IE ↓.
- Shielding effect ↑ → nuclear attraction on valence electron ↓ → IE ↓.
- Effective nuclear charge ↑ → nuclear attraction ↑ → IE ↑.
Q. Explain four important factors affecting ionization enthalpy.
Solution:
- Atomic size: Larger size decreases IE.
- Effective nuclear charge: Higher effective nuclear charge increases IE.
- Shielding effect: Greater shielding decreases IE.
- Electronic configuration: Stable half-filled and completely filled subshells generally have higher IE.
Q. Explain the factors affecting ionization enthalpy with suitable examples.
Solution:
Ionization enthalpy depends mainly on atomic size, effective nuclear charge, shielding effect, orbital penetration and electronic configuration.
- Atomic size: Ionization enthalpy decreases as atomic size increases. Example: IE decreases down Group 1.
- Effective nuclear charge: Higher effective nuclear charge increases IE because the electron is held more strongly.
- Shielding effect: More inner electrons shield the valence electron and lower IE.
- Penetration: s electrons are more penetrating than p, d and f electrons.
- Electronic configuration: Half-filled and completely filled subshells are relatively stable and can show higher IE.
Q. Explain in detail the various factors affecting ionization enthalpy and discuss their effects.
Detailed Solution
Ionization enthalpy depends on how strongly the outermost electron is attracted towards the nucleus. Several factors determine this attraction.
1. Atomic Size
With increase in atomic size, the valence electron moves farther from the nucleus. The attractive force decreases and ionization becomes easier.
2. Effective Nuclear Charge
Higher effective nuclear charge means stronger attraction between the nucleus and the valence electron. Hence, more energy is required for its removal.
3. Shielding Effect
Inner electrons shield the valence electron from the nucleus. As shielding increases, effective nuclear attraction decreases and IE falls.
4. Penetration Effect
The penetration ability of orbitals follows:
More penetrating electrons are held more strongly by the nucleus and require more energy for removal.
5. Electronic Configuration
Half-filled and completely filled subshells have additional stability. Therefore, removal of an electron from such configurations may require higher energy.
For example:
because Be has a stable 2s² configuration while B loses a higher-energy 2p electron.
6. Charge on the Species
As positive charge increases, the remaining electrons are held more strongly by the nucleus. Therefore, ionization enthalpy generally increases.
Ionization enthalpy is mainly controlled by atomic size, effective nuclear charge, shielding effect, orbital penetration, electronic configuration and charge.
Size ↑ → IE ↓ | Zeff ↑ → IE ↑ | Shielding ↑ → IE ↓
🎯 Quick Revision
2. Effective Nuclear Charge: ↑ → IE ↑
3. Shielding Effect: ↑ → IE ↓
4. Penetration: s > p > d > f
5. Stable Configuration: IE generally ↑
6. Positive Charge: ↑ → IE generally ↑
