Define Electron Gain Enthalpy.

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Definition

Electron Gain Enthalpy is the enthalpy change accompanying the addition of an electron to an isolated neutral gaseous atom to form a gaseous negative ion.

X(g) + e⁻ → X⁻(g)

It is usually expressed in kJ mol⁻¹.

🔹 Example

When a chlorine atom gains one electron, a chloride ion is formed:

Cl(g) + e⁻ → Cl⁻(g)

Energy is released in this process, so the electron gain enthalpy of chlorine is negative.

⚛️ Important Point

A more negative electron gain enthalpy indicates that an atom has a greater tendency to accept an electron.

Electron Gain Enthalpy	Meaning
More negative	Greater tendency to gain electron
Less negative / Positive	Lower tendency to gain electron

📌 First and Second Electron Gain Enthalpy

The first electron gain enthalpy involves addition of an electron to a neutral atom:

X(g) + e⁻ → X⁻(g)

The second electron gain enthalpy involves adding an electron to an already negatively charged ion:

X⁻(g) + e⁻ → X²⁻(g)

The second electron gain enthalpy is generally positive because the incoming electron is repelled by the already negatively charged ion.

✅ Final Answer

Electron Gain Enthalpy is the enthalpy change accompanying the addition of an electron to an isolated neutral gaseous atom to form a gaseous negative ion.

X(g) + e⁻ → X⁻(g)

It is generally negative when energy is released during electron addition.

20 MCQs with Answers

1. Electron gain enthalpy is associated with:

A) Removal of electron

B) Addition of electron

C) Removal of proton

D) Addition of neutron

✅ Answer

B) Addition of electron

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2. Electron gain enthalpy is measured for:

A) Solid atoms

B) Liquid atoms

C) Isolated gaseous atoms

D) Molecules only

✅ Answer

C

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3. Formation of a negative ion involves:

A) Loss of electron

B) Gain of electron

C) Loss of proton

D) Gain of neutron

✅ Answer

B

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4. The general process is:

A) X → X⁺ + e⁻

B) X + e⁻ → X⁻

C) X → X²⁺

D) X⁻ → X + e⁻

✅ Answer

B

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5. Electron gain enthalpy is commonly expressed in:

A) J

B) kJ mol⁻¹

C) kg

D) mol

✅ Answer

B) kJ mol⁻¹

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6. A negative electron gain enthalpy generally indicates:

A) Energy is released

B) Energy is always absorbed

C) No electron is added

D) Nuclear decay

✅ Answer

A

---

7. Which element has a strong tendency to gain an electron?

A) Noble gas

B) Halogen

C) Alkali metal

D) Alkaline earth metal

✅ Answer

B) Halogen

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8. Chlorine gains an electron to form:

A) Cl⁺

B) Cl²⁺

C) Cl⁻

D) Cl²⁻

✅ Answer

C) Cl⁻

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9. First electron gain enthalpy involves:

A) Neutral atom + electron

B) Cation + electron only

C) Anion + proton

D) Neutral atom + proton

✅ Answer

A

---

10. Second electron gain enthalpy is generally:

A) Negative

B) Positive

C) Always zero

D) Undefined

✅ Answer

B) Positive

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11. Why is second electron addition difficult?

A) Electron-electron repulsion

B) Proton-proton attraction

C) No nucleus

D) No electrons

✅ Answer

A

---

12. Greater tendency to gain an electron is generally associated with:

A) More negative electron gain enthalpy

B) More positive electron gain enthalpy

C) Zero atomic number

D) Larger mass only

✅ Answer

A

---

13. Which group generally has very low tendency to gain electrons?

A) Group 17

B) Group 18

C) Group 16

D) Group 15

✅ Answer

B) Group 18

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14. Which particle is formed when an atom gains one electron?

A) Cation

B) Anion

C) Proton

D) Neutron

✅ Answer

B) Anion

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15. Electron gain enthalpy is a:

A) Nuclear property only

B) Periodic property

C) Mechanical property

D) Optical property

✅ Answer

B

---

16. Which generally has more negative electron gain enthalpy?

A) Halogens

B) Noble gases

C) Alkali metals

D) Alkaline earth metals

✅ Answer

A

---

17. In X(g) + e⁻ → X⁻(g), X⁻ is:

A) Cation

B) Anion

C) Neutral atom

D) Proton

✅ Answer

B) Anion

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18. Electron gain enthalpy differs from ionization enthalpy because it involves:

A) Addition of electron

B) Removal of electron

C) Removal of proton

D) Addition of neutron

✅ Answer

A

---

19. Which is more likely to accept an electron?

A) An atom with more negative electron gain enthalpy

B) An atom with zero nuclear charge

C) An atom with no valence electrons

D) None

✅ Answer

A

---

20. Electron gain enthalpy is related mainly to the tendency of an atom to:

A) Lose an electron

B) Gain an electron

C) Lose a proton

D) Gain a neutron

✅ Answer

B) Gain an electron

Q. Define electron gain enthalpy.

Solution: Electron gain enthalpy is the enthalpy change accompanying the addition of an electron to an isolated neutral gaseous atom to form a gaseous negative ion.

X(g) + e⁻ → X⁻(g)

Q. Define electron gain enthalpy and explain its sign.

Solution:

Electron gain enthalpy is the enthalpy change when an isolated gaseous neutral atom accepts an electron to form a gaseous anion.

X(g) + e⁻ → X⁻(g)

For many atoms, energy is released during the first electron addition, so electron gain enthalpy is negative. The second electron gain enthalpy is generally positive because the incoming electron is repelled by the already negative ion.

Q. Explain electron gain enthalpy with an example.

Solution:

Electron gain enthalpy is the enthalpy change accompanying the addition of an electron to an isolated neutral gaseous atom.

Cl(g) + e⁻ → Cl⁻(g)

• Chlorine accepts one electron.
• A chloride ion is formed.
• Energy is released in the process.
• Therefore, the first electron gain enthalpy of chlorine is negative.

More negative electron gain enthalpy → Greater tendency to gain an electron.

Q. Explain first and second electron gain enthalpy.

Solution:

First Electron Gain Enthalpy

It is the enthalpy change when one electron is added to a neutral gaseous atom.

X(g) + e⁻ → X⁻(g)

It is generally negative when the process releases energy.

Second Electron Gain Enthalpy

It is the enthalpy change when an electron is added to a gaseous negative ion.

X⁻(g) + e⁻ → X²⁻(g)

This process is generally endothermic because the incoming electron is repelled by the negatively charged ion.

First EGE → generally negative
Second EGE → generally positive

Q. Define electron gain enthalpy and explain its significance, first and second electron gain enthalpy with suitable examples.

Detailed Solution

Electron gain enthalpy is the enthalpy change accompanying the addition of an electron to an isolated neutral gaseous atom to form a gaseous negative ion.

X(g) + e⁻ → X⁻(g)

First Electron Gain Enthalpy

The first electron is added to a neutral atom. For atoms that readily accept electrons, energy is usually released and the value is negative.

Cl(g) + e⁻ → Cl⁻(g)

Second Electron Gain Enthalpy

The second electron is added to an already negatively charged ion. Considerable energy is generally required because the incoming electron is repelled by the negative ion.

Cl⁻(g) + e⁻ → Cl²⁻(g)

Therefore, second electron gain enthalpy is generally positive.

Significance

• It indicates the tendency of an atom to accept an electron.
• More negative value generally indicates greater electron-accepting tendency.
• Halogens generally have highly negative electron gain enthalpies.
• Noble gases have very low tendency to accept an additional electron.

Final Answer:

Electron gain enthalpy is the enthalpy change associated with addition of an electron to an isolated neutral gaseous atom to form a gaseous anion.

X(g) + e⁻ → X⁻(g)

🎯 Quick Revision

Electron Gain Enthalpy: Addition of electron
Process: X(g) + e⁻ → X⁻(g)
Unit: kJ mol⁻¹
First EGE: Generally negative
Second EGE: Generally positive
More negative EGE: Greater tendency to gain electron

Arrange C,N,O,F in increasing order of their first IE.

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Solution

The elements C, N, O and F belong to the 2nd period. Generally, first ionization enthalpy increases from left to right across a period because effective nuclear charge increases and atomic size decreases.

C    →    N    →    O    →    F

However, there is an important exception between N and O. Nitrogen has a stable half-filled 2p³ configuration, whereas oxygen has 2p⁴ configuration with one pair of electrons in a 2p orbital.

⚛️ Electronic Configurations

Element	Electronic Configuration	Special Feature
C	1s² 2s² 2p²	2p²
N	1s² 2s² 2p³	Half-filled 2p³
O	1s² 2s² 2p⁴	Paired 2p electron
F	1s² 2s² 2p⁵	High effective nuclear charge

🔹 Why is IE of N higher than O?

Nitrogen has the configuration 2p³, in which the three 2p orbitals are singly occupied. This is a relatively stable half-filled subshell.

Oxygen has 2p⁴. In one of its 2p orbitals, two electrons are paired. The electron-electron repulsion within this paired orbital makes one electron comparatively easier to remove.

N (2p³) → More Stable → Higher IE

O (2p⁴) → Electron Pairing Repulsion → Lower IE

📊 Increasing Order

Considering the general periodic trend and the N–O exception:

C < O < N < F

Therefore, the increasing order of first ionization enthalpy is:

C < O < N < F

✅ Final Answer

Increasing order:

C < O < N < F

The unusual position of O and N is due to the greater stability of the half-filled 2p³ configuration of nitrogen.

20 MCQs with Answers

1. Which has the highest first IE among C, N, O and F?

A) C

B) N

C) O

D) F

✅ Answer

D) F

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2. Which has the lowest first IE among C, N, O and F?

A) C

B) N

C) O

D) F

✅ Answer

A) C

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3. Correct increasing order is:

A) C < O < N < F

B) C < N < O < F

C) F < N < O < C

D) O < C < N < F

✅ Answer

A) C < O < N < F

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4. C, N, O and F belong to which period?

A) 1st

B) 2nd

C) 3rd

D) 4th

✅ Answer

B) 2nd

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5. Nitrogen has which valence configuration?

A) 2s²2p¹

B) 2s²2p²

C) 2s²2p³

D) 2s²2p⁴

✅ Answer

C) 2s²2p³

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6. Oxygen has which valence configuration?

A) 2s²2p²

B) 2s²2p³

C) 2s²2p⁴

D) 2s²2p⁵

✅ Answer

C) 2s²2p⁴

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7. Why is IE of N higher than O?

A) N has more shells

B) N has a stable half-filled 2p³ subshell

C) O has no electrons

D) N is a metal

✅ Answer

B

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8. In oxygen, lower IE is partly due to:

A) Electron pairing repulsion

B) Absence of nucleus

C) Larger number of shells

D) Zero effective charge

✅ Answer

A

---

9. Which element has a half-filled 2p subshell?

A) C

B) N

C) O

D) F

✅ Answer

B) N

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10. Across a period, effective nuclear charge generally:

A) Increases

B) Decreases

C) Becomes zero

D) Remains unchanged

✅ Answer

A) Increases

---

11. Atomic size across a period generally:

A) Increases

B) Decreases

C) Remains constant

D) Doubles

✅ Answer

B) Decreases

---

12. Which has configuration 1s²2s²2p⁵?

A) C

B) N

C) O

D) F

✅ Answer

D) F

---

13. Which has configuration 1s²2s²2p²?

A) C

B) N

C) O

D) F

✅ Answer

A) C

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14. The first IE of N is higher than O because:

A) N has 2p³ stability

B) O has more shells

C) N has fewer protons

D) O has no nucleus

✅ Answer

A

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15. Which order follows the general increase in IE across the period?

A) C → N → O → F

B) F → O → N → C

C) O → C → F → N

D) N → C → O → F

✅ Answer

A

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16. The exception in the C–N–O–F sequence occurs between:

A) C and N

B) N and O

C) O and F

D) C and F

✅ Answer

B) N and O

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17. The atomic number of nitrogen is:

A) 5

B) 6

C) 7

D) 8

✅ Answer

C) 7

---

18. The atomic number of oxygen is:

A) 6

B) 7

C) 8

D) 9

✅ Answer

C) 8

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19. The correct order of first IE is:

A) F > N > O > C

B) C > O > N > F

C) N > F > O > C

D) O > N > F > C

✅ Answer

A

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20. Which principle explains the higher IE of N compared with O?

A) Hund's rule and half-filled stability

B) Boyle's law

C) Avogadro's law

D) Conservation of mass

✅ Answer

A

Q. Arrange C, N, O and F in increasing order of first ionization enthalpy.

Solution: The general IE trend increases across the period, but N has a stable half-filled 2p³ configuration and therefore has higher IE than O.

C < O < N < F

Q. Why is the first ionization enthalpy of nitrogen greater than that of oxygen?

Solution:

• N has configuration 2p³, which is half-filled and relatively stable.
• O has configuration 2p⁴, containing a paired electron.
• Electron-electron repulsion in the paired orbital makes removal of an electron from O easier.

Therefore, IE(N) > IE(O).

Q. Explain the increasing order of first ionization enthalpy of C, N, O and F.

Solution:

All four elements belong to the second period. Effective nuclear charge increases from C to F, so IE generally increases. However, nitrogen has a stable half-filled 2p³ configuration, while oxygen has 2p⁴ with electron pairing repulsion.

C < O < N < F

Thus, the expected C < N < O < F sequence is modified because IE(N) > IE(O).

Q. Explain the periodic trend in first ionization enthalpy and arrange C, N, O and F in increasing order.

Solution:

1. First ionization enthalpy generally increases from left to right across a period.
2. This is mainly due to increasing effective nuclear charge and decreasing atomic size.
3. C, N, O and F are elements of the second period.
4. N has a stable half-filled 2p³ configuration.
5. O has 2p⁴ configuration and electron pairing causes additional repulsion.

C < O < N < F

Q. Discuss the first ionization enthalpy trend of C, N, O and F and give the correct increasing order with reasons.

Detailed Solution

Carbon, nitrogen, oxygen and fluorine belong to the second period. As we move from left to right, nuclear charge increases while electrons are added to the same principal shell. Therefore, effective nuclear charge generally increases and atomic radius decreases. Consequently, first ionization enthalpy generally increases.

Electronic Configurations

C = 1s² 2s² 2p²
N = 1s² 2s² 2p³
O = 1s² 2s² 2p⁴
F = 1s² 2s² 2p⁵

Important Exception: N and O

Nitrogen has a half-filled 2p³ subshell. This configuration is relatively stable because the three 2p orbitals contain one electron each.

Oxygen has 2p⁴ configuration. One of its 2p orbitals contains a pair of electrons. Repulsion between the paired electrons makes one electron easier to remove.

IE(N) > IE(O)

Final Order

C < O < N < F

Therefore, the increasing order of first ionization enthalpy is C < O < N < F.

🎯 Quick Revision

C: 2p²
N: 2p³ → Half-filled → Higher IE
O: 2p⁴ → Pairing repulsion → Lower than N
F: 2p⁵ → Highest among these four

C < O < N < F

What are the factors affecting IE?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 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.

Ionization Enthalpy = Energy required to remove an electron

The main factors affecting ionization enthalpy are:

1. Atomic size
2. Effective nuclear charge
3. Shielding or screening effect
4. Penetration of orbital
5. Electronic configuration
6. 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.

Atomic Size ↑ → IE ↓

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.

Effective Nuclear Charge ↑ → IE ↑

🔹 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.

Shielding Effect ↑ → Nuclear Attraction ↓ → IE ↓

🔹 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:

s > p > d > f

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.

IE(Be) > IE(B)

🔹 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.

Cation → Stronger Attraction → Higher IE

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.

Size ↑ → IE ↓   |   Z_eff ↑ → IE ↑   |   Shielding ↑ → IE ↓

20 MCQs with Answers

1. 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

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2. Increase in atomic size generally causes IE to:

A) Increase

B) Decrease

C) Remain constant

D) Become infinite

✅ Answer

B) Decrease

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3. Effective nuclear charge increases the:

A) Ease of electron removal

B) Attraction of electron to nucleus

C) Atomic radius always

D) Shielding effect

✅ Answer

B

---

4. Greater shielding effect generally causes:

A) Higher IE

B) Lower IE

C) No change

D) Zero IE

✅ Answer

B

---

5. Which orbital has the greatest penetration?

A) s

B) p

C) d

D) f

✅ Answer

A) s

---

6. Correct order of penetration is:

A) s > p > d > f

B) f > d > p > s

C) p > s > d > f

D) d > f > s > p

✅ Answer

A

---

7. Higher effective nuclear charge generally means:

A) Lower IE

B) Higher IE

C) Zero IE

D) No effect

✅ Answer

B

---

8. Which configuration is relatively more stable?

A) Completely filled subshell

B) Random configuration

C) Empty nucleus

D) None

✅ Answer

A

---

9. Half-filled subshells are generally:

A) Relatively stable

B) Extremely unstable

C) Empty

D) Impossible

✅ Answer

A

---

10. Which has higher IE generally?

A) Larger atom

B) Smaller atom

C) Both always equal

D) Neither

✅ Answer

B) Smaller atom

---

11. Down a group, atomic size generally:

A) Decreases

B) Increases

C) Remains fixed

D) Becomes zero

✅ Answer

B

---

12. Down a group, IE generally:

A) Increases

B) Decreases

C) Remains constant

D) Becomes infinite

✅ Answer

B

---

13. Across a period, effective nuclear charge generally:

A) Increases

B) Decreases

C) Becomes zero

D) Does not change

✅ Answer

A

---

14. Across a period, IE generally:

A) Decreases

B) Increases

C) Remains zero

D) Has no trend

✅ Answer

B

---

15. Which is a major cause of lower IE down a group?

A) Decreased atomic size

B) Increased atomic size and shielding

C) Increased nuclear attraction only

D) Decreased number of shells

✅ Answer

B

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16. Be has higher IE than B mainly because:

A) B has no electrons

B) B loses a higher-energy 2p electron

C) Be has more shells

D) B is a metal

✅ Answer

B

---

17. Which factor makes an electron easier to remove?

A) Strong nuclear attraction

B) High shielding

C) Small atomic size

D) High effective nuclear charge

✅ Answer

B

---

18. A highly positive ion generally has:

A) Lower attraction for electrons

B) Stronger attraction for remaining electrons

C) No nucleus

D) No ionization enthalpy

✅ Answer

B

---

19. Which factor is directly related to the distance of the valence electron from nucleus?

A) Atomic size

B) Neutron mass

C) Isotopic abundance

D) Melting point

✅ Answer

A

---

20. Which combination generally gives lower IE?

A) Small size + high effective nuclear charge

B) Large size + high shielding

C) Small size + low shielding

D) High nuclear attraction + small size

✅ Answer

B

Q. Name any two factors affecting ionization enthalpy.

Solution:

1. Atomic size: Larger atomic size generally lowers IE.
2. 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 ↑.

Therefore, atomic size and shielding lower IE, while effective nuclear charge increases IE.

Q. Explain four important factors affecting ionization enthalpy.

Solution:

1. Atomic size: Larger size decreases IE.
2. Effective nuclear charge: Higher effective nuclear charge increases IE.
3. Shielding effect: Greater shielding decreases IE.
4. 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.

Thus, IE is determined by the strength with which the nucleus holds the outermost electron.

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.

Size ↑ → IE ↓

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.

Z_eff ↑ → IE ↑

3. Shielding Effect

Inner electrons shield the valence electron from the nucleus. As shielding increases, effective nuclear attraction decreases and IE falls.

Shielding ↑ → IE ↓

4. Penetration Effect

The penetration ability of orbitals follows:

s > p > d > f

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:

IE(Be) > IE(B)

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.

Final Conclusion:

Ionization enthalpy is mainly controlled by atomic size, effective nuclear charge, shielding effect, orbital penetration, electronic configuration and charge.

Size ↑ → IE ↓   |   Z_eff ↑ → IE ↑   |   Shielding ↑ → IE ↓

🎯 Quick Revision

1. Atomic Size: ↑ → IE ↓
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 ↑