Why Electron Gain Enthalpy of Noble Gases are Positive while Be, Mg, N, P are Almost Zero

Anand Classes explains the concept of electron gain enthalpy of noble gases, beryllium (Be), magnesium (Mg), nitrogen (N), and phosphorus (P) in detail for JEE, NEET, and CBSE examinations. While most elements have negative electron gain enthalpy due to their tendency to attract electrons, noble gases exhibit positive electron gain enthalpy because of their stable octet configuration (ns²np⁶). On the other hand, Be and Mg with completely filled s-orbitals, and N and P with half-filled p-orbitals, show very low or nearly zero electron gain enthalpy values due to their extra stability. Understanding these exceptions is crucial for mastering trends in the periodic table and is a frequently asked concept in competitive exams.

🔹 What is Electron Gain Enthalpy ?

• Definition: Electron gain enthalpy is the amount of energy released (negative value) or absorbed (positive value) when an atom in the gaseous state gains an extra electron to form a negative ion.
• It gives us an idea of how strongly an atom can attract and accommodate an additional electron.

Normally, elements with high effective nuclear charge (like halogens) have highly negative electron gain enthalpies. But in some cases, the values are positive or nearly zero due to special electronic configurations.

Why Noble Gases have Positive Electron Gain Enthalpy Values ?

• Electronic Configuration:
  Noble gases have the general electronic configuration: ns²np⁶ which corresponds to a completely filled octet (stable configuration).
• Reason for Positive Values:
  • Since their outermost shell is already full, they have no tendency to accept an electron.
  • If we try to add an electron, it has to go into the next higher principal quantum number (n+1) orbital.
  • The incoming electron is not attracted effectively by the nucleus because the stable closed shell repels it.
  • To push an electron into such a system, external energy must be supplied, making the process endothermic.

👉 Therefore, noble gases show positive electron gain enthalpy values.

Why Beryllium (Be) and Magnesium (Mg) have Almost Zero Electron Gain Enthalpy Value ?

• Electronic Configurations:
  • Be : 1s² 2s²
  • Mg : 1s² 2s² 2p⁶ 3s²
• Reason for Very Low Values:
  • Both Be and Mg have completely filled ss-orbitals (2s² and 3s²).
  • A filled subshell has extra stability due to symmetrical distribution of electrons.
  • Adding an electron would mean disturbing this stability by entering a new, higher energy orbital.

👉 Hence, Be and Mg do not favor gaining electrons, and their electron gain enthalpies are very low (close to zero).

Why Nitrogen (N) and Phosphorus (P) have Almost Zero Electron Gain Enthalpy Value ?

• Electronic Configurations:
  • N : 1s² 2s² 2p_x¹ 2p_y¹ 2p_z¹
  • P : 1s² 2s² 2p⁶ 3s² 3p_x¹ 3p_y¹ 3p_z¹
• Reason for Very Low Values:
  • Both N and P have half-filled p-orbitals (2p_x¹ 2p_y¹ 2p_z¹ for N and 3p_x¹ 3p_y¹ 3p_z¹ for P).
  • Half-filled subshells are relatively stable due to:
    • Symmetrical distribution of electrons.
    • Exchange energy stabilization (electrons in degenerate orbitals can exchange positions, lowering energy).
  • Adding an extra electron would disrupt this stable half-filled state.

👉 Thus, N and P show very low electron gain enthalpy values (close to zero).

🔹 Conceptual Examination Asked Questions Answers

Q1. Why do noble gases have positive electron gain enthalpy?

Answer: Noble gases have a stable electronic configuration ns²np⁶. Adding an extra electron means forcing it into the next higher energy level, where it experiences negligible attraction from the nucleus. Since energy is required for this, their electron gain enthalpy is positive.

Q2. Why are the electron gain enthalpies of Be and Mg almost zero?

Answer: Be (1s² 2s²) and Mg (1s² 2s² 2p⁶ 3s²) have completely filled s-orbitals, which are especially stable. They show very little tendency to gain an electron, so their electron gain enthalpy is almost zero.

Q3. Why are the electron gain enthalpies of N and P almost zero despite being non-metals?

Answer: N (1s² 2s² 2p_x¹ 2p_y¹ 2p_z¹) and P (1s² 2s² 2p⁶ 3s² 3p_x¹ 3p_y¹ 3p_z¹) have half-filled p-orbitals, which are stabilized by exchange energy. Adding another electron would disturb this stable arrangement, hence their electron gain enthalpy is very low (almost zero).

Q4. Which group of elements shows the most negative electron gain enthalpy?

Answer: Halogens (Group 17) show the most negative electron gain enthalpy, because they need just one electron to complete their stable octet configuration.

Q5. Why is the electron gain enthalpy of chlorine more negative than that of fluorine?

Answer: Though fluorine is more electronegative, its small atomic size creates strong inter-electronic repulsion when an extra electron is added. In chlorine, the atomic size is slightly larger, so incoming electrons experience less repulsion. Hence, chlorine has more negative electron gain enthalpy than fluorine.

Comparison Table

✅ Key Point for Exams:

• Noble gases → positive Electron Gain Enthalpy (require energy).
• Be, Mg, N, P → almost zero Electron Gain Enthalpy (no tendency to gain electrons).
• Halogens → most negative Electron Gain Enthalpy (strong tendency to gain electrons).

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