Electronic Configuration — Revision Notes

⚡ 30-Second Revision

Electronic Configuration: — Distribution of electrons in atomic orbitals.
Rules:

- Aufbau Principle: Fill lowest energy orbitals first ($1s < 2s < 2p < 3s < 3p < 4s < 3d < ...$). Use $(n+l)$ rule. - Pauli Exclusion Principle: Max 2 electrons per orbital, with opposite spins ($\uparrow\downarrow$). - Hund's Rule: Degenerate orbitals filled singly with parallel spins first ($\uparrow\uparrow\uparrow$).

Noble Gas Configuration:

- Helium (He, Z=2): $1s^2$ (stable duplet) - Others (Ne, Ar, Kr, Xe, Rn, Og): $ns^2np^6$ (stable octet)

Properties: — High stability, chemical inertness, very high ionization enthalpy, positive/slightly negative electron gain enthalpy.

2-Minute Revision

Electronic configuration describes how electrons are arranged in an atom's orbitals, following three key rules. The Aufbau principle dictates filling orbitals from lowest to highest energy, often determined by the $(n+l)$ rule (e.

g., $4s$ before $3d$). The Pauli exclusion principle states that each orbital can hold a maximum of two electrons, but they must have opposite spins. Hund's rule applies to degenerate orbitals (same energy, like $p_x, p_y, p_z$), requiring them to be filled singly with parallel spins before any pairing occurs.

For Group 18 elements, the noble gases, their electronic configurations are exceptionally stable. Helium has a $1s^2$ configuration (a stable duplet), while all other noble gases (Neon, Argon, Krypton, Xenon, Radon) possess a general valence shell configuration of $ns^2np^6$, representing a stable octet.

This complete filling of their outermost electron shells is the reason for their characteristic chemical inertness, very high ionization energies, and positive or slightly negative electron gain enthalpies, making them highly unreactive under normal conditions.

Understanding these configurations is crucial for predicting chemical behavior and periodic trends.

5-Minute Revision

Electronic configuration is the blueprint of an atom's electron arrangement, governed by fundamental quantum mechanical principles. The Aufbau principle guides the sequential filling of orbitals based on increasing energy.

Remember the energy order: $1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < ...$. A quick way to remember this is the $(n+l)$ rule: lower $(n+l)$ means lower energy; if $(n+l)$ is equal, the orbital with lower 'n' is lower in energy (e.

g., $4s$ ($4+0=4$) fills before $3d$ ($3+2=5$)). The Pauli exclusion principle limits each atomic orbital to a maximum of two electrons, which must possess opposite spins ($\uparrow\downarrow$). This ensures each electron has a unique quantum state.

Finally, Hund's rule addresses degenerate orbitals (orbitals of the same energy, like the three $p$ orbitals). It states that electrons will first occupy each degenerate orbital singly with parallel spins before any orbital receives a second electron with opposite spin.

This minimizes electron-electron repulsion and enhances stability.

For Group 18 elements (Noble Gases), their electronic configurations are the epitome of stability. Helium (Z=2) has $1s^2$, a completely filled first shell (a stable duplet). All other noble gases (Neon, Argon, Krypton, Xenon, Radon) share a general valence shell configuration of $ns^2np^6$.

This 'octet' configuration signifies a completely filled outermost s and p subshells. This inherent stability explains their defining characteristics: extremely low chemical reactivity (inertness), very high ionization enthalpies (difficult to remove an electron), and positive or near-zero electron gain enthalpies (unfavorable to add an electron).

For example, Neon (Z=10) is $1s^22s^22p^6$, and Argon (Z=18) is $1s^22s^22p^63s^23p^6$. Recognizing these configurations and their implications is vital for NEET, as it underpins concepts of periodicity, chemical bonding, and the reactivity of other elements striving to achieve such noble gas stability.

Prelims Revision Notes

1
Definition: — Electronic configuration is the distribution of electrons in atomic orbitals.
2
Rules for Filling Orbitals:

* Aufbau Principle: Electrons occupy the lowest energy orbitals first. The approximate order is $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p$. Use the $(n+l)$ rule: lower $(n+l)$ means lower energy.

If $(n+l)$ is same, lower 'n' means lower energy. * Pauli Exclusion Principle: An atomic orbital can hold a maximum of two electrons, and these two electrons must have opposite spins ($m_s = +1/2$ and $m_s = -1/2$).

No two electrons in an atom can have all four quantum numbers identical. * Hund's Rule of Maximum Multiplicity: For degenerate orbitals (orbitals of the same energy, e.g., $p_x, p_y, p_z$), electrons will first occupy each orbital singly with parallel spins before any orbital is doubly occupied.

This maximizes spin multiplicity and minimizes electron-electron repulsion.

1
Electronic Configuration of Group 18 Elements (Noble Gases):

* Helium (He, Z=2): $1s^2$. This is a stable duplet. * Neon (Ne, Z=10): $1s^22s^22p^6$ or $[He]2s^22p^6$. * Argon (Ar, Z=18): $1s^22s^22p^63s^23p^6$ or $[Ne]3s^23p^6$. * Krypton (Kr, Z=36): $[Ar]4s^23d^{10}4p^6$. * Xenon (Xe, Z=54): $[Kr]5s^24d^{10}5p^6$. * Radon (Rn, Z=86): $[Xe]6s^24f^{14}5d^{10}6p^6$. * General Valence Configuration (except He): $ns^2np^6$. This is a stable octet.

1
Significance of Noble Gas Configuration:

* Stability: Completely filled valence shell leads to exceptional stability. * Inertness: Very low chemical reactivity due to stable configuration. * Ionization Enthalpy: Very high, as it's difficult to remove an electron from a stable configuration. * Electron Gain Enthalpy: Positive or slightly negative, indicating an unfavorable process to add an electron.

1
NEET Focus: — Be able to write configurations, identify elements from configurations, explain properties based on configurations, and understand the order of orbital filling.

Vyyuha Quick Recall

For the Aufbau order (up to $4p$): Some People Say People Should Dance Perfectly. (1s 2s 2p 3s 3p 4s 3d 4p)