Electronic Configuration — Revision Notes

⚡ 30-Second Revision

Aufbau Principle: — Fill lowest energy orbitals first. Order: $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, dots$
Pauli's Exclusion Principle: — Max 2 electrons per orbital, with opposite spins ($m_s = \pm 1/2$).
Hund's Rule: — For degenerate orbitals, fill singly with parallel spins before pairing.
Quantum Numbers: — $n$ (shell), $l$ (subshell, $0 \dots n-1$), $m_l$ (orbital orientation, $-l \dots +l$), $m_s$ (spin, $\pm 1/2$).
Exceptions: — Cr ($[Ar] 3d^5 4s^1$), Cu ($[Ar] 3d^{10} 4s^1$).
Ions: — Remove/add electrons from/to outermost shell (highest 'n') first for cations, lowest available for anions.

2-Minute Revision

Electronic configuration is the electron arrangement in orbitals, governed by three rules. The Aufbau principle dictates filling orbitals from lowest to highest energy, following the $(n+l)$ rule (e.

g., $4s$ before $3d$). Pauli's exclusion principle limits each orbital to two electrons, which must have opposite spins. Hund's rule states that degenerate orbitals (like $p_x, p_y, p_z$) are first filled singly with parallel spins before any pairing occurs, maximizing stability.

Remember exceptions for elements like Chromium ($[Ar] 3d^5 4s^1$) and Copper ($[Ar] 3d^{10} 4s^1$) due to the stability of half-filled or fully-filled subshells. For ions, electrons are removed from the outermost shell (highest 'n') first for cations, and added to the lowest available orbital for anions.

This configuration determines an atom's chemical properties, magnetic behavior, and position in the periodic table.

5-Minute Revision

Mastering electronic configuration is crucial for NEET. Start by understanding the fundamental principles: Aufbau principle, which dictates the filling order ($1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \dots$).

The $(n+l)$ rule helps predict this order: lower $(n+l)$ fills first; if $(n+l)$ is same, lower 'n' fills first. For example, $4s (4+0=4)$ fills before $3d (3+2=5)$. Next, Pauli's exclusion principle states that an orbital can hold a maximum of two electrons, and they must have opposite spins (e.

g., $\uparrow\downarrow$). Finally, Hund's rule of maximum multiplicity applies to degenerate orbitals (same energy, e.g., $p_x, p_y, p_z$). Electrons will first occupy each degenerate orbital singly with parallel spins before pairing up (e.

g., for $2p^3$, it's $\uparrow \quad \uparrow \quad \uparrow$, not $\uparrow\downarrow \quad \uparrow \quad \_$).

Crucial points for NEET include: Exceptions like Chromium (Cr, Z=24) which is $[Ar] 3d^5 4s^1$ (not $3d^4 4s^2$) and Copper (Cu, Z=29) which is $[Ar] 3d^{10} 4s^1$ (not $3d^9 4s^2$). These occur due to the extra stability of half-filled or fully-filled subshells.

For ions, remember to remove electrons from the outermost shell (highest principal quantum number 'n') first when forming cations. For example, for $Fe^{2+}$ (Fe is $[Ar] 4s^2 3d^6$), electrons are removed from $4s$, giving $[Ar] 3d^6$.

For anions, add electrons to the lowest available orbital. Practice drawing orbital diagrams to visualize electron distribution and determine the number of unpaired electrons, which dictates magnetic properties (paramagnetic if unpaired, diamagnetic if all paired).

Also, be proficient with quantum numbers and their allowed ranges ($l < n$, $-l \le m_l \le +l$).

Prelims Revision Notes

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

* Aufbau Principle: Electrons fill orbitals in increasing order of energy. The general order is $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p$. * **$(n+l)$ Rule:** For orbitals with the same $(n+l)$ sum, the one with lower 'n' is filled first (e.

g., $3p$ before $4s$, $4s$ before $3d$). * Pauli's Exclusion Principle: An orbital can hold a maximum of two electrons, and these two electrons must have opposite spins ($+1/2$ and $-1/2$). No two electrons in an atom can have the same set of all four quantum numbers.

* Hund's Rule of Maximum Multiplicity: For degenerate orbitals (orbitals of the same energy within a subshell, e.g., $p_x, p_y, p_z$), electrons occupy each orbital singly with parallel spins before any orbital is doubly occupied.

This maximizes stability.

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Quantum Numbers:

* Principal (n): Energy shell ($1, 2, 3, dots$). * Azimuthal (l): Subshell shape ($0$ for s, $1$ for p, $2$ for d, $3$ for f). Range: $0$ to $n-1$. * Magnetic (m_l): Orbital orientation (e.g., $p_x, p_y, p_z$). Range: $-l$ to $+l$. * Spin (m_s): Electron spin ($\pm 1/2$).

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Exceptions to Aufbau Principle:

* Chromium (Cr, Z=24): $[Ar] 3d^5 4s^1$ (not $3d^4 4s^2$) - half-filled stability. * Copper (Cu, Z=29): $[Ar] 3d^{10} 4s^1$ (not $3d^9 4s^2$) - fully-filled stability. * Similar exceptions exist for Mo, Ag, Au.

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Electronic Configuration of Ions:

* Cations (positive ions): Remove electrons from the outermost shell (highest 'n' value) first. If multiple orbitals have the same highest 'n', remove from 'p' then 's'. For transition metals, remove from $ns$ before $(n-1)d$. * Example: $Fe (Z=26): [Ar] 4s^2 3d^6 \implies Fe^{2+}: [Ar] 3d^6$ (remove from $4s$). * Anions (negative ions): Add electrons to the lowest energy available orbital according to Aufbau. * Example: $O (Z=8): 1s^2 2s^2 2p^4 \implies O^{2-}: 1s^2 2s^2 2p^6$.

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Magnetic Properties:

* Paramagnetic: Contains one or more unpaired electrons (attracted to magnetic field). * Diamagnetic: All electrons are paired (repelled by magnetic field). * Use Hund's rule to determine unpaired electrons from orbital diagrams.

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Periodic Table Connection: — Valence shell configuration determines group and period. s-block, p-block, d-block, f-block elements are classified based on the subshell of the last electron.

Vyyuha Quick Recall

To remember the three rules of electronic configuration: All People Have Spin.

Aufbau: Fill from lowest energy.
Pauli: Max 2 electrons per orbital, opposite spins.
Hund's: Single occupancy in degenerate orbitals first, then pair up.
Spin: Refers to the spin quantum number and the opposite spins in an orbital.