Inorganic Chemistry Cheat Sheet: Periodic Trends, Slater's Rules & VSEPR Geometry - NEET 2027

Welcome to this high-yield, LibreTexts-style reference module for NEET 2027 Inorganic Chemistry. Master the key principles of Effective Nuclear Charge (\(Z^*\)), Slater's Shielding Rules, Periodic Trends, and VSEPR Molecular Geometries based on the latest NTA exam pattern.

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Chapter 1: Effective Nuclear Charge & Slater's Rules

The effective nuclear charge (\(Z^*\)) experienced by an electron is given by:

\[ Z^* = Z - S \]

where \(Z\) is the atomic number and \(S\) is the shielding (or screening) constant determined by Slater's Rules:

Rules for \(s\) and \(p\) Electrons

• Same Shell (\(n\)): Each other electron in the same \(ns, np\) valence shell contributes \(0.35\) to \(S\) (except in \(1s\), where the other electron contributes \(0.30\)).
• Penultimate Shell (\(n-1\)): Each electron in the \((n-1)\) shell contributes \(0.85\) to \(S\).
• Inner Shells (\(n-2\) or lower): Each electron in lower shells contributes \(1.00\) to \(S\).

Rules for \(d\) and \(f\) Electrons

• Electrons in the same \(d\) or \(f\) group contribute \(0.35\) each.
• Electrons in all groups lying to the left (lower principal quantum numbers or inner subshells) contribute \(1.00\) each.

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Chapter 2: Periodic Trends & Anomalies

Understanding general trends across periods and down groups is critical for solving multi-statement questions in NEET 2027.

Key Trends Overview

• Atomic Radius: Decreases across a period (left to right) due to increasing \(Z^*\); increases down a group due to addition of electron shells.
• Ionization Energy (\(IE\)): Increases across a period, decreases down a group.
• Electron Affinity (\(EA\)): Most negative for Halogens (Group 17). Note that Chlorine has a more negative electron gain enthalpy than Fluorine: \(EA(\text{Cl}) > EA(\text{F})\) due to strong inter-electronic repulsions in the compact \(2p\) orbital of Fluorine.
• Electronegativity (\(\chi\)): Follows the Pauling scale order: \(\text{F} (4.0) > \text{O} (3.5) > \text{N} = \text{Cl} (3.0) > \text{Br} (2.8) > \text{C} = \text{S} = \text{I} (2.5) > \text{H} = \text{P} (2.1)\).

Crucial Ionization Energy Anomalies

• \(IE_1(\text{Be}) > IE_1(\text{B})\): Beryllium has a stable fully-filled subshell (\(2s^2\)), whereas Boron has a single electron in \(2p^1\).
• \(IE_1(\text{N}) > IE_1(\text{O})\): Nitrogen possesses a stable half-filled electronic configuration (\(2p^3\)), making electron removal harder compared to Oxygen (\(2p^4\)).

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Chapter 3: VSEPR Theory & Molecular Geometries

The Valence Shell Electron Pair Repulsion (VSEPR) model predicts the three-dimensional geometry of molecules based on electron-pair repulsions around a central atom.

Steric Number Formula

\[ \text{SN} = \frac{1}{2}\bigl(V + M - C + A\bigr) \]

where \(V\) is the number of valence electrons on the central atom, \(M\) is the number of monovalent bonding atoms, \(C\) is the cationic charge, and \(A\) is the anionic charge.

Summary Table of VSEPR Shapes

Steric No. (SN)	Bond Pairs (bp)	Lone Pairs (lp)	Molecular Geometry	Examples
2	2	0	Linear (\(180^\circ\))	$\text{BeCl}_2, \text{CO}_2$
3	3	0	Trigonal Planar (\(120^\circ\))	$\text{BF}_3$
3	2	1	Bent (\(<120^\circ\))	$\text{SO}_2, \text{NO}_2^-$
4	4	0	Tetrahedral (\(109.5^\circ\))	$\text{CH}_4, \text{NH}_4^+$
4	3	1	Trigonal Pyramidal	$\text{NH}_3, \text{PCl}_3$
4	2	2	Bent (\(104.5^\circ\))	$\text{H}_2\text{O}, \text{OF}_2$
5	5	0	Trigonal Bipyramidal	$\text{PCl}_5$
5	4	1	Seesaw	$\text{SF}_4$
5	3	2	T-shaped	$\text{ClF}_3$
5	2	3	Linear	$\text{XeF}_2, \text{I}_3^-$

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NTA Pattern & Chapter Weightage Analysis

In the NEET entrance examination, Inorganic Chemistry contributes approximately 34% to 36% of the chemistry section (15–18 questions out of 50). Periodic Properties and Chemical Bonding are the foundational pillars from which direct formula-based, assertion-reason, and matching-type questions are crafted by NTA.

• Chemical Bonding & VSEPR: 3-4 direct questions per year. High probability of questions asking for lone pair count, shape match, and bond angle comparisons.
• Periodic Trends & Anomalies: 2-3 direct questions per year focused on electron gain enthalpy exceptions ($\text{Cl} vs \text{F}$) and relative first/second ionization enthalpy orders.

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PYQ Practice & Structural Concept Check

Question: Predict the steric number, electron pair geometry, and hybridisation state for phosphorus in the given below molecule, which features a expanded octet with T-shaped arrangement under VSEPR theory:

```smiles FP(F)F ```

Explanation: The given structure represents phosphorus trifluoride/interhalogen analogue geometry ($\text{PF}_3$ or $\text{ClF}_3$ framework). For phosphorus trifluoride ($\text{PF}_3$), valence electrons $V = 5$, monovalent fluorine atoms $M = 3$. Steric Number $\text{SN} = \frac{1}{2}(5 + 3) = 4$. It possesses 3 bonding pairs and 1 lone pair, yielding a Trigonal Pyramidal molecular geometry with $sp^3$ hybridisation.

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People Also Ask (FAQ)

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