Inorganic Chemistry Cheat Sheet - NDA

Welcome to this LibreTexts-style comprehensive study guide designed specifically for National Defence Academy (NDA) aspirants. This quick-review module synthesizes fundamental inorganic chemistry concepts—ranging from atomic structure and periodic trends to chemical bonding, VSEPR configurations, coordination complexes, and acid-base chemistry—tailored according to the latest NTA and UPSC NDA exam patterns.

1. Atomic Structure & Periodicity

Quantum Numbers

Quantum numbers define the spatial distribution, energy levels, and spin orientations of electrons within an atom:

• Principal Quantum Number (\(n\)): Determines the energy level and overall size of the atomic orbital (\(n = 1, 2, 3, \dots\)).
• Azimuthal Quantum Number (\(l\)): Defines the orbital shape or subshell (\(l = 0\) to \(n-1\)), corresponding to \(s, p, d, f\) subshells.
• Magnetic Quantum Number (\(m_l\)): Indicates the orientation of the orbital in space (\(m_l = -l\) to \(+l\)).
• Spin Quantum Number (\(m_s\)): Specifies the direction of intrinsic electron spin (\(m_s = \pm \frac{1}{2}\)).

Periodic Trends Summary

Understanding periodic trends is vital for solving trend-ordering questions in the NDA exam paper:

• Atomic Radius: Decreases across a period (\(\to\)) due to increased effective nuclear charge; increases down a group (\(\downarrow\)) as new shells are added.
• Ionization Energy (IE): Increases across a period (\(\to\)); decreases down a group (\(\downarrow\)).
• Electron Affinity (EA): Becomes more negative/exothermic across a period (\(\to\)); becomes less negative down a group (\(\downarrow\)).
• Electronegativity: Increases across a period (\(\to\)); decreases down a group (\(\downarrow\)).

2. Chemical Bonding & VSEPR Theory

VSEPR Geometries by Steric Number (SN)

Valence Shell Electron Pair Repulsion (VSEPR) theory determines molecular geometry based on electron pair repulsion around a central atom:

• SN = 2: Linear structure with a bond angle of \(180^\circ\) (e.g., \(\text{BeCl}_2\)).
• SN = 3: Trigonal Planar structure with a bond angle of \(120^\circ\) (e.g., \(\text{BF}_3\)).
• SN = 4: Tetrahedral structure with a bond angle of \(109.5^\circ\) (e.g., \(\text{CH}_4\)). Here is the structural representation of methane:

```smiles C ```

• SN = 5: Trigonal Bipyramidal structure with bond angles of \(90^\circ\) and \(120^\circ\) (e.g., \(\text{PCl}_5\)).
• SN = 6: Octahedral structure with bond angles of \(90^\circ\) (e.g., \(\text{SF}_6\)).

Formal Charge Calculation

Formal charge helps evaluate the most stable Lewis structure for a molecule or polyatomic ion:

\[\text{FC} = V - N - \frac{B}{2}\]

where \(V\) represents total valence electrons, \(N\) represents non-bonding valence electrons, and \(B\) represents total bonding electrons.

3. Coordination Chemistry & Field Theory

IUPAC Naming Conventions

• Name the cation before the anion.
• Name ligands in alphabetical order before the central metal atom/ion.
• Anionic ligands end with '-o' (e.g., chloro, cyano).
• Neutral ligands maintain special names: \(\text{NH}_3\) (ammine), \(\text{H}_2\text{O}\) (aqua), \(\text{CO}\) (carbonyl).
• Specify the oxidation state of the central metal using Roman numerals enclosed in parentheses.

Crystal Field Stabilization Energy (CFSE)

For an octahedral transition metal complex, CFSE is calculated as:

\[\text{CFSE} = -0.4 n_{t_{2g}} + 0.6 n_{e_g} \Delta_0 + P\]

where \(n_{t_{2g}}\) and \(n_{e_g}\) are electron populations in the respective sub-levels, \(\Delta_0\) is the octahedral splitting energy, and \(P\) is the electron pairing energy.

4. Acids, Bases & HSAB Principle

The Hard and Soft Acids and Bases (HSAB) principle states that Hard Acids preferentially coordinate with Hard Bases, and Soft Acids coordinate with Soft Bases:

• Hard Acids: \(\text{H}^+\), \(\text{Li}^+\), \(\text{Na}^+\), \(\text{Mg}^{2+}\), \(\text{Al}^{3+}\), \(\text{Ti}^{4+}\)
• Soft Acids: \(\text{Cu}^+\), \(\text{Ag}^+\), \(\text{Hg}^{2+}\), \(\text{Pd}^{2+}\), \(\text{Pt}^{2+}\)
• Hard Bases: \(\text{H}_2\text{O}\), \(\text{OH}^-\), \(\text{F}^-\), \(\text{Cl}^-\), \(\text{NH}_3\)
• Soft Bases: \(\text{I}^-\), \(\text{CN}^-\), \(\text{CO}\), \(\text{S}_2\text{O}_3^{2-}\)

5. NDA Exam Weightage & PYQ Analysis

In the General Ability Test (GAT) section of the NDA exam, Chemistry contributes approximately 15 questions (60 marks). Inorganic Chemistry accounts for roughly 40-50% of these questions. Recent UPSC trends show a heavy emphasis on VSEPR geometries, periodic trends, acid-base character, and basic formulas. Mastering simple direct calculations like formal charge and understanding coordination ligand types yields high-accuracy scoring potential.

6. Frequently Asked Questions (FAQ)