Class 11 Chemistry begins with understanding the atom at a deeper level than you have seen before. Atomic structure and chemical bonding are the two chapters that form the foundation for organic chemistry, inorganic chemistry, and physical chemistry in Class 12 and beyond.
What Is the Modern Model of the Atom?
The atomic model has evolved over centuries. Here is a quick comparison:
| Model | Key Idea | Limitation |
|---|---|---|
| Dalton's | Atoms are indivisible spheres | Could not explain subatomic particles |
| Thomson's | Positive sphere with embedded electrons (plum pudding) | Could not explain Rutherford's scattering results |
| Rutherford's | Nucleus at centre, electrons orbit around it | Could not explain line spectra or atom stability |
| Bohr's | Electrons in fixed circular orbits with quantised energy | Only works for hydrogen-like atoms |
| Quantum mechanical | Electrons exist in orbitals (probability regions) | More complex but most accurate |
What Are the Four Quantum Numbers?
Every electron in an atom is described by four quantum numbers:
| Quantum Number | Symbol | Values | What It Tells You |
|---|---|---|---|
| Principal | n | 1, 2, 3, ... | Shell (energy level) |
| Azimuthal | l | 0 to (n-1) | Subshell shape (s, p, d, f) |
| Magnetic | ml | -l to +l | Orbital orientation |
| Spin | ms | +1/2 or -1/2 | Electron spin direction |
How Do You Write Electron Configuration?
Follow these three rules:
- Aufbau principle — Fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
- Pauli exclusion principle — Each orbital holds a maximum of 2 electrons with opposite spins
- Hund's rule — In a subshell, electrons fill empty orbitals first with parallel spins before pairing up
Note the exceptions: Chromium (Cr) is [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) and Copper (Cu) is [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²). Half-filled and fully filled d-orbitals are extra stable.
What Types of Chemical Bonds Exist?
Chemical bonding explains how atoms combine to form molecules. The three main types are:How Does VSEPR Theory Predict Molecular Shape?
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the shape of a molecule based on the number of bonding pairs (BP) and lone pairs (LP) around the central atom. Electron pairs repel each other and arrange themselves to minimise repulsion.| BP + LP | Shape | Bond Angle | Example |
|---|---|---|---|
| 2 + 0 | Linear | 180° | BeCl₂, CO₂ |
| 3 + 0 | Trigonal planar | 120° | BF₃ |
| 3 + 1 | Pyramidal | ~107° | NH₃ |
| 2 + 2 | Bent (V-shape) | ~104.5° | H₂O |
| 4 + 0 | Tetrahedral | 109.5° | CH₄ |
| 5 + 0 | Trigonal bipyramidal | 90°, 120° | PCl₅ |
| 6 + 0 | Octahedral | 90° | SF₆ |
What Is Hybridisation?
Hybridisation is the mixing of atomic orbitals to form new hybrid orbitals of equal energy. The type of hybridisation determines the molecular geometry:| Hybridisation | Orbitals Mixed | Shape | Example |
|---|---|---|---|
| sp | 1s + 1p | Linear | BeCl₂, C₂H₂ |
| sp² | 1s + 2p | Trigonal planar | BF₃, C₂H₄ |
| sp³ | 1s + 3p | Tetrahedral | CH₄, NH₃, H₂O |
| sp³d | 1s + 3p + 1d | Trigonal bipyramidal | PCl₅ |
| sp³d² | 1s + 3p + 2d | Octahedral | SF₆ |
Key Takeaways
Frequently Asked Questions
Why is the quantum mechanical model better than Bohr's model?
Bohr's model only works accurately for hydrogen and hydrogen-like atoms (single electron). The quantum mechanical model describes multi-electron atoms correctly by treating electrons as waves and using probability distributions instead of fixed orbits.
How do you quickly find the hybridisation of a molecule?
Count the number of atoms bonded to the central atom plus the number of lone pairs on it. If the total is 2, it is sp. If 3, it is sp². If 4, it is sp³. If 5, it is sp³d. If 6, it is sp³d².
What is the difference between sigma and pi bonds?
A sigma bond is formed by head-on overlap of orbitals along the bond axis. It is stronger and allows free rotation. A pi bond is formed by sideways overlap of p-orbitals above and below the bond axis. It is weaker and prevents rotation. A single bond is one sigma bond. A double bond is one sigma plus one pi. A triple bond is one sigma plus two pi.
Why are half-filled and fully filled d-orbitals extra stable?
Half-filled (d⁵) and fully filled (d¹⁰) configurations have maximum exchange energy due to the symmetrical distribution of electrons. This extra stability is why chromium and copper have exceptional electron configurations.
Practice Class 11 Chemistry concepts with our Chemistry Grade 11 quizzes covering atomic structure, bonding, and more.