Structure of Atom
Easy Overview
Ever tried to take a selfie of your soul? That's basically what scientists did when they tried to figure out what an atom looks like inside. This chapter takes you from simple 'billiard ball' models to the quantum view — where electrons act like waves, exist in orbitals, and can be described by weird-sounding things like quantum numbers. Buckle up.
Evolution of Atomic Models
Dalton said atoms are indestructible spheres. Thomson discovered electrons and made the 'plum pudding' model. Rutherford shot alpha particles at gold foil and found a tiny nucleus. Bohr said electrons orbit in fixed energy levels. Each model improved on the last — like phone upgrades, but over a century.
Quantum Numbers — The Electron's Address
Every electron in an atom has a unique address. Principal quantum number (n) tells you the energy level — like what floor of a building. Azimuthal (l) tells you the shape of the orbital — like whether it's a hallway or a room. Magnetic (m) tells you orientation — which direction the room faces. Spin (s) tells you whether the electron spins clockwise or counterclockwise.
Shapes of Orbitals
s orbitals are spherical — like a balloon around the nucleus. p orbitals are dumbbell-shaped — like two balloons tied together. d orbitals are even fancier — clover shapes. f orbitals are just weird. Higher l means more complicated shapes.
Electronic Configuration — Aufbau and Friends
Electrons fill orbitals in a specific order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p... Aufbau principle says fill from lowest energy first. Pauli's exclusion principle says no two electrons in an atom can have identical quantum numbers — so at most 2 per orbital. Hund's rule says electrons spread out before pairing up, like people on an empty bus.
de Broglie and Heisenberg — The Quantum Weirdness
de Broglie said electrons behave like waves too. Heisenberg said you can't know both where an electron is and how fast it's going at the same time — the uncertainty principle. It's not about bad measurement; it's a fundamental limit of nature. Electrons are fuzzy, not tiny planets.
Key Points
- •Rutherford's gold foil experiment proved the nucleus exists
- •Bohr model works for hydrogen but fails for multi-electron atoms
- •Quantum numbers: n (shell), l (subshell), m (orbital), s (spin)
- •s orbital = spherical, p orbital = dumbbell, d orbital = clover, f = complex
- •Aufbau: fill from lowest energy; Pauli: max 2 electrons per orbital; Hund: spread before pairing
- •de Broglie: matter has wave nature. Heisenberg: can't know both position and momentum exactly
- •Electronic configuration of Cu: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ (exception due to stability)
Practice Questions
- What are quantum numbers? Assign all four quantum numbers to the last electron in chlorine (Z = 17).
- Explain Heisenberg's uncertainty principle. Why does it not apply to macroscopic objects?
- Write the electronic configuration of Fe (Z = 26) and Fe²⁺.
- What is the shape of s, p, and d orbitals? Draw them.
- State and explain Hund's rule of maximum multiplicity with an example.