Structure of Atom — Part 2: Quantum mechanical model and electronic configuration: visual lesson, CBSE Class 11 Chemistry

Lesson

The quantum mechanical atom

Part 2 — electrons as waves, orbitals instead of orbits, and how electrons fill them.

In this lesson you will

Use de Broglie’s relation and Heisenberg’s uncertainty principle
Assign quantum numbers and describe the shapes and nodes of s, p and d orbitals
Write electronic configurations using Aufbau, Pauli and Hund’s rules, including Cr and Cu

Concept

Limitations of the Bohr model

Bohr’s model works for H, He⁺, Li²⁺ but fails elsewhere.

Table of the limitations of Bohr’s model of the atom
Limitation	Explanation
Many-electron atoms	Cannot account for their spectra
Fine structure	Cannot explain splitting of lines in a magnetic or electric field (Zeeman and Stark effects)
Wave nature of the electron	Treats it only as a particle in a fixed orbit
Uncertainty principle	Gives exact position and speed, which cannot be known together

Concept

Dual nature of matter: de Broglie’s relation

de Broglie proposed that a moving particle has a wavelength: $\lambda = \dfrac{h}{mv} = \dfrac{h}{p}$ . It is significant only for microscopic particles; for large objects $\lambda$ is far too small to detect.

Table of de Broglie wavelengths of an electron, a cricket ball and a bullet
Object	Mass / kg	Speed / m s⁻¹	Wavelength / m
Electron	$9.11 \times 10^{-31}$	$1.0 \times 10^{6}$	$7.3 \times 10^{-10}$
Cricket ball	0.16	40	$1.0 \times 10^{-34}$
Bullet	0.050	300	$4.4 \times 10^{-35}$

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