Class 10 Computer Science Chapter 1: Office Tools (Notes & Online Test) - PSEB

STRUCTURE OF ATOM

NCERT Masterclass & NEET Study Module

1. Discovery of Subatomic Particles

Atoms are made up of three fundamental particles: electrons, protons, and neutrons.

• Electron: Discovered by J.J. Thomson via cathode ray tube experiment. $e/m$ ratio $= 1.758820 \times 10^{11} \text{ C kg}^{-1}$. Charge on electron was determined by R.A. Millikan (Oil drop experiment).
• Proton: Discovered by Goldstein (canal rays).
• Neutron: Discovered by James Chadwick by bombarding a thin sheet of beryllium by $\alpha$-particles.

Important Terms:

• Isotopes: Atoms with identical atomic number ($Z$) but different atomic mass number ($A$).
• Isobars: Atoms with same mass number ($A$) but different atomic number ($Z$).
• Isotones: Atoms with the same number of neutrons ($A - Z$).

2. Electromagnetic Radiation & Photoelectric Effect

Planck's Quantum Theory: Energy is emitted or absorbed not continuously, but discontinuously in the form of small discrete packets called quanta (or photons in case of light). $E = h\nu = \frac{hc}{\lambda}$

Photoelectric Effect: Ejection of electrons when light of certain minimum frequency ($\nu_0$) strikes a metal surface.

$$h\nu = h\nu_0 + \frac{1}{2}m_e v^2 \implies E = W_0 + K.E._{max}$$

3. Bohr's Model for Hydrogen Atom

Electrons revolve in discrete stationary orbits without radiating energy. Angular momentum is quantized: $mvr = \frac{nh}{2\pi}$.

• Radius of $n^{th}$ orbit: $r_n = 52.9 \times \frac{n^2}{Z} \text{ pm}$
• Energy of $n^{th}$ orbit: $E_n = -2.18 \times 10^{-18} \times \frac{Z^2}{n^2} \text{ J/atom} = -13.6 \times \frac{Z^2}{n^2} \text{ eV/atom}$

Hydrogen Spectrum: When an electron drops from higher energy level ($n_2$) to lower ($n_1$), a photon is emitted.
Wave number $\bar{\nu} = \frac{1}{\lambda} = R_H \left( \frac{1}{n_1^2} - \frac{1}{n_2^2} \right) Z^2$

4. Quantum Mechanical Model & Dual Behavior

de Broglie's Relationship: All matter exhibits wave-like behavior. $\lambda = \frac{h}{mv} = \frac{h}{p}$

Heisenberg's Uncertainty Principle: It is impossible to determine simultaneously, the exact position and exact momentum (or velocity) of an electron. $\Delta x \cdot \Delta p \ge \frac{h}{4\pi}$

Quantum Numbers:

1. Principal Q.N. ($n$): Shell/Energy level. ($n = 1, 2, 3...$)
2. Azimuthal Q.N. ($l$): Subshell/Shape. ($l = 0$ to $n-1$). ($l=0 \rightarrow s, l=1 \rightarrow p, l=2 \rightarrow d, l=3 \rightarrow f$).
3. Magnetic Q.N. ($m_l$): Orbital orientation. ($m_l = -l$ to $+l$).
4. Spin Q.N. ($m_s$): Electron spin. ($+\frac{1}{2}, -\frac{1}{2}$).

Total nodes = $n - 1$. Radial nodes = $n - l - 1$. Angular nodes = $l$.

5. Rules for Filling Electrons

• Aufbau Principle: Electrons are added progressively to the various orbitals in their order of increasing energy (Based on $(n+l)$ rule). $1s < 2s < 2p < 3s < 3p < 4s < 3d...$
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. (An orbital holds max 2 electrons with opposite spins).
• Hund's Rule of Maximum Multiplicity: Electron pairing in orbitals of the same subshell ($p, d, f$) does not take place until each orbital is singly occupied.

NEET Exceptions: Half-filled and fully-filled subshells are extra stable due to symmetrical distribution and maximum exchange energy.
Chromium (Cr, Z=24): $[Ar] 4s^1 3d^5$ (not $4s^2 3d^4$)
Copper (Cu, Z=29): $[Ar] 4s^1 3d^{10}$ (not $4s^2 3d^9$)

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🚀 NEET ATOMIC STRUCTURE MEGA QUIZ (100 MCQ)

Solve the 5 parts below to master Bohr's Model, Quantum Mechanics, and Electronic Config.

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