Structure of Atom

JKBOSE/CBSE
Structure of Atom Class 11 Chemistry Notes
Structure of Atom - Class 11 Chemistry

Structure of Atom

Overview: Discovery of subatomic particles, atomic models, quantum mechanical model, and electronic configuration.

1. Subatomic Particles

  • Electron: Discovered by J.J. Thomson (Cathode Ray). Charge -1.6 × 10-19 C. Mass 9.1 × 10-31 kg.
  • Proton: Discovered by Goldstein (Canal Rays). Charge +1.6 × 10-19 C. Mass 1.672 × 10-27 kg.
  • Neutron: Discovered by Chadwick. Neutral. Mass 1.674 × 10-27 kg.

2. Atomic Models

Rutherford's Model

Alpha scattering experiment. Discovered Nucleus. Flaw: Could not explain stability (Maxwell theory) and line spectra.

Bohr's Model

Electrons revolve in fixed orbits (stationary states). Quantization of angular momentum.

mvr = nh / 2π

Energy of orbit:

En = -13.6 Z² / n² (eV)

3. Dual Nature & Uncertainty

de Broglie Relationship

Matter has both particle and wave nature.

λ = h / mv = h / p

Heisenberg Uncertainty Principle

Impossible to determine exact position and momentum simultaneously.

Δx · Δp ≥ h / 4π

4. Quantum Numbers

  • Principal (n): Shell. Size and Energy. (1, 2, 3...)
  • Azimuthal (l): Subshell. Shape. (0 to n-1). s=0, p=1, d=2, f=3.
  • Magnetic (ml): Orbital. Orientation. (-l to +l).
  • Spin (ms): Spin direction. (+1/2, -1/2).

5. Filling of Orbitals

  • Aufbau Principle: Fill lowest energy orbitals first (n+l rule).
  • Pauli Exclusion Principle: No two electrons can have same 4 quantum numbers.
  • Hund's Rule: Pairing starts only after each degenerate orbital has one electron.

Half-filled/Full-filled Stability: Cr (3d5 4s1) and Cu (3d10 4s1) are exceptions.

Numericals - Structure of Atom

Numericals

Energy of Photon
Q1. Calculate energy of one photon of light with wavelength 400 nm.
E = hc / λ
h = 6.626 × 10-34 J s
λ = 400 × 10-9 m
E = (6.626 × 10-34 × 3 × 108) / (4 × 10-7)
E = 4.97 × 10-19 J
Bohr Energy
Q2. Calculate energy associated with 5th orbit of Hydrogen.
E_n = -2.18 × 10-18 / n² J
E_5 = -2.18 × 10-18 / 25
E_5 = -8.72 × 10-20 J
de Broglie Wavelength
Q3. Calculate wavelength of electron moving with v = 2.05 × 107 m/s.
λ = h / mv
m = 9.1 × 10-31 kg
λ = 6.626 × 10-34 / (9.1 × 10-31 × 2.05 × 107)
λ = 3.55 × 10-11 m
Uncertainty Principle
Q4. Microscope locates electron within 0.1 Angstrom. Cal uncertainty in velocity.
Δx = 0.1 A = 10-11 m.
Δx Δv ≥ h / 4πm
Δv = h / (4πm Δx)
Δv = 6.626×10-34 / (4 × 3.14 × 9.1×10-31 × 10-11)
Δv ≈ 5.79 × 106 m/s
Rydberg Formula
Q5. Wavelength of first line of Balmer series (n2=3 to n1=2).
1/λ = R (1/n1² - 1/n2²)
1/λ = 109677 (1/4 - 1/9)
1/λ = 109677 (5/36)
λ = 656 nm (Red light)
Photoelectric Effect
Q6. Threshold freq is 7 × 1014 Hz. If freq 1 × 1015 Hz hits metal, find KE.
KE = h (v - v0)
KE = 6.626×10-34 (10 - 7) × 1014
KE = 6.626 × 3 × 10-20
KE = 1.988 × 10-19 J
Quantum Numbers
Q7. How many electrons in n=4 shell?
Number of electrons = 2n²
= 2 (4)² = 32 electrons.
Nodes
Q8. Calculate radial and angular nodes for 3p orbital.
n=3, l=1 (p).
Angular nodes = l = 1.
Radial nodes = n - l - 1 = 3 - 1 - 1 = 1.
Total nodes = 2.
Effective Nuclear Charge
Q9. Which has more negative electron gain enthalpy: O or F?
F has higher Zeff and smaller size.
It attracts incoming electron more strongly.
F (-328 kJ/mol) is more negative than O.
Isoelectronic
Q10. Which is isoelectronic with Na+?
Na+ has 11-1 = 10 electrons.
Examples: Ne (10), F- (9+1=10), Mg2+ (12-2=10).
Formulas & Facts - Structure of Atom

Equations & Formulas

ConceptFormula
Energy PhotonE = hν = hc/λ
Photoelectric Eqhν = hν0 + KE
Bohr Radius (H)rn = 0.529 n² (Angstrom)
Bohr Energy (H)En = -13.6 / n² (eV)
Rydberg Eq1/λ = R (1/n1² - 1/n2²)
de Broglieλ = h / mv
UncertaintyΔx Δp ≥ h / 4π
Orbital Ang MomL = √[l(l+1)] h/2π
Spin Magnetic Momμ = √[n(n+2)] BM
Total NodesN - 1

50 NEET Facts

Key points for Atomic Structure.

1. e/m Ratio e/m for electron is constant. Independent of gas and electrode material.
2. Canal Rays Positively charged. Depends on nature of gas in tube.
3. Atomic Number (Z) Number of protons. Characteristic of element.
4. Mass Number (A) Protons + Neutrons.
5. Isobars Same A, different Z (e.g. C-14, N-14).
6. Isotopes Same Z, different A (e.g. H-1, H-2, H-3).
7. Isotones Same number of neutrons (e.g. C-13, N-14).
8. Shortest Wavelength (R) Lyman series (UV region). Transition infinity to 1.
9. Visible Series Balmer series (n2 -> n1=2).
10. Infrared Series Paschen, Brackett, Pfund.
11. Bohr Limit Applicable only to single electron species (H, He+, Li2+).
12. Splitting of Lines Zeeman Effect (Magnetic field) and Stark Effect (Electric field) - Bohr couldn't explain.
13. Black Body Radiation Plank's Quantum Theory explained it. Energy is discretized.
14. Work Function Min energy required to eject electron. Characteristic of metal. (Cs has lowest).
15. Kinetic Energy Photoelectron Depends only on Frequency of incident light. Not Intensity.
16. Photocurrent Depends on Intensity of light.
17. KE of e in Bohr orbit KE = - Total Energy. PE = 2 × Total Energy.
18. Velocity in Orbit v ∝ Z/n.
19. Radius of Orbit r ∝ n²/Z.
20. Time Period T ∝ n³/Z².
21. Macroscopic Object Wavelength is negligible. (e.g., throwing a ball).
22. Microscopic Object Wavelength is significant. (Electron).
23. Orbital Region of high probability (90-95%) of finding electron.
24. s-orbital Spherical. Non-directional.
25. p-orbital Dumbbell. Directional (px, py, pz).
26. d-orbital Double Dumbbell (except dz2).
27. dz2 shape Doughtnut/Baby soother shape.
28. Nodal Plane Plane where probability is zero.
29. s-orbital nodes No angular nodes. Only radial nodes (n-1).
30. p-orbital nodes 1 angular node (nodal plane).
31. Principal QN (n) Determines Energy (mainly) and Size.
32. Azimuthal QN (l) Determines Shape. Energy in multi-electron atoms.
33. Magnetic QN (m) Determines Orientation in space.
34. Spin QN (s) Intrinsic spin. Not derived from Schrodinger Eq.
35. (n+l) Rule Lower (n+l) -> Lower energy. If same, lower n -> lower energy.
36. Degenerate Orbitals Orbitals with same energy (e.g. px, py, pz).
37. H-atom Degeneracy Energy depends only on n. 2s = 2p. 3s = 3p = 3d.
38. Cr Configuration [Ar] 3d5 4s1. (Half filled stability).
39. Cu Configuration [Ar] 3d10 4s1. (Full filled stability).
40. Exchange Energy Parallel spins exchange positions releasing energy. Maximized in half/full filled.
41. Paramagnetic Unpaired electrons. Attracted by magnet.
42. Diamagnetic All paired electrons. Repelled by magnet.
43. Magnetic Moment Depends on number of unpaired electrons (n).
44. Fe3+ 3d5. n=5. Magnetic moment = 5.9 BM.
45. Zn2+ 3d10. n=0. Diamagnetic. Colorless.
46. Color of Ions Due to d-d transition of unpaired electrons.
47. Sc3+ d0. Colorless.
48. Psi vs Psi^2 Psi is amplitude (no physical meaning). Psi^2 is probability density.
49. Boundary Surface Diagram enclosing region of 90% probability.
50. Max electrons in subshell s:2, p:6, d:10, f:14. Formula 2(2l+1).
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