Class 10 Chemistry โ Chapter 2: Atomic Structure
Sindh Board โ Complete solved notes, definitions, solved short & long questions, and quick revision points for Chapter 2.
Important Definitions
- Atom: The smallest particle of an element that can take part in a chemical reaction.
- Atomic Number (Z): Number of protons in the nucleus of an atom.
- Mass Number (A): Total number of protons and neutrons in the nucleus.
A = Z + N. - Isotopes: Atoms of the same element with the same atomic number but different mass numbers (e.g., Cโ12, Cโ14).
- Isobars: Atoms of different elements having the same mass number but different atomic numbers.
- Orbitals: Regions around the nucleus where electrons are most likely to be found (s, p, d, f).
- Quantum Numbers: Four numbers (n, l, m, s) that describe the energy and position of an electron.
- Electronic Configuration: Distribution of electrons in shells and subshells (eg. Na:
1sยฒ 2sยฒ 2pโถ 3sยน).
Rutherfordโs Atomic Model (Summary)
- Most of the atom is empty space.
- All positive charge and most mass are concentrated in a small, dense nucleus.
- Electrons move around the nucleus.
Limitations: Could not explain atomic stability or spectral lines of hydrogen; predicted orbiting electrons should lose energy and fall into nucleus.
Bohrโs Model (Key Points)
- Electrons move in fixed circular orbits with quantized energies.
- Electrons do not radiate energy while in permitted orbits.
- Energy is emitted or absorbed when electrons jump between orbits:
ฮE = hฮฝ. - Explained the hydrogen atom spectrum successfully but failed for multi-electron atoms.
Quantum Numbers (n, l, m, s)
- Principal (n): 1,2,3,… indicates main energy level and size of orbital.
- Azimuthal (l): 0 to nโ1 defines subshell (0=s,1=p,2=d,3=f).
- Magnetic (m): โl … 0 … +l defines orientation of orbital.
- Spin (s): +ยฝ or โยฝ describes electron spin; maximum two electrons per orbital with opposite spins.
Example: For 3p orbital: n=3, l=1, m can be โ1,0,+1.
Electronic Configuration & Aufbau Principle
Electrons fill the lowest energy orbitals first (Aufbau principle), obey Pauli exclusion principle and Hund’s rule.
| Element | Atomic No. | Electronic Configuration |
|---|---|---|
| Hydrogen (H) | 1 | 1sยน |
| Carbon (C) | 6 | 1sยฒ 2sยฒ 2pยฒ |
| Sodium (Na) | 11 | 1sยฒ 2sยฒ 2pโถ 3sยน |
| Chlorine (Cl) | 17 | 1sยฒ 2sยฒ 2pโถ 3sยฒ 3pโต |
Isotopes & Isobars
Isotopes: Same Z, different A. Example: ^{12}C and ^{14}C. Uses: Cโ14 dating, medical tracers.
Isobars: Different elements, same mass number. Example: ^{40}Ca (Z=20) and ^{40}Ar (Z=18).
Solved Short Questions
1sยฒ 2sยฒ 2pโถ 3sยน.Solved Long Questions
Rutherford proposed a tiny dense positively charged nucleus with electrons moving around it. Limitations: could not explain atomic stability, electron energy levels, or hydrogen spectrum lines.
Bohr introduced quantized orbits with fixed energy. Electrons jump between orbits emitting/absorbing photons: ฮE = hv. Limitations: only explains hydrogen-like atoms; fails for multi-electron atoms and doesn’t account for fine spectral details.
Principal n=2, Azimuthal l=1 (p), Magnetic m=โ1,0,+1, Spin s=+ยฝ or โยฝ. Example: For 2p electron, n=2, l=1.
Used in medicine (diagnosis & therapy), archaeology (radiocarbon dating), industry (tracers), and research.
Quick Revision Points
- Z = number of protons; A = protons + neutrons.
- Isotopes: same Z, different A; Isobars: different Z, same A.
- Electrons fill orbitals by Aufbau principle; Pauli exclusion limits two electrons per orbital.
- Quantum numbers uniquely identify an electron in an atom.
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