Atomic Structure

2.1 Sub-atomic Particles

The atom comprises three fundamental particles:

Particle	Symbol	Charge	Mass (a.m.u.)	Location
Proton	p	+1	1.00728	Nucleus
Neutron	n	0	1.00867	Nucleus
Electron	e⁻	−1	0.000549	Orbitals around nucleus

Atomic Number (Z): Number of protons in the nucleus. Z uniquely identifies an element. For a neutral atom, Z also equals the number of electrons.

Mass Number (A): A = Z + N (number of neutrons). The mass number is not exact atomic mass — it rounds to the nearest integer.

2.2 Bohr's Model (1913)

Niels Bohr's model (1913) proposed:

1. Electrons orbit the nucleus in fixed circular paths called shells or energy levels (K, L, M, N…).
2. Each shell has a maximum capacity: K = 2, L = 8, M = 18, N = 32 (formula: 2n²).
3. Electrons do not emit radiation while in these shells. When an electron jumps from higher to lower shell, it emits a photon of specific wavelength (explains atomic spectra).

Limitation: Bohr's model works well for hydrogen (1 electron) but fails for multi-electron atoms.

2.3 Isotopes, Isobars, Isotones

Type	Definition	Example
Isotopes	Same Z, different A	¹²C and ¹⁴C (carbon-12 and carbon-14)
Isobars	Same A, different Z	⁴⁰Ar and ⁴⁰Ca
Isotones	Same N (neutrons), different Z	³H (tritium) and ⁴He

Key isotope applications:

• Carbon-14 (half-life 5,730 years): Radiocarbon dating of organic materials up to ~50,000 years old.
• Uranium-235: Nuclear fission fuel in reactors and weapons.
• Cobalt-60: Gamma radiation source for cancer radiotherapy.
• Iodine-131 (half-life 8 days): Diagnosis and treatment of thyroid disorders.

2.4 Electronic Configuration and Valency

Electrons fill shells in order K → L → M → N. Valency is determined by the number of electrons in the outermost shell (valence shell).

Elements with the same valency in the same group behave similarly. Noble gases (He, Ne, Ar, Kr) have full outer shells — valency 0 — explaining their inertness.

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