Atomic Structure
Chemistry · परमाणु संरचना
📋Quick Overview
An atom is the smallest particle of an element that takes part in a chemical reaction. The word 'atom' comes from the Greek word 'atomos' meaning indivisible. John Dalton proposed the Atomic Theory in 1808. Later, J.J. Thomson discovered the electron (1897), Rutherford discovered the nucleus and proton (1911), and Chadwick discovered the neutron (1932).
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Atom has 3 sub-atomic particles: Proton (+ve, in nucleus), Neutron (neutral, in nucleus), Electron (-ve, orbits around nucleus)
📖Sub-Atomic Particles
| Particle | Symbol | Charge | Mass (amu) | Location | Discoverer |
|---|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 | Nucleus | Rutherford (1919) |
| Neutron | n⁰ | 0 | 1 | Nucleus | Chadwick (1932) |
| Electron | e⁻ | -1 | 1/1836 | Orbits/Shells | J.J. Thomson (1897) |
📖Atomic Number & Mass Number
- •Atomic Number (Z) = Number of protons in the nucleus = Number of electrons in a neutral atom
- •Mass Number (A) = Protons + Neutrons (A = Z + N)
- •Number of Neutrons = A - Z (Mass Number minus Atomic Number)
- •Example: Carbon-12 → Z=6, A=12, so Neutrons = 12-6 = 6
📖Isotopes, Isobars & Isotones
| Type | Definition | Same | Different | Example |
|---|---|---|---|---|
| Isotopes | Same element, different mass number | Atomic number (Z) | Mass number (A) | ¹H, ²H (Deuterium), ³H (Tritium); C-12, C-14 |
| Isobars | Different elements, same mass number | Mass number (A) | Atomic number (Z) | ⁴⁰Ar, ⁴⁰K, ⁴⁰Ca (all A=40) |
| Isotones | Different elements, same number of neutrons | Number of neutrons | Atomic number & mass number | ¹⁴C and ¹⁵N (both have 8 neutrons) |
📖Atomic Models
| Model | Scientist | Year | Key Idea | Limitation |
|---|---|---|---|---|
| Plum Pudding | J.J. Thomson | 1904 | Atom is a +ve sphere with electrons embedded like plums in pudding | Failed after Rutherford's experiment |
| Nuclear Model | Rutherford | 1911 | Atom is mostly empty space; nucleus is +ve center; electrons revolve around | Could not explain stability of atom |
| Bohr Model | Niels Bohr | 1913 | Electrons revolve in fixed orbits (shells K,L,M,N) with definite energy | Could not explain multi-electron atoms well |
📖Electron Configuration — Shells
| Shell | Letter | n | Max Electrons (2n²) |
|---|---|---|---|
| 1st Shell | K | 1 | 2 |
| 2nd Shell | L | 2 | 8 |
| 3rd Shell | M | 3 | 18 |
| 4th Shell | N | 4 | 32 |
- •Formula: Maximum electrons in a shell = 2n² (where n = shell number)
- •Outermost shell can have maximum 8 electrons (Octet Rule)
- •Valence electrons = electrons in outermost shell → decide chemical properties
- •Example: Sodium (Na, Z=11) → 2, 8, 1 → Valence electrons = 1
- •Example: Chlorine (Cl, Z=17) → 2, 8, 7 → Valence electrons = 7
📖Rutherford's Gold Foil Experiment
- •Alpha particles fired at thin gold foil (1911)
- •Most particles passed straight through → Atom is mostly empty space
- •Some particles deflected at small angles → Positive charge in center
- •Very few bounced back (1 in 20000) → Nucleus is very small but dense and +ve