Structure of the Atom
An atom is the smallest unit of an element that retains its chemical properties. It consists of a dense nucleus (protons + neutrons) surrounded by electrons in energy levels.
A. Sub-atomic Particles
Proton, Neutron & Electron
| Particle | Symbol | Charge | Mass (amu) | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 | Nucleus |
| Neutron | n⁰ | 0 (neutral) | 1 | Nucleus |
| Electron | e⁻ | −1 | ~0 (1/1836 amu) | Orbitals around nucleus |
Proton
Symbolp⁺
Charge+1
Mass (amu)1
LocationNucleus
Neutron
Symboln⁰
Charge0 (neutral)
Mass (amu)1
LocationNucleus
Electron
Symbole⁻
Charge−1
Mass (amu)~0 (1/1836 amu)
LocationOrbitals around nucleus
Key Definitions
| Term | Definition | Example |
|---|---|---|
| Atomic Number (Z) | Number of protons in the nucleus — defines the element | Carbon: Z = 6 (6 protons) |
| Mass Number (A) | Total protons + neutrons in the nucleus | Carbon-12: A = 12 |
| Neutrons | Mass Number − Atomic Number = number of neutrons | ¹²C: 12 − 6 = 6 neutrons |
| Isotopes | Same element (same Z), different number of neutrons (different A) | ¹²C and ¹⁴C — both carbon, different mass |
| Ions | Atoms that gain or lose electrons — acquire charge | Na⁺ (lost 1e⁻), Cl⁻ (gained 1e⁻) |
| Valence Electrons | Electrons in the outermost (highest) shell — determine reactivity | Na (2,8,1): 1 valence electron |
Neutrons
Neutrons = Mass Number (A) − Atomic Number (Z)
⚡ MCQ Tip Atomic number = protons = electrons (in neutral atom). Mass number = protons + neutrons. Isotopes differ ONLY in neutron number — same element, same Z.
B. Atomic Models (Historical Development)
Evolution of the Atomic Model
| Scientist | Year | Model | Key Idea |
|---|---|---|---|
| John Dalton | 1808 | Solid sphere | Atoms are tiny, indivisible, indestructible solid spheres. All atoms of same element are identical. |
| J.J. Thomson | 1897 | Plum pudding | Discovered electrons. Atom = positive sphere with electrons embedded like plums in a pudding. |
| Ernest Rutherford | 1911 | Nuclear model | Gold foil experiment: discovered the small, dense, positive nucleus. Electrons orbit outside at distance. |
| Niels Bohr | 1913 | Planetary model | Electrons move in fixed circular orbits (energy levels/shells). Each orbit has a fixed energy. |
| Quantum Mechanical | Modern | Electron cloud | Electrons exist in probability clouds called orbitals (s, p, d, f). Cannot predict exact position. |
John Dalton
Year1808
ModelSolid sphere
Key IdeaAtoms are tiny, indivisible, indestructible solid spheres. All atoms of same element are identical.
J.J. Thomson
Year1897
ModelPlum pudding
Key IdeaDiscovered electrons. Atom = positive sphere with electrons embedded like plums in a pudding.
Ernest Rutherford
Year1911
ModelNuclear model
Key IdeaGold foil experiment: discovered the small, dense, positive nucleus. Electrons orbit outside at distance.
Niels Bohr
Year1913
ModelPlanetary model
Key IdeaElectrons move in fixed circular orbits (energy levels/shells). Each orbit has a fixed energy.
Quantum Mechanical
YearModern
ModelElectron cloud
Key IdeaElectrons exist in probability clouds called orbitals (s, p, d, f). Cannot predict exact position.
⚡ MCQ Tip Rutherford = gold foil experiment = discovered nucleus. Thomson = discovered electron. Bohr = fixed orbits/energy levels. Dalton = indivisible atoms. Modern = electron cloud/orbitals.
C. Electronic Configuration
Shell Capacity & Filling Rules
- Electrons fill shells (energy levels) from the innermost outward
- Shell 1 (K): maximum 2 electrons
- Shell 2 (L): maximum 8 electrons
- Shell 3 (M): maximum 18 electrons (but fills to 8 before M starts filling for elements 1–20)
- The number of valence electrons determines an element's group in the periodic table and its reactivity
Shell Filling Order (first 20 elements)
2 → 8 → 8 → 2
K shell fills first (max 2), then L (max 8), then M (max 8 for elements up to Ca)
H
Hydrogen
Z = 1
1
C
Carbon
Z = 6
2, 4
Na
Sodium
Z = 11
2, 8, 1
Cl
Chlorine
Z = 17
2, 8, 7
Ca
Calcium
Z = 20
2, 8, 8, 2
Mg
Magnesium
Z = 12
2, 8, 2
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Atomic number defines | Number of protons (= electrons in neutral atom) |
| Mass number = | Protons + Neutrons |
| Isotopes differ in | Number of neutrons only — same element, same Z |
| Rutherford's experiment | Gold foil — discovered the nucleus (1911) |
| Thomson discovered | The electron — plum pudding model (1897) |
| Bohr's contribution | Fixed circular orbits (energy levels) for electrons (1913) |
| Na (Z=11) configuration | 2, 8, 1 — 1 valence electron |
| Cl (Z=17) configuration | 2, 8, 7 — 7 valence electrons |
| Shell 1 max electrons | 2 |
| Shell 2 max electrons | 8 |
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