John Dalton proposed the first scientific atomic theory. Main postulates: (1) All matter is made of atoms — extremely small, indivisible particles. (2) Atoms of a given element are identical in mass and properties. Atoms of different elements have different masses. (3) Compounds are formed when atoms of different elements combine in simple whole number ratios. (4) Chemical reactions are rearrangements (combination/separation/rearrangement) of atoms. Atoms are neither created nor destroyed in chemical reactions. Laws explained: Law of conservation of mass (atoms indestructible). Law of definite proportions (fixed atom ratio per compound). Law of multiple proportions (same elements in different ratios). Laws NOT fully explained: Gay-Lussac's law of combining volumes. Anomalies later discovered: Isotopes (same element, different mass — violates postulate 2). Sub-atomic particles (atoms divisible — violates postulate 1). Nuclear reactions (atoms can change — violates postulate 4).

When gases react, the volumes of reactants and products (at same T and P) are in simple whole number ratios. Examples: $H_2(g) + Cl_2(g) \to 2HCl(g)$: 1:1:2 volumes. $2H_2(g) + O_2(g) \to 2H_2O(g)$: 2:1:2 volumes. $N_2(g) + 3H_2(g) \to 2NH_3(g)$: 1:3:2 volumes. Dalton's problem: he only had atoms, not molecules. He could not explain how 1 volume of H₂ and 1 volume of Cl₂ give 2 volumes of HCl without splitting the H and Cl atoms. If HCl molecule has 1 H and 1 Cl, you need 2 HCl molecules from 1 H₂ molecule (split the H₂ into 2 H atoms). Dalton rejected atom-splitting, so he struggled with this law. Resolution: Avogadro (1811) proposed that gases consist of molecules (diatomic for H₂, Cl₂, etc.) and equal volumes contain equal numbers of molecules (Avogadro's hypothesis). This resolved the problem perfectly.

Avogadro proposed: equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Key difference from Dalton: Avogadro introduced the concept of a molecule (group of atoms bonded together) as distinct from an atom. Diatomic molecules: H₂, O₂, N₂, Cl₂, Br₂, F₂, I₂. Explanation of Gay-Lussac: H₂ + Cl₂ → 2HCl. 1 molecule H₂ + 1 molecule Cl₂ → 2 molecules HCl. 1 volume + 1 volume → 2 volumes (since each volume has same number of molecules). Avogadro's hypothesis was largely ignored for 50 years (Dalton opposed it). Cannizzaro (1858) revived it and used it to establish consistent atomic masses. This became the foundation of modern chemistry. Avogadro's number $N_A = 6.022 \times 10^{23}$: number of molecules in one mole (his hypothesis finally quantified centuries later).

J.J. Thomson (1897): cathode ray tube experiments. Evacuated tube with high voltage → rays travel from cathode to anode. These rays: deflect in electric and magnetic fields (negatively charged). Same regardless of cathode material (universal particle). Specific charge $e/m = 1.76 \times 10^{11}$ C/kg (always same). Conclusion: electrons are fundamental negatively charged particles present in all atoms. Thomson's "plum pudding" model: positive sphere (pudding) with electrons (plums) embedded. Millikan oil drop experiment (1909): measured charge of electron $e = 1.602 \times 10^{-19}$ C. From $e$ and $e/m$: mass of electron $m_e = 9.11 \times 10^{-31}$ kg $= 1/1836$ of proton mass.

Geiger-Marsden experiment: α particles (from Ra) fired at thin gold foil. Expected (Thomson model): α particles should pass through with slight deflection (positive charge spread out). Observed: most α pass through undeflected (large empty space). Few deflect at large angles. Very few (1 in 20,000) deflect back. Conclusion: most mass concentrated in tiny nucleus (diameter ~10⁻¹⁵ m vs atom 10⁻¹⁰ m). Nucleus is positive. Electrons orbit outside. Nuclear model: positive nucleus + electrons outside. Failures: (1) Could not explain stability of atom (orbiting electron should radiate energy and spiral into nucleus — Maxwell's equations predict this). (2) Could not explain discrete line spectra of hydrogen (orbit of any radius should be possible). Bohr (1913) resolved these by quantizing orbits.

Bohr (1913) postulated: (1) Electrons orbit nucleus in specific allowed orbits without radiating. (2) Angular momentum quantized: $L = mvr = n\hbar$. (3) Energy emitted/absorbed only during transitions: $h\nu = E_1 - E_2$. Results for hydrogen: $r_n = n^2 a_0$ (a₀ = 0.529 Å). $E_n = -13.6/n^2$ eV. Explained: hydrogen spectrum perfectly (Lyman, Balmer, Paschen series). Rydberg constant $R_H$ predicted from first principles. Failures: (1) Cannot explain spectra of multi-electron atoms. (2) Cannot explain fine structure (Zeeman effect). (3) Violates Heisenberg uncertainty principle. Modern quantum mechanics (Schrödinger equation, 1926) overcame all these. Still useful as first approximation.

Heisenberg uncertainty: $\Delta x \cdot \Delta p \geq \hbar/2$. Cannot simultaneously determine exact position and momentum of electron. de Broglie: $\lambda = h/(mv)$. Schrödinger (1926): $\hat{H}\psi = E\psi$ (time-independent). $\psi$ = wave function, $|\psi|^2$ = probability density. Quantum numbers: $n$ (principal), $l$ (azimuthal), $m_l$ (magnetic), $m_s$ (spin). Orbitals: regions of space with 90-95% probability of finding electron. Shapes: s (spherical), p (dumbbell), d (complex), f (more complex). Aufbau, Hund, Pauli rules for electronic configuration. This model fully explains: spectral lines (including fine structure), chemical bonding (MO theory), magnetic properties, periodic trends.

Electron: charge $-e = -1.602\times10^{-19}$ C, mass $9.109\times10^{-31}$ kg. Discovered by J.J. Thomson. Proton: charge $+e$, mass $1.673\times10^{-27}$ kg $= 1836 m_e$. Discovered by Rutherford (1918) from H⁺. Neutron: no charge, mass $1.675\times10^{-27}$ kg ≈ proton mass. Discovered by Chadwick (1932). In atomic nucleus. Essential for nuclear stability (proton-proton repulsion overcome by nuclear force involving neutrons). Positron ($e^+$): same mass as electron, charge $+e$. Antiparticle of electron. Detected by Anderson (1932). Neutrino ($\nu$): near-massless, neutral. Emitted in beta decay. Detected (1956) after 26 years of theoretical prediction. Quarks: fundamental particles. Proton = 2 up + 1 down quark. Neutron = 1 up + 2 down. 6 types (flavours): up, down, charm, strange, top, bottom. Held together by gluons (strong nuclear force).