Atomic Theory and Structure

Early Atomic Theory

• John Dalton proposed the first atomic theory
• Dalton believed atoms were the smallest, indivisible particles
• Dalton's theory stated that all atoms of a particular element have the same weight
• Different elements have different atomic weights
• Atoms combine in simple whole-number ratios to form compounds

Discovery of Subatomic Particles

• Cathode ray experiments revealed negatively charged particles called electrons, traveling from cathode to anode in low-pressure gas tubes.
• Anode rays, traveling from anode to cathode, carried a positive charge, leading to the discovery of the proton.
• Further experiments revealed the neutron, a neutral particle within the atom.

Atomic Structure

• The electron's mass is 9.1 x 10^-31 kg.
• Protons and neutrons have similar masses, approximately 1.67 x 10^-27 kg.
• Electron charge is -1.6 x 10^-19 Coulombs.
• Proton charge is +1.6 x 10^-19 Coulombs.

Atomic Models

• J.J. Thomson's plum pudding model depicted a positively charged sphere with negatively charged electrons embedded within.
• Rutherford's alpha-scattering experiment demonstrated that atoms are mostly empty space, with a small, positively charged nucleus at the center.
• Rutherford proposed electrons orbiting the nucleus due to electrostatic forces.
• Rutherford's model was unstable, as orbiting electrons would lose energy and spiral into the nucleus.

Quantum Mechanics and Atomic Spectra

• Max Planck proposed that light energy is in discrete packets called quanta.
• Energy of a quantum is E = hν, where h is Planck's constant (6.6 x 10^-34 J·s) and ν is the light frequency.
• Einstein explained the photoelectric effect using Planck's quantum theory.
• Atoms emit and absorb light at specific wavelengths, forming unique atomic spectra.
• Balmer formulated a formula predicting spectral lines in the visible region of the hydrogen spectrum.
• Rydberg generalized Balmer's formula for all hydrogen spectral lines.

Bohr's Model of the Atom

• Bohr's model applies only to one-electron systems (hydrogen, helium+, lithium²⁺).
• The model posits electrons orbiting the nucleus in specific energy levels.
• Electron angular momentum is quantized: mvr = nh/2π.
• Bohr's model derived formulas for electron energy (E = -13.6Z² / n² eV), orbital radius (r = a₀n² / Z), and electron velocity (v = 2.18 x 10⁶Z / n m/s).

Modern Atomic Theory

• de Broglie proposed that all particles exhibit wave-like properties.
• Heisenberg's uncertainty principle states that precise simultaneous measurement of a particle's momentum and position is impossible.
• Schrödinger's wave equation defines electron behavior in atoms.
• The wave function (ψ) indicates the probability of finding an electron in a specific region.
• Orbitals represent regions of space where electron probability is high.

Quantum Numbers

• Quantum numbers describe electron properties in atoms.
• Principal quantum number (n): energy level (1 to ∞).
• Azimuthal (angular momentum) quantum number (l): orbital shape (0 to n-1).
• Magnetic quantum number (ml): orbital orientation in space (-l to +l).
• Spin quantum number (ms): electron spin (+1/2 or -1/2).

Electronic Configuration

• Aufbau principle: electrons fill orbitals in increasing energy order.
• Hund's rule: orbitals are singly occupied before pairing up.
• Pauli exclusion principle: no two electrons can have the same four quantum numbers.

Orbitals and Nodes

• Electron probability density is represented by ψ².
• Nodes are regions where ψ² = 0 (zero electron probability).
• Radial nodes occur along the atom's radius.
• Angular nodes occur at specific angles.
• Number of radial nodes is n - l - 1.
• Number of angular nodes is l.
• Total number of nodes in an orbital is n - 1.

Summary:

• Atomic structure understanding progressed from Dalton's theory to modern quantum mechanics.
• Key discoveries (electron, proton, neutron, quantized energy levels) revolutionized atomic understanding.
• Quantum mechanics and quantum numbers form the basis for understanding atomic structure and behavior.