structure_of_atom

The Structure of the Atom

Overview

• The diversity of chemical behavior among elements is due to differences in internal atomic structure.

• Objectives include discovering subatomic particles, atomic models, and key principles related to atomic structure and configurations.

Historical Background

• Early Theories: Philosophers proposed atoms as indivisible. Derived from Greek "a-tomio" meaning "uncuttable".

• John Dalton (1808): Proposed the atomic theory, explaining laws of conservation of mass and constant composition, but couldn't explain electrical charge phenomena.

Discovery of Sub-atomic Particles

1. Discovery of Electrons

• Michael Faraday (1830): Demonstrated electricity through electrolytic solutions, suggesting a particulate nature of electricity.

• Cathode Ray Tubes:

  • Created by scientists in the mid-1850s to study electrical discharges in gases at low pressure.

  • Cathode rays travel from cathode to anode and strike phosphorescent materials, revealing their negatively charged nature (electrons).

• Observations:

  • Cathode rays are invisible and deflect in magnetic/electric fields, indicating they are negatively charged.

2. Charge to Mass Ratio of Electrons

• J.J. Thomson (1897): Measured the e/m ratio of electrons using electric/magnetic fields, determining it to be approximately 1.758820 x 10^11 C/kg.

3. Charge on Electrons

• R.A. Millikan (1906-14): Used the oil drop experiment to determine the charge of an electron as -1.602176 x 10^-19 C, leading to a measured mass of 9.1094 x 10^-31 kg.

4. Discovery of Protons and Neutrons

• Canal Rays: Positive particles discovered in modified cathode ray tubes, leading to the identification of protons.

• James Chadwick (1932): Discovered neutrons by bombarding beryllium with alpha particles, identifying electrically neutral particles with mass nearly equal to protons.

Atomic Models

1. Thomson’s Model (Plum Pudding Model)

• Proposed a uniform distribution of positive charge, with electrons embedded, explaining overall neutrality but not stability or behavior in experiments.

2. Rutherford’s Model (Nuclear Model)

• Gold Foil Experiment: Alpha particles bombard thin foil, revealing most pass through, but some are deflected, indicating a small, dense, positively charged nucleus.

• Conclusions:

  • Atom consists mostly of empty space.

  • Nucleus contains positive charge and most of the atom's mass.

  • Electrons orbit at considerable distances due to electrostatic attraction.

3. Bohr’s Model of the Hydrogen Atom

• Bohr quantized energy levels and described electron orbits. Improved on Rutherford by introducing stable energy levels where electrons circle the nucleus at fixed distances.

Quantum Mechanical Model

1. Schrödinger’s Equation

• Developed a theoretical framework incorporating wave-particle duality, leading to quantized energy levels and atomic orbitals, allowing prediction of electron distribution.

2. Quantum Numbers and Orbitals

• Each atomic orbital is described by three quantum numbers (n, l, ml) indicating size, shape, and orientation.

• The fourth quantum number (ms) accounts for electron spin.

3. Electron Configuration

• Aufbau principle governs electron filling in orbitals based on increasing energy levels.

• Pauli exclusion principle states no two electrons can have the same set of four quantum numbers.

• Hund’s rule states that every orbital in a subshell is singly occupied before any is doubly occupied, maximizing total spin.

Summary of Electronic Configuration Principles

• Electrons occupy orbitals according to energy levels, following specific filling orders.

• Stability arises from symmetry and exchange energy in half-filled and fully filled subshells.

• The ordering of orbitals also depends on effective nuclear charge and electron shielding effects.

• Hund's Rule: Electrons will fill degenerate orbitals singly before pairing up, maximizing total spin. This principle is crucial for understanding the electronic configuration of elements, as it influences their chemical properties and reactivity.