Democritus to Quantum Models: Atomic Theory Explained

Atomic Theory: From Democritus to Quantum Models

Democritus' Atomic Theory

Democritus, in ancient Greece, founded the atomist school. This school, taking a pluralistic approach, proposed that atoms were the fundamental particles. The core principles of atomism are:

1. Atoms move in a vacuum.
2. The shape and size of atoms determine the substance.

Aristotle, conversely, believed that matter was a single, continuous entity.

Dalton's Atomic Theory

John Dalton proposed his atomic theory in the early 19th century. Its key postulates are:

1. Elements are composed of indivisible and indestructible particles called atoms.
2. Atoms of the same element have the same mass and properties.
3. Atoms of different elements have different masses and properties.
4. Chemical compounds are formed by the union of atoms of different elements in simple numerical ratios.
5. The relative number and kind of atoms are constant in a given compound.

Rutherford's Model

Rutherford proposed a planetary model of the atom, featuring:

1. The mass of an atom is concentrated in a positively charged nucleus.
2. Electrons orbit the nucleus.
3. Most of the atom is empty space.

Bohr's Model

Bohr refined Rutherford's model, particularly for the hydrogen atom, with these postulates:

• Electrons revolve around the nucleus in specific, well-defined circular orbits without emitting or absorbing energy.
• Only certain orbits are allowed, each with a specific energy value. These energy values are quantized. The lowest energy state corresponds to the electron being in the first orbit. More distant orbits represent excited states.
• An electron transitions between orbits by absorbing or emitting energy. Moving to a higher energy orbit requires energy absorption, while moving to a lower energy orbit releases energy in the form of electromagnetic radiation.

Quantum Models

Quantum models incorporate:

• De Broglie hypothesis (wave-particle duality of matter).
• Heisenberg Uncertainty Principle (simultaneous precise determination of position and momentum is impossible).
• Schrödinger equation.

Orbitals

An orbital is a region of space around the nucleus where there is a high probability of finding an electron.

Electronic Configurations

Electronic configurations are governed by:

• Pauli Exclusion Principle (no two electrons in an atom can have the same set of four quantum numbers).
• Aufbau Principle (electrons fill orbitals in order of increasing energy).
• Hund's Rule (electrons individually occupy orbitals of equal energy before pairing up).