Atomic Theory and Models of the Atom

Atomic Theory

• Democritus hypothesized that matter could be divided into smaller and smaller pieces until a point where it could no longer be divided.
• He called this indivisible piece an atom.
• Dalton stated that all matter is composed of extremely small particles called atoms.
• Atoms of the same element are identical in size, mass, and chemical properties.
• The atoms of different elements are different.
• Atoms cannot be subdivided or destroyed during chemical reactions.
• Atoms are combined, separated, or rearranged in chemical reactions.
• Atoms of different elements combine to form compounds that are always found in definite proportions.

J.J. Thomson's Model

• Discovered negatively charged particles called electrons.
• Proposed the plum pudding model, where electrons are embedded in a positively charged sphere.

Rutherford's Model

• Discovered a dense, positively charged nucleus at the center of the atom.
• Electrons orbit the nucleus in an empty space around the positively charged nucleus.
• Protons are found within the nucleus and are positively charged.
• Neutrons are also found in the nucleus and are neutral.

Origin of Quantum Theory

• Max Planck observed that excited gases produce a bright-line spectrum, meaning energy is emitted at specific wavelengths.
• Planck proposed that energy is emitted or absorbed in discrete, specific quantities called quanta.

Bohr's Model

• Bohr hypothesized that electrons orbit the nucleus in specific, allowed paths.
• Electrons in specific orbits have definite amounts of energy (quantized).

Einstein's Explanation of the Photoelectric Effect

• Einstein explained the photoelectric effect (light striking a metal and ejecting electrons).
• He suggested that light behaves as a stream of particles called photons, and each photon possesses energy.
• Photons with enough energy can cause electrons to jump to higher energy levels.

Louis de Broglie's Hypothesis

• De Broglie proposed that electrons exhibit both wave-like and particle-like properties (wave-particle duality).

Schrodinger's Model

• Schrodinger developed a model describing electron orbital clouds.
• The probability of finding an electron at a specific location around the nucleus is calculated.
• The location of an electron cannot be precisely determined; only the probability can be determined.
• This is because the act of measuring the electron affects its speed and position (Heisenberg's Uncertainty Principle).