**Atom and Atomic Model History**
**Early Philosophical Ideas (Ancient Greece):**
* **Democritus (c. 460-370 BC):**
* Proposed the concept of "atomos" (indivisible).
* Believed that all matter is composed of tiny, indivisible particles.
* His ideas were largely philosophical and lacked experimental evidence.
* **Aristotle (384-322 BC):**
* Rejected Democritus's atomic theory.
* Proposed that all matter is composed of four elements: earth, water, air, and fire.
* Aristotle's views dominated scientific thought for centuries.
**The Revival of Atomic Theory (17th-19th Centuries):**
* **John Dalton (1766-1844):**
* Developed the first modern atomic theory based on experimental evidence.
* Dalton's Atomic Theory (1803):
* Elements are made of extremely small particles called atoms.
* Atoms of a given element are identical in size, mass, and other properties.
* Atoms cannot be subdivided, created, or destroyed.
* Atoms of different elements combine in simple whole-number ratios to form
chemical compounds.
* In chemical reactions, atoms are combined, separated, or rearranged.
* Dalton's model: A solid, indivisible sphere.
* **J.J. Thomson (1856-1940):**
* Discovered the electron (1897) using cathode ray tubes.
* Proposed the "plum pudding" model:
* Atoms are positively charged spheres with negatively charged electrons
embedded within them.
* Like plums in a pudding.
* **Ernest Rutherford (1871-1937):**
* Gold foil experiment (1909):
* Directed alpha particles at a thin gold foil.
* Most particles passed through, but some were deflected at large angles.
* This led to the discovery of the nucleus.
* Rutherford's model (1911):
* Atoms have a small, dense, positively charged nucleus at the center.
* Electrons orbit the nucleus like planets around the sun.
* Most of the atom is empty space.
* **Niels Bohr (1885-1962):**
* Bohr model (1913):
* Electrons orbit the nucleus in specific energy levels or shells.
* Electrons can jump between energy levels by absorbing or emitting photons of
light.
* Explained the line spectra of hydrogen.
* **Louis de Broglie (1892-1987):**
* Proposed that electrons have wave-particle duality (1924).
* Electrons can behave as both particles and waves.
* **Werner Heisenberg (1901-1976):**
* Heisenberg Uncertainty Principle (1927):
* It is impossible to know both the position and momentum of an electron
simultaneously.
* **Erwin Schrödinger (1887-1961):**
* Developed the Schrödinger equation (1926) to describe the wave-like behavior of
electrons.
* Quantum mechanical model (electron cloud model):
* Electrons do not orbit the nucleus in definite paths.
* Electrons exist in regions of probability called orbitals.
* Orbitals have different shapes and energy levels.
* The nucleus is surrounded by an electron cloud.
* **James Chadwick (1891-1974):**
* Discovered the neutron (1932).
* Neutrons are neutral particles located in the nucleus.
* This completed the basic atomic model.
**Modern Atomic Theory:**
* The atom consists of a nucleus containing protons and neutrons, surrounded by an
electron cloud.
* Electrons occupy orbitals, which are regions of probability where electrons are likely to
be found.
* Quantum mechanics provides the most accurate description of atomic structure and
behavior.
* The standard model of particle physics further refines our understanding of the
subatomic particles that make up protons, neutrons, and other particles.
Credits to: Gemini
