Atoms themselves are not directly observable with conventional optical microscopes or the naked eye due to their incredibly small size. Atoms are on the scale of angstroms (10^-10 meters), which is much smaller than the wavelength of visible light. Therefore, scientists have developed indirect methods to infer the existence and properties of atoms based on experimental evidence and theoretical models.
Atomic theory is a scientific model that describes the nature and behavior of atoms. It originated from the work of several notable scientists over centuries, and its development can be traced through the following key milestones:
Ancient Greece: The concept of atoms can be traced back to ancient Greek philosophers such as Leucippus and Democritus in the 5th century BCE. They proposed that matter is composed of indivisible particles called atoms, derived from the Greek word "atomos" meaning "uncuttable" or "indivisible."
Law of Conservation of Mass and Chemical Elements: In the late 18th century, Antoine Lavoisier and others conducted chemical experiments and formulated the Law of Conservation of Mass, which states that mass is conserved during chemical reactions. Lavoisier also identified several chemical elements and performed quantitative experiments to study their compositions.
Dalton's Atomic Theory: In the early 19th century, John Dalton proposed his Atomic Theory, which built upon the concept of atoms. Dalton suggested that elements consist of tiny, indivisible particles called atoms, and that compounds are formed by combining atoms in fixed ratios. He also introduced the concept of atomic weights and explained chemical reactions in terms of rearrangements of atoms.
Discoveries of Subatomic Particles: Towards the end of the 19th century, experiments revealed the existence of subatomic particles within atoms. J.J. Thomson discovered the electron in 1897 using cathode ray tube experiments, and later Ernest Rutherford conducted the famous gold foil experiment in 1909, which led to the identification of the atomic nucleus.
Bohr's Model of the Atom: In 1913, Niels Bohr proposed a model of the atom based on quantum theory. Bohr's model described electrons as orbiting the nucleus in specific energy levels or shells, and it successfully explained the stability of atoms and the emission and absorption of light.
Modern Quantum Mechanical Model: In the 1920s, quantum mechanics emerged as a more comprehensive theory to describe the behavior of atoms. Scientists such as Erwin Schrödinger and Werner Heisenberg developed mathematical equations and principles that described the wave-like behavior of particles, including electrons. The quantum mechanical model introduced the concept of electron probability clouds, where the location of an electron is described by a probability distribution rather than a definite path.
Since then, atomic theory has continued to evolve with advancements in experimental techniques and theoretical models. The development of powerful tools like electron microscopy and spectroscopy has allowed scientists to directly observe and manipulate individual atoms, confirming many aspects of atomic theory. Today, atomic theory forms the foundation of chemistry and provides a fundamental understanding of the properties and interactions of matter at the atomic scale.