Atomic Models: Historical Development

Study Notes

Development of Atomic Models

Early Ideas: The idea of atoms dates back to Ancient Greece, with the word "atom" coming from the Greek word "atomos" meaning indivisible. Philosophers believed matter was made of tiny, indivisible particles.
Dalton's Atomic Theory (1803): John Dalton formalized the concept of atoms with his atomic theory, based on experimental observations. He proposed that all matter is made up of atoms, which are indivisible and indestructible. All atoms of a given element are identical in mass and properties. Compounds are formed by combinations of two or more different kinds of atoms. Chemical reactions involve the rearrangement of atoms.
Thomson's Plum Pudding Model (1903): J.J. Thomson discovered electrons in 1897 and proposed the "plum pudding" model. This model depicted the atom as a sphere of positive charge with negatively charged electrons scattered throughout, like plums in a pudding.
Rutherford's Nuclear Model (1911): Ernest Rutherford conducted the famous gold foil experiment, where he bombarded a thin sheet of gold foil with positively charged alpha particles. Most alpha particles passed straight through, but some were deflected at large angles. This led to the conclusion that atoms have a small, dense, positively charged center called the nucleus, while the rest of the atom is mostly empty space.
Bohr's Planetary Model (1913): Niels Bohr expanded on Rutherford's model by proposing that electrons move in orbits of fixed sizes and energies around the nucleus. Each orbit has a specific energy level, and electrons can jump between these levels by absorbing or emitting energy.
Schrödinger's Quantum Model (1926): Erwin Schrödinger further developed the model of the atom, proposing that electrons don't move in defined orbits, but rather exist as probability clouds (orbitals) where the likelihood of finding electrons is higher.

Quiz Time!

Thomson proposed that negative particles are mixed in with positively charged material.
Rutherford discovered the atom has a small, dense, positively charged center.
Rutherford named the positive center of the atom the "nucleus."
Thomson discovered that the atom has parts, meaning it can be divided.
Dalton believed that elements differed from one another because their atoms are different.