Historical Development of Atomic Concept and Atomic Models

Democritus

465

1.All matter consists of invisible particles called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position, and arrangement.

Dalton

Jan 5, 1787

1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms.

Thomson

Jan 5, 1903

1. An atom consists of a sphere of positive charge with negatively charged electron embedded in it,
2. The positive and the negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no over all negative or positive charge.

We can compare the Thomson’s atomic theory model with a water melon. The red edible part of a water melon represents the sphere of a positive charge where as the black seeds embedded in it is like the electrons.

Rutherford

Jan 5, 1911

However, it was not clear how these protons and electrons were arranged within the atom. J.J. Thomson suggested the"plum pudding" model. In this model the electrons and protons are uniformly mixed throughout the atom
Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. Then, if he shot high velocity alpha particles (helium nuclei) at an atom then there

Bhor

Jan 5, 1913

Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons

Modern Atomic Theory

Jan 5, 1921

Bohr's model of the atom is important because it introduced the concept of the quantum in explaining atomic properties. However, Bohr's model ultimately needed revision becuase it failed to explain the natue of atoms more complicated than hydrogen. It took roughly another decade before a new more complete atomic theory was developed - the modern atomic theory.